2023 overview

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2023 Outfall
Monitoring
Overview 
Massachusetts Water
Resources Authority
Environmental Quality 
Department Report 202 4 -09

Citation: 
Werme C, Libby PS, Rogers JB, Carroll SR, Goodwin C. 2024. 2023 outfall monitoring
overview. Boston: Massachusetts Water Resources Authority. Report 2024-09. 63 p. 
Cover photos: 
Top: 2023 Fish and shellfish survey at Deer Island Flats: 
Michael Moore, Woods Hole Oceanographic Institution 
Bottom: Sea stars feeding on barnacles: 
Normandeau Associates, Semper Diving & Marine

2023
Outfall Monitoring Overview 
prepared by 
Christine Werme 
Independent Consultant 
P. Scott Libby 
Battelle
72 Main Street
Topsham, ME 04086 
Jennifer B. Rogers, Sally R. Carroll, and Christopher Goodwin 
Massachusetts Water Resources Authority
Environmental Quality Department
Deer Island
33 Tafts Avenue
Boston, MA 02128 
November 12, 2024

2023 Outfall Monitoring Panel and Committees 
Outfall Monitoring Science Advisory Panel 
Judith Pederson, Massachusetts Institute of Technology Sea Grant (chair)
Peter Burn, Suffolk University
Virginia P. Edgcomb, Woods Hole Oceanographic Institution
Loretta Fernandez, Northeastern University
Robert Kenney, University of Rhode Island
Mark Patterson, Northeastern University
Jeff Rosen, Corona Environmental Consulting
Juliet Simpson, Massachusetts Institute of Technology Sea Grant
Juanita Urban-Rich, University of Massachusetts, Boston 
Inter-Agency Advisory Committee 
Massachusetts Bays National Estuary Partnership
Massachusetts Coastal Zone Management
Massachusetts Department of Environmental Protection
Massachusetts Division of Marine Fisheries
NOAA Fisheries
Stellwagen Bank National Marine Sanctuary
US Army Corps of Engineers
US Environmental Protection Agency
US Geological Survey 
Public Interest Advisory Committee 
Association to Preserve Cape Cod
Boston Harbor Now
Cape Cod Commission
Center for Coastal Studies
Conservation Law Foundation
Massachusetts Audubon Society
Massachusetts Bays National Estuary Partnership
MWRA Advisory Board
MWRA Wastewater Advisory Committee
Safer Waters in Massachusetts
Save the Harbor/Save the Bay

2023 O UTFALL M ONITORING O VERVIEW : E XECUTIVE S UMMARY 
i 
Executive Summary 
The Massachusetts Water Resources Authority (MWRA) provides wastewater
services to more than three million people in Greater Boston. MWRA works with
industrial dischargers to prevent pollutants from entering the waste stream and
provides primary and secondary sewage treatment to remove solid material and
ensure that remaining pollutants meet permitted limits before discharge into
Massachusetts Bay. The sewage effluent discharge is at depth, below the sunlit
portion of the water column, where plant growth occurs, preventing nutrients from
overstimulating algal growth.
MWRA’s permit to discharge treated effluent requires monitoring the sewage effluent
before discharge from Deer Island Treatment Plant and the water column, sea floor,
and living communities in Massachusetts Bay. In the more than two decades that
MWRA has discharged treated wastewater into Massachusetts Bay, there have been
no unanticipated effects or findings of environmental concern. 
Deer Island Treatment Plant effluent meets permit conditions 
Producing the cleanest possible sewage effluent is the most important way that
MWRA strives to protect the marine environment. In 2023, MWRA earned a
Platinum Peak Performance Award from the National Association of Clean Water
Agencies for a 17th consecutive year with no exceedances of its Deer Island
Treatment Plant effluent permit limits.
One important measure of effluent quality is the suspended solids load, that is, the
total amount of solid material discharged each year. Primary treatment removes most
of the solid material from sewage as sludge, which is processed into fertilizer, but
small particles remain suspended in the liquid effluent. The 2023 effluent solids load
remained consistent with the past two decades, far lower than the amounts once
discharged into Boston Harbor (Figure i, top). Discharges of toxic metals have
followed similar patterns, remaining low for more than 20 years (Figure i, bottom).

2023 O UTFALL M ONITORING O VERVIEW : E XECUTIVE S UMMARY 
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Figure i. Annual discharges of sewage solids (top) and metals (bottom), 1990–2023. Source
reduction, including industrial pretreatment, ending sludge discharges, and secondary effluent
treatment dramatically decreased solids and metals discharges, and those decreases have been
sustained for more than 20 years. TSS = total suspended solids, Kg = kilogram.

2023 O UTFALL M ONITORING O VERVIEW : E XECUTIVE S UMMARY 
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MWRA’s discharge has not led to eutrophication 
Sewage treatment removes some, but not all,
of the nutrients in wastewater, including
nitrogen, phosphorus, and silica, so MWRA
water-column monitoring focuses on
potential effects of nutrient inputs.
Monitoring ensures that nutrients do not lead
to eutrophication, a condition caused by
excessive growth and decay of phytoplankton, the microscopic algae that live in the
water column (Figure ii), or stimulate harmful algal blooms. 
Figure ii. Typical phytoplankton from Massachusetts Bay. Phytoplankton include chains of
individual diatoms and larger dinoflagellates, such as this Tripos muelleri , which bloomed throughout
New England in 2023. Photo credit David Borkman, Pausacaco Plankton, Saunderstown, Rhode
Island. 
The MWRA discharge has not increased phytoplankton growth or lowered dissolved
oxygen levels, even at stations closest to the outfall. In 2023, there was an unusual
large bloom of one phytoplankton species, the dinoflagellate Tripos muelleri , which
did affect phytoplankton biomass and dissolved oxygen levels. The bloom occurred
from Maine to Rhode Island, beyond any possible influence of the MWRA
wastewater discharge. Fortunately, dissolved oxygen concentrations did not reach
extremely low hypoxic levels, and marine life was not noticeably affected. The bloom
ended quickly during the summer and did not recur in 2024. 
Diatom chains
Dinoflagellate 
Eutrophication occurs when
nutrients overstimulate
phytoplankton growth, leading to
lower levels of oxygen when the
phytoplankton die and
decompose.

2023 O UTFALL M ONITORING O VERVIEW : E XECUTIVE S UMMARY 
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Seafloor communities remain healthy and diverse 
MWRA’s monitoring of the sediments and animals at the sea floor has shown that
bottom-dwelling communities are influenced by sediment types and water depth
rather than by the effluent discharge. The total abundance of animals, numbers of
species, and the species makeup of bottom-dwelling communities have remained
within the ranges of natural variability. Even the outfall diffusers provide habitat to
bottom-dwelling animal communities (Figure iii). 
Figure iii. Blue mussels and sea anemones growing on an active outfall diffuser in 2023. Robust
communities continue to thrive near the Massachusetts Bay outfall. 
Outfall plume 
Blue mussels 
Sea anemones

2023 O UTFALL M ONITORING O VERVIEW : E XECUTIVE S UMMARY 
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Flounder have low rates of liver disease and are cancer-free 
Winter flounder, popular commercial and recreational fish that live and feed at the sea
floor, are susceptible to contaminant-related physical abnormalities and health risks,
such as fin disease, ulcers, and liver conditions. Flounder health has improved both
near the outfall and at the former sewage effluent outfall in Boston Harbor (Figure
iv), a result of pollution prevention and sewage treatment. Cancerous tumors have
never been found in fish taken from near the Massachusetts Bay outfall. 
Figure iv. Winter flounder tumor precursors, 1991–2023. Incidence of the tumor precursor CHV
has declined throughout the region. Only the outfall site and Deer Island Flats in Boston Harbor were
sampled in 2023. Nantasket Beach and East Cape Cod Bay provided additional geographic coverage in
previous years. CHV = centrotubular hydropic vacuolation, a key tumor precursor and mild liver
condition associated with exposure to pollutants. 
Stellwagen Bank Marine Sanctuary remains healthy 
MWRA’s permit requires an assessment of monitoring results relevant to the
Stellwagen Bank National Marine Sanctuary, about 2000 protected square kilometers,
located offshore from the outfall. The Sanctuary’s nearest boundary is about 20
kilometers to the east of the outfall. Stellwagen Bank supports rich recreational and
commercial fisheries and is a major feeding ground for whales. There have been no
measurable effects of the MWRA outfall discharge on the sanctuary’s water column
or sea floor.
The 2023 bottom-water dissolved oxygen concentrations in Stellwagen Basin reached
unusually low levels, a result of the wide-spread Tripos muelleri phytoplankton
bloom and changing physical conditions throughout the region. No environmental
harm was observed.

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Environmental conditions are changing throughout the region 
While MWRA has not found any unexpected or harmful effects of its outfall on
marine life, the more than 30 years of monitoring have detected changes. Waters are
warmer than they used to be (Figure v), and changes in wind speeds and directions
have altered the flow of ocean water into Massachusetts Bay. These physical changes
affect summertime dissolved oxygen levels and the biology and ecology of the
region. Other scientists and government agencies throughout New England have
detected similar changes. MWRA is paying close attention to their own
measurements and results reported by other research organizations throughout New
England and the Gulf of Maine to better understand regional changes and how they
might interact with the outfall in the future. 
Figure v. Increases in summertime surface-water temperatures in Massachusetts Bay, 2001-
2023. Maximum, mean, and minimum values recorded at an instrumented buoy off Cape Ann at the
northern limit of the bay.

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Table of Contents 
Executive Summary............................................................................................................. i 
1. Introduction ....................................................................................................................1 
2. Effluent ....................................................................................................................2 
2023 Effluent Characterization .......................................................................................3 
Effluent Contingency Plan Thresholds............................................................................8 
3. Water Column .................................................................................................................9 
Physical Conditions .......................................................................................................11 
Water Quality ................................................................................................................14 
Nutrients ....................................................................................................................15 
Phytoplankton Biomass .............................................................................................18 
Dissolved Oxygen .....................................................................................................19 
Plankton Communities ..................................................................................................20 
Phytoplankton Communities .....................................................................................22 
Zooplankton Communities ........................................................................................24 
Stellwagen Bank National Marine Sanctuary ...............................................................26 
Boston Harbor Water Quality .......................................................................................27 
Water-Column Contingency Plan Thresholds...............................................................28 
Chlorophyll Exceedances ..........................................................................................29 
Dissolved Oxygen Exceedances................................................................................30 
4. Sea Floor ..................................................................................................................32 
Sediment Characteristics and Tracers ...........................................................................35 
Soft-bottom Communities .............................................................................................37 
Hard-bottom Communities ............................................................................................41 
Stellwagen Bank National Marine Sanctuary ...............................................................43 
Boston Harbor Seafloor Monitoring .............................................................................43 
Seafloor Contingency Plan Thresholds .........................................................................46 
5. Fish and Shellfish ..........................................................................................................47 
Flounder Characteristics ................................................................................................49 
Flounder Health .............................................................................................................51 
Winter Flounder Contingency Plan Thresholds ............................................................52 
6. Special Topics ...............................................................................................................53 
Regional Environmental Trends....................................................................................53 
2023 Tripos muelleri Bloom .........................................................................................55 
7. Update on the NPDES Permit .......................................................................................59 
References ..................................................................................................................60 
List of Acronyms ...............................................................................................................63

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List of Figures 
Figure i. Annual discharges of sewage solids (top) and metals (bottom), 1990–2023....... ii 
Figure ii. Typical phytoplankton from Massachusetts Bay ............................................... iii 
Figure iii. Blue mussels and sea anemones growing on an active outfall diffuser in 2023.
.................................................................................................................. iv 
Figure iv. Winter flounder tumor precursors, 1991–2023. ..................................................v 
Figure v. Increases in summertime surface-water temperatures in Massachusetts Bay,
2001-2023 ................................................................................................. vi 
Figure 2-1. Annual and average rainfall in Boston, 1990–2023 .........................................2 
Figure 2-2. Annual full secondary-treated effluent flows and primary-blended flows,
1999–2023 ..................................................................................................3 
Figure 2-3. Monthly full secondary-treated and primary-blended flows and rainfall in
2023. ...........................................................................................................4 
Figure 2-4. Average annual daily discharges of sewage solids, 1990–2023 .......................4 
Figure 2-5. Annual nitrogen discharges, 1996–2023 ..........................................................6 
Figure 2-6. Annual metals discharges, 1990–2023 .............................................................7 
Figure 3-1. Water-column monitoring stations and instrumented buoys in Massachusetts
and Cape Cod Bays ..................................................................................10 
Figure 3-2. Discharges from the Merrimack (top) and Charles Rivers. ............................12 
Figure 3-3. Surface- and bottom-water temperature and salinity at Station N18 just to the
south of the outfall in 2023 compared to prior years................................13 
Figure 3-4. Stratification of the water column in 2023, compared to prior years. ............14 
Figure 3-5. Seasonal trends of major water-column nutrient concentrations at Station N18
just to the south of the outfall in 2023 compared to prior years ...............15 
Figure 3-6. Ammonium concentrations at selected stations in 2023 compared to prior
years.......................................................................................................... 17 
Figure 3-7. Surface- and bottom-water ammonium during mixed and stratified conditions
..................................................................................................................18 
Figure 3-8. Monthly areal chlorophyll concentrations at selected stations in 2023
compared to prior years ............................................................................19 
Figure 3-9. Near-bottom water dissolved oxygen concentrations in 2023 compared to
prior years .................................................................................................20 
Figure 3-10. Typical phytoplankton from Massachusetts Bay. .........................................21 
Figure 3-11. Total phytoplankton abundance at selected stations in 2023 compared to
prior years. ................................................................................................22

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Figure 3-12. Tripos muelleri abundance at selected stations in 2023 compared to prior
years.......................................................................................................... 23 
Figure 3-13. A lexandrium catenella abundance in 2023 compared to prior years ...........24 
Figure 3-14. Total zooplankton abundance at selected stations in 2023 compared to prior
years.......................................................................................................... 25 
Figure 3-15. Surface chlorophyll measured at the NERACOOS Buoy A01 and nearby
MWRA station F22 in 2023 .....................................................................26 
Figure 3-16. Dissolved oxygen concentrations at the NERACOOS Buoy A01 and at
nearby deepwater MWRA survey Station F22 in 2023............................27 
Figure 3-17. Summer (top) and annual (bottom) average chlorophyll concentrations,
1992–2023 ................................................................................................29 
Figure 3-18. Bottom dissolved oxygen concentrations and percent saturation in the
nearfield and Stellwagen Basin, 2023 ......................................................31 
Figure 4-1. Soft-bottom monitoring stations .....................................................................33 
Figure 4-2. Hard-bottom monitoring stations. ...................................................................34 
Figure 4-3. Concentrations of the effluent tracer Clostridium perfringens spores in 2023
..................................................................................................................35 
Figure 4-4. Total organic carbon at selected stations, 1992–2023 ....................................36 
Figure 4-5. Total abundance of soft-bottom organisms by region, 1992–2023.. ..............37 
Figure 4-6. Polychaete worms dominate the soft-bottom habitats in Massachusetts Bay.38 
Figure 4-7. Ordination plot of 2023 Massachusetts Bay samples by location ..................40 
Figure 4-8. Percent fine sediments superimposed on the ordination plot of the 2023
samples .....................................................................................................40 
Figure 4-9. Top: Large numbers of sea stars feeding on barnacles in 2023. Bottom: Blue
mussels and sea anemones growing on an active outfall diffuser ............42 
Figure 4-10. Total abundance of soft-bottom animals (top) and number of species
(bottom) per sample in Boston Harbor samples, 1991–2023 ...................44 
Figure 4-11. Example of a sediment-profile image, showing a large mantis shrimp burrow
at Station R07, at Deer Island Flats, near the location of the former
Boston Harbor effluent outfall. .................................................................45 
Figure 5-1. Fish-and-shellfish monitoring stations............................................................48 
Figure 5-2. Top: Lost lobster traps and other “ghost” fishing gear interfere with flounder
collection. Bottom: Excess kelp also interferes with sampling. ...............50 
Figure 5-3. Annual prevalence of the tumor precursor centrotubular hydropic vacuolation
in winter flounder from the Massachusetts Bay outfall site and Deer
Island Flats, 1991–2023............................................................................51 
Figure 5-4. Annual prevalence of liver tumors in winter flounder, 1991–2023 ................52

2023 O UTFALL M ONITORING O VERVIEW : T ABLE OF C ONTENTS 
x 
Figure 6-1. Increases in summertime (July and August) water temperatures in
Massachusetts Bay, 2001-2023. ...............................................................54 
Figure 6-2. Wind stress, 1989-2023 ..................................................................................55 
Figure 6-3. A plankton tow near the mouth of the North River, Marshfield,
Massachusetts, during the 2023 Tripos muelleri bloom ...........................56 
Figure 6-4. Average abundance of Tripos muelleri in 2023 MWRA samples compared to
1992–2022 ................................................................................................57 
Figure 6-5. Tripos muelleri is much larger, contains more biomass, and grows more
slowly than other types of phytoplankton. ...............................................57 
Figure 6-6. Tripos muelleri abundance in June compared to water-column stratification
during May, 1992–2023 ...........................................................................58 
List of Tables 
Table 2-1. Contingency Plan threshold values and 2023 results for effluent monitoring ...8 
Table 3-1. Contingency Plan threshold values and 2023 results for water-column
monitoring. ...............................................................................................28 
Table 4-1. Contingency Plan threshold values and 2023 results for seafloor monitoring
..................................................................................................................46 
Table 5-1. Contingency Plan threshold values and 2023 results for winter flounder
monitoring ................................................................................................52

2023 O UTFALL M ONITORING O VERVIEW : I NTRODUCTION 
1 
1. Introduction 
Since 1984, the Massachusetts Water Resources Authority (MWRA) has worked to
minimize adverse effects of municipal wastewater on the marine environment through
a vigorous pretreatment program, a carefully maintained treatment plant, and vigilant
effluent and environmental monitoring. One of the most important milestones in the
well-known Boston Harbor Project occurred in September 2000, when the MWRA
Deer Island Treatment Plant wastewater effluent discharge was diverted from the
shallow, enclosed waters of Boston Harbor to deeper, more open waters in
Massachusetts Bay. Since then, the Massachusetts Bay outfall has operated under a
National Pollutant Discharge Elimination System (NPDES) permit, jointly issued by
the U.S. Environmental Protection Agency (EPA) and the Massachusetts Department
of Environmental Protection. An independent Outfall Monitoring Science Advisory
Panel has assisted EPA in addressing scientific questions about potential
environmental effects of the outfall on Massachusetts Bay.
The NPDES permit requires MWRA to measure effluent quality before discharge and
to monitor the seawater, sea floor, and marine life in Massachusetts Bay. A
Contingency Plan attached to the permit requires comparing measured conditions
against “caution” and “warning” thresholds, developed from state standards and
baseline monitoring data. If a caution threshold is exceeded, MWRA must investigate
the issue. A warning threshold is a higher level of concern and could require both
investigation and a responsive action. Background information about the monitoring
program (Werme et al. 2012), the most current monitoring plan (MWRA 2021) and
Contingency Plan (MWRA 2001), past plans and overviews, and study-specific
technical reports are available on MWRA’s Technical Reports List at
www.mwra.com/our-environment/water-quality-reports.
This 2023 Outfall Monitoring Overview fulfills a requirement of the NPDES permit,
which stipulates that MWRA produce an annual summary of monitoring results for
Massachusetts Bay, including results relevant to the Stellwagen Bank National
Marine Sanctuary; the sanctuary’s western boundary is about 20 kilometers offshore
from the MWRA outfall.
This overview also discusses special topics selected in response to permit conditions
and environmental concerns, often including research completed in cooperation with
other agencies and research organizations. This year’s special topics section includes
a discussion of changing environmental conditions throughout the broader region of
Massachusetts Bay and the Gulf of Maine and a report on a large bloom of a marine
phytoplankton species, Tripos muelleri , which occurred throughout the region during
the summer of 2023. The bloom extended well beyond any possible influence of the
MWRA outfall but led to several Contingency Plan threshold exceedances.

2023 O UTFALL M ONITORING O VERVIEW : E FFLUENT 
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2. Effluent 
MWRA’s NPDES permit and Contingency Plan set limits on effluent parameters and
pollutants. Contaminants of concern include pathogens, suspended solids, organic
material, nutrients, and toxic contaminants . Because wastewater entering Deer
Island Treatment Plant includes not only municipal sewage but also groundwater
infiltration and stormwater inflow, rainfall is an important factor in determining
annual wastewater flows and contaminant concentrations in the treated effluent. The
Boston area received 50 inches of rain in 2023, more than the 1990–2023 average of
44 inches (Figure 2-1). 
Figure 2-1. Annual and average rainfall in Boston, 1990–2023. Boston received almost 50 inches of
rain in 2023. Data are from the National Weather Service rain gauge at Boston Logan International
Airport. 
Deer Island Treatment Plant continued to meet permit conditions through 2023,
marking 17 consecutive years of 100% compliance with permit limits and earning
MWRA a National Association of Clean Water Agencies Platinum Peak Performance
Award. MWRA’s compliance report for Fiscal Year 2023 summarizes data from July
2022 through June 2023 (Davis 2023); this Outfall Monitoring Overview reports on
data for January through December 2023. 
Effective wastewater treatment protects Massachusetts Bay. 
MWRA’s industrial pretreatment program and its primary and secondary
sewage treatment minimize the contaminants discharged into Massachusetts
Bay. In 2023, the effluent met all permit and Contingency Plan limits.
.

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2023 Effluent Characterization 
The high rainfall in 2023 led to an annual
effluent flow comparable to other recent wet
years (Figure 2-2). Flow averaged 337 million
gallons per day, just slightly higher than the
average 327 million gallons per day since 1999.
As in past years, almost all effluent in 2023
received full secondary treatment, with less than
1% of the total flow consisting of primary-only
treated effluent blended into secondary-treated
effluent prior to disinfection and discharge
(Figure 2-3).
Primary treatment uses physical process to remove solid material through settling.
Secondary treatment adds biological processes that remove most of the remaining
organic material and toxic pollutants. Blending occurs during intense rainfall, when
flow to the treatment plant exceeds what the secondary treatment process is designed
to handle. Diverting some flow around the secondary treatment process prevents
street flooding and water backups into homes and businesses. It also protects the
secondary treatment system, preventing washout of the beneficial bacteria that are a
critical part of the process. Massachusetts regulations require MWRA to notify
regulators and the public whenever a blending event occurs. Historically, blended
flows have fully met permit limits. 
Figure 2-2. Annual full secondary-treated effluent flows and primary-blended flows, 1999–2023 .
During large storms, flow exceeding the secondary capacity of the plant is diverted around the
secondary process; this primary-treated flow is blended with the fully treated flow before disinfection
and discharge. Historically, blended flows have been minimal and met all permit limits. MGD =
million gallons per day. 
Flow refers to the rate at which
wastewater travels through the
treatment plant. It may refer to
flow entering the plant (influent),
but often refers to outflow from
the plant (effluent). Once
effluent is released to the
environment, it is often called
the discharge .

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Figure 2-3. Monthly full secondary-treated and primary-blended flows (bars) and rainfall
(dashed line) in 2023. Months with primary-blended flows corresponded with months of greater
rainfall and snow melt. MGD = million gallons per day. 
Most solid material is removed from sewage influent as sludge, and the remaining
particles are called total suspended solids. In 2023, about 16 metric tons of total
suspended solids were discharged per day, only slightly more than in 2022, despite
much higher flow (Figure 2-4). Over the past two decades, the total solids discharges
have been only about 10% of what had been discharged to the harbor in 1990–1991. 
Figure 2-4. Average annual daily discharges of sewage solids, 1990–2023. Solids discharges have
remained low. TSS = total suspended solids.

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Organic material in effluent consumes
oxygen as it decays. Biochemical
oxygen demand (BOD), a measure of
the amount of oxygen needed to
decompose organic material,
remained well below levels that would be expected to affect dissolved oxygen in the
environment in 2023. MWRA has consistently met its BOD permit limit. 
MWRA monitors the effluent for fecal
coliform bacteria, one indicator of a variety of
pathogens that can make shellfish unhealthy
for human consumption. Fecal coliform
bacteria levels in the effluent have always
been well below permit limits. MWRA also
monitors fecal coliform bacteria in
Massachusetts Bay, and results at all locations, even those closest to the outfall, meet
the stringent shellfishing standards set by the state.
MWRA’s permit requires reporting on
nutrients, toxic metals, and organic
compounds. Nutrient measurements
include individual components of total
nitrogen (ammonium, nitrate, and
nitrite). Toxic metals with reporting requirements include zinc, copper, lead, nickel,
silver, and mercury. Organic compounds include selected pesticides and
polychlorinated biphenyls (PCBs).
The Contingency Plan attached to MWRA’s permit includes thresholds for nitrogen
discharges. The total nitrogen load for 2023 was 12,103 metric tons, below the
Contingency Plan caution threshold of 12,500 metric tons per year (Figure 2-5). This
threshold was set conservatively at about 90% of the warning threshold, 14,000
metric tons, which was the amount expected to be discharged annually by 2020,
based on population growth projections for the MWRA service area made in the
1990s. Actual population growth has been somewhat higher than projected, but
nitrogen loads have remained well below that anticipated amount, a result of water
conservation, better controls on groundwater infiltration, and good removal
efficiencies during treatment. The caution threshold was exceeded once, in 2019,
when the population size of the MWRA service area was about 6% greater than had
been predicted for 2020. However, modeling has suggested that nitrogen loads even
higher than the more stringent warning threshold would not harm the environment
(Deltares 2022). MWRA updates a report on potential nitrogen-removal strategies
annually (Davis et al. 2024). 
Organic material , a major constituent
of untreated sewage that consumes
oxygen as it decays, remains at safe
levels. 
Nutrients control algal growth and, in
excess, can lead to lower oxygen
concentrations or stimulate nuisance
algal blooms. 
Pathogens include disease-
causing bacteria, viruses, and
protozoa, and are found in
human and animal waste.
Effluent bacteria levels continue
to meet permit limits.

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Figure 2-5. Annual nitrogen discharges, 1996–2023. The warning threshold, 14,000 metric tons per
year, was set as the load anticipated for 2020; the caution threshold was set at 90% of the warning
threshold. Actual loads have remained lower than anticipated. TN = total nitrogen. 
Ammonium is the form of nitrogen most readily taken up by phytoplankton and a
good effluent tracer. During the late 1990s and early 2000s, ammonium discharges
became a larger component of the total nitrogen loading, likely a result of the
evolving treatment processes for both sewage sludge and effluent. Those increases
have plateaued in more recent years.
Metals discharges remained low in 2023
(Figure 2-6). Zinc continued to be the most
abundant metal, followed by copper. Both
are present in water pipes and fixtures.
Other notable sources of zinc to wastewater
include hair salons, automobile-repair
shops, and hospitals; residential household
products, such as shampoos, ointments, and laundry detergent; and street runoff
(MWRA, unpublished data). 
Toxic contaminants in the effluent
include heavy metals, such as
copper and lead, and organic
compounds, including some now-
banned pesticides and
polychlorinated biphenyls (PCBs).

2023 O UTFALL M ONITORING O VERVIEW : E FFLUENT 
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Figure 2-6. Annual metals discharges, 1990–2023. Zinc and copper from water pipes and fixtures
remain the most abundant metals. Source reduction, industrial pretreatment, and secondary treatment
have dramatically decreased metals loads in the effluent. Kg = kilogram. 
Total loads of all metals remained small percentages of what had been anticipated
during planning for the Massachusetts Bay outfall. Copper discharges remained less
than 20% of what had been predicted would be discharged in 2020, and all other
metals discharges were less than 10% of the predictions (summarized in Werme et al.
2021). Except for copper, all metals meet water quality standards even prior to
discharge. Initial dilution (the amount of mixing within a strictly defined area near the
outfall) further reduces concentrations. Copper discharges, which meet the standard
after initial dilution, have declined over time, a result of drinking-water corrosion
control, which decreases leaching from water pipes. Removing lead from pipes and
service lines has significantly reduced lead levels in the effluent discharge.
Mercury discharges were only 1% of what had been anticipated during planning for
the outfall, a result of source-reduction efforts in dental and medical facilities. Silver,
once considered an effluent tracer, is now often at levels below detection limits, due
to the decline in film-based photography, as well as efficient removal by secondary
treatment.
Discharges of organic contaminants have varied slightly from year to year but have
been well below loads historically discharged into Boston Harbor. Annual polycyclic
aromatic hydrocarbons (PAH) discharges, for example, were the lowest ever
measured in 2023, less than 2% of the 3,000 kilograms of PAHs estimated to have
been discharged in wastewater effluent into Boston Harbor in 1991 (Rex et al. 1992).
PAH concentrations now meet water quality standards at discharge. Effluent data for
some other organic compounds, such as 4,4’-DDE, one of the most prevalent
breakdown products of the pesticide DDT, show evidence of the very slow declines
anticipated in the 1970s, when PCBs and chlorinated pesticides were banned.

2023 O UTFALL M ONITORING O VERVIEW : E FFLUENT 
8 
Effluent Contingency Plan Thresholds 
There were no effluent Contingency Plan exceedances in 2023 (Table 2-1).
Exceedances of effluent permit limits have been rare, and none have occurred since
2006. The total nitrogen thresholds, set in the Contingency Plan attached to the
permit, were also not exceeded in 2023. Similar to other effluent constituents, total
nitrogen discharges have been less than were expected during planning for the
treatment plant and the outfall. 
Table 2-1. Contingency Plan threshold values and 2023 results for effluent monitoring. 
Parameter Baseline Caution Level Warning Level 2023 Results 
Permit Condition and Contingency Plan Thresholds 
pH NA NA <6 or >9 Not exceeded
Fecal coliform NA NA >14,000 fecal
coliforms/100 mL Not exceeded
Chlorine, residual NA NA >631 μg/L daily,
>456 μg/L monthly Not exceeded
Suspended solids NA NA >45 mg/L weekly
>30 mg/L monthly Not exceeded
cBOD NA NA >40 mg/L weekly,
>25 mg/L monthly Not exceeded
Acute toxicity NA NA LC50 <50% Not exceeded
Chronic toxicity NA NA NOEC <1.5%
effluent Not exceeded
PCBs NA Aroclor>0.045 ng/L NA Not exceeded
Plant performance NA 5 violations/year Compliance <95%
of the time 100% compliance
Flow NA NA >436 MGD
average dry days Not exceeded
Oil and grease NA NA >15 mg/L weekly Not exceeded 
Contingency Plan Thresholds 
Total nitrogen load NA >12,500
mtons/year
>14,000
mtons/year 12,103 mtons 
NA = not applicable
cBOD = carbonaceous biological oxygen demand
LC50 = 50% mortality concentration
NOEC = no observable effect concentration
PCB = polychlorinated biphenyl
Aroclor = total for a specified suite of PCB compounds (called Aroclors)
Plant performance = compliance with permit conditions
mL = milliliter
μg/L = micrograms per liter
mg/L = milligrams per liter
ng/L = nanograms per liter
MGD = million gallons per day
mtons = metric tons

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
9 
3. Water Column 
MWRA’s water-column monitoring program measures and evaluates physical
oceanographic conditions, water quality, and phytoplankton and zooplankton
communities at stations in Massachusetts Bay, at the mouth of Boston Harbor, and in
Cape Cod Bay. Ship-based field surveys are augmented by data from instrumented
buoys and satellite imagery. Organic material, pathogens, and toxic contaminants are
largely removed in wastewater treatment, so are mostly undetectable in the water, and
unlikely to have any harmful effects. Therefore, water-column monitoring focuses
mainly on potential effects of excess nutrients, which are less completely removed by
treatment.
Excess nutrients can cause eutrophication or
promote growth of nuisance or toxic algal
blooms. One nuisance, but not toxic,
phytoplankton species, Phaeocystis
pouchetii , was a focus in past years, but its
presence and abundance have been shown to
vary in response to physical oceanographic
conditions rather than the outfall, and it is no longer a focus of the monitoring
program. Potentially toxic species include Alexandrium catenella , which occurs
regularly in Massachusetts Bay, and Pseudo-nitzschia , a species group that is often
present, but usually in low numbers and in its nontoxic forms.
Each monitoring year includes nine ship-based surveys, completed monthly between
February and October. Five stations are in the “nearfield,” a 12×10-kilometer area
centered on the outfall, where some effects of the effluent were expected and have
been observed. Nine stations are in the more distant “farfield,” including the mouth of
Boston Harbor, Cape Cod Bay, and near Stellwagen Bank National Marine Sanctuary
(Figure 3-1). Nearfield Station N18, located just to the south of the outfall, is often
considered representative of the nearfield, as it has been consistently monitored over
time and would be likely to show any effects, should they occur. 
Water-column monitoring focuses on potential eutrophication. 
For many parameters, 2023 was a typical year in the water column, following
well-established seasonal patterns. However, a large regional bloom of the
phytoplankton species Tripos muelleri resulted in widespread high summer
chlorophyll concentrations and low dissolved oxygen levels. 
Eutrophication occurs when
nutrients overstimulate
phytoplankton growth, leading to
lower levels of oxygen when the
phytoplankton die and
decompose.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
10 
Figure 3-1. Water-column monitoring stations and instrumented buoys in Massachusetts and
Cape Cod Bays. Data are obtained from surveys, the NERACOOS A01 and NDBC 44013 buoys, and
satellite imagery. Also shown are the outfall and the Stellwagen Bank National Marine Sanctuary.
Nearfield stations include a designation N, while farfield stations include a designation F.
NERACOOS = Northeastern Regional Association of Coastal Ocean Observing Systems; NDBC =
National Data Buoy Center.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
11
Additional surveys may be triggered by elevated abundance of the potentially toxic
phytoplankton species Alexandrium catenella . These “ Alexandrium Rapid-Response
Study” surveys (Libby et al. 2013) provide in situ hydrographic data and water
samples for measuring nutrients and Alexandrium catenella abundance at up to 19
stations. No additional surveys were necessary in 2023.
The monitoring program benefits from collaboration with the Center for Coastal
Studies in Provincetown, Massachusetts, which samples the three stations in Cape
Cod Bay and near Stellwagen Bank National Marine Sanctuary (Figure 3-1, red
points). In 2023, all sampling by MWRA and the Center for Coastal Studies occurred
within 48-hour windows.
Surveys are supplemented by measurements at two instrumented buoys: the
Northeastern Regional Association of Coastal and Ocean Observing Systems
(NERACOOS) Buoy A01 off Cape Ann at the northern limit of Massachusetts Bay,
and the National Oceanic and Atmospheric Administration (NOAA) National Data
Buoy Center (NDBC) Buoy 44013 in central Massachusetts Bay (Figure 3-1, white
diamonds). The National Aeronautics and Space Administration provides Moderate
Resolution Imaging Spectroradiometer satellite imagery of chlorophyll fluorescence,
a measure of phytoplankton biomass. 
Physical Conditions 
Circulation and physical properties in Massachusetts Bay are driven by the larger
patterns of water flow in the Gulf of Maine. Water typically enters Massachusetts Bay
from the north, circulates in a counter-clockwise direction, and exits by Race Point, at
the tip of Cape Cod. Consequently, Station F22, to the north and east of the outfall, is
considered a reference station for water entering the bay, while stations to the south
of the outfall are more likely to have detectable outfall influences. Water flows vary
from week-to-week and depend on the strength of the current and on wind speed and
direction. The coastal current is strongest during spring runoff from rivers and
streams.
As in other temperate waters, there are
seasonal patterns to the physical structure
of the water column, extending from the
surface to the sea floor. In the winter, the
water column is well mixed, and dissolved
oxygen levels are high throughout the
water column. Warm water is less dense
than cold water, so as surface waters warm, the water column stratifies, effectively
separating surface from bottom waters. Bottom waters become isolated from the
atmosphere and oxygen production by phytoplankton, which leads to lowered
bottom-water dissolved oxygen levels in late summer and fall. In the fall, cooling
surface waters and strong winds promote mixing, ending the stratified season and re-
oxygenating bottom waters. 
Stratification separates warm,
surface water from colder, denser
bottom water. The boundary
between layers is called the
thermocline or pycnocline.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
12 
Stratification is also affected by rainfall and river flow to Massachusetts Bay. Inputs
of freshwater, which is lighter than seawater, contribute to stronger stratification. The
Merrimack River, which enters the Gulf of Maine just to the north of Massachusetts
Bay, is the largest river in the region and plays an important role in freshwater
delivery to Massachusetts Bay. River flow from the smaller Charles, Mystic, and
Neponsit Rivers into Boston Harbor also influence conditions in the bay.
Following a dry year in 2022, 2023 was relatively wet, particularly during the
summer, when river flows are usually at their lowest (Libby et al. 2024). Flows from
both the Merrimack and Charles Rivers were greater than average in the winter,
summer, and fall (Figure 3-2), leading to lower-than-average surface salinities and
promoting stronger stratification. 
Figure 3-2. Discharges from the Merrimack (top) and Charles (bottom) Rivers. The Charles River
flows into Boston Harbor and the much larger Merrimack River flows into the Gulf of Maine just
south of the New Hampshire border. The quarterly percentiles represent the 2023 flows in comparison
to the entire record. m 3 /s = cubic meters per second. 
Winter air temperatures measured at the NERACOOS A01 buoy at the northern
boundary of Massachusetts Bay, were unusually warm in the winter of 2023. Winter
water temperatures, measured at the NDBC 44013 buoy in the middle of the bay,
were also warm, and those warm water temperatures persisted into the summer, with
a brief cooling episode in August. The same temperature pattern was observed during
MWRA surveys. Surface- and bottom-water temperatures were warmer than prior
years during most survey months, for example, at Station N18 just south of the outfall
(Figure 3-3, top two panels).

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
13
Surface- and bottom-water salinities at Station N18 reflected the high river discharges
in 2023 (Figure 3-3, bottom panels). Low surface-water salinity was a major factor in
establishing especially strong summer stratification throughout Massachusetts Bay
(Figure 3-4). 
Figure 3-3. Surface- and bottom-water temperature and salinity at Station N18 just to the south
of the outfall in 2023 compared to prior years. PSU = practical salinity units or parts per thousand.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
14 
Figure 3-4. Stratification of the water column in 2023, compared to prior years. Greater
stratification indicates greater separation between surface and bottom waters. Strong stratification
developed early throughout Massachusetts Bay in 2023. kg/m 3 = kilograms per cubic meter. 
Wind speeds and directions, which can exert large effects on conditions in the bay,
were mostly typical through 2023. No storms produced high winds or large waves
capable of resuspending and transporting bottom sediments, which occurs in some
years. Winds from the south in July and August promoted upwelling of bottom water
to the surface, temporarily disrupting stratification and cooling the surface waters. 
Water Quality 
Water quality measurements include quantification of nutrients (nitrogen,
phosphorus, and silica), phytoplankton biomass (chlorophyll and particulate organic
carbon), and dissolved oxygen (concentration and percent saturation). These
measurements address the concern that excess nutrients from the effluent discharge
could stimulate phytoplankton growth (increase biomass) and exacerbate the low
bottom-water oxygen conditions that occur naturally in the late summer and fall.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
15
Results for 2023 continued to confirm measurable outfall influence on some nutrient
concentrations at stations near the outfall (Libby et al. 2024). Unlike previous years,
plankton biomass reached unusually high levels throughout the bay in April until
June, a result of the unprecedented bloom of the dinoflagellate Tripos muelleri . This
bloom preceded unusually low summer dissolved oxygen levels throughout the bay. 
Nutrients 
The most important nutrients that fuel phytoplankton growth in Massachusetts Bay
are two forms of nitrogen (nitrate and ammonium), phosphate, and silicate. Typical of
temperate, coastal waters, nutrient concentrations follow seasonal patterns, naturally
varying with phytoplankton uptake, seasonal stratification, water exchange with the
Gulf of Maine, and river flow. Concentrations of some of the nutrients near the outfall
followed the typical patterns in 2023, but there were also some variations (Figure 3-
5), due to effluent inputs, physical conditions, and the Tripos muelleri bloom. 
Figure 3-5. Seasonal trends of major water-column nutrient concentrations at Station N18 just to
the south of the outfall in 2023 compared to prior years. μM = micromoles.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
16
Following the typical coastal pattern, nitrate and phosphate concentrations were
relatively high at the beginning of the survey season, and decreased through the
spring, as phytoplankton production and biomass increased. These typical springtime
declines were especially steep in 2023, and nitrate levels remained especially low or
depleted from April through July.
Silicate concentrations generally follow similar though less pronounced seasonal
patterns. In 2023, silicate concentrations were somewhat lower than most years at the
start of the survey season, generally an indication that a winter diatom bloom has
occurred prior to the February survey. Diatoms are phytoplankton that use silicate to
build glass-like, shells, and the first MWRA surveys of the year often occur after the
early blooms. Silicate concentrations were unusually high in May to July, with
concentrations exceeding the historic maxima at many stations. This anomaly may
have resulted from the high river runoff, a major source of silicate. Another factor
could be the increasing presence of Tripos muelleri , which may have outcompeted
diatom species. Tripos species are in a different phytoplankton group, dinoflagellates,
and do not use silicate to produce exterior shells.
Ammonium concentrations have varied from the typical seasonal pattern at stations
nearest the outfall since the discharge began. Ammonium is the largest fraction of the
total nitrogen in wastewater (see Figure 2-5, page 6), and ambient concentrations are
relatively low for most of the year, making it a good effluent tracer. Since the
discharge was relocated from the harbor to the bay in late 2000, variable and elevated
levels of ammonium have been detected at stations nearest and immediately to the
south of the outfall, in the direction of the predominant currents (Figure 3-6).
Especially high ammonium concentrations at stations closest to the outfall are
sometimes a result of a sample being taken directly in the effluent plume before it is
completely mixed into the surrounding water. For example, for most of the year,
ammonium levels were particularly high at Station N21, located at the outfall.
However, during some surveys, ammonium levels were lower at Station N21 and
higher at Station N18, just to the south of the outfall. These results indicate possible
shifts in water flow, sometimes pushing the plume to the south as it mixes into the
water.
Ammonium levels remained lower than the baseline concentrations at Station F23, at
the mouth of Boston Harbor. Ammonium concentrations dropped precipitously at
Station F23 and at all stations sampled as part of MWRA’s separate Boston Harbor
monitoring program as soon as effluent discharge in Boston Harbor ended and the
Massachusetts Bay outfall discharge began.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
17 
Figure 3-6. Ammonium concentrations at selected stations in 2023 compared to prior years. 
Increased and more variable concentrations of the sewage tracer ammonium are found at stations
closest to the outfall, while stations further away have maintained concentrations similar to pre-outfall
levels. Ammonium concentrations at Station F23 (red point) at the mouth of Boston Harbor decreased
abruptly when discharge to Boston Harbor ended and have remained low. μM = micromoles. 
As is typical, the effluent plume’s ammonium signature was evident in surface waters
at stations closest to the outfall during the winter surveys, when the water column was
relatively well-mixed (Figure 3-7). During the summer, stratified season, the plume
was confined beneath the pycnocline, below depths where maximum phytoplankton
growth occurs. The plume could be detected as far as 20 kilometers from the outfall
under both mixed and stratified conditions.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
18 
Figure 3-7. Surface- and bottom-water ammonium during mixed and stratified conditions. 
During winter, well-mixed conditions, the plume was evident in surface waters. The plume was
confined below the pycnocline (yellow line), below the depth of most phytoplankton growth, during
the summer. Station N21 is directly over the outfall. m = meters; μM = micromoles. 
Phytoplankton Biomass 
More than two decades of monitoring have shown that the localized ammonium
concentration increases from the outfall have not stimulated phytoplankton growth or
resulted in increased phytoplankton biomass at any station. Phytoplankton biomass in
the water column is determined from chlorophyll concentrations and fluorescence and
from particulate organic carbon levels. Unusually elevated chlorophyll concentrations
were observed throughout the region in the late spring and early summer surveys,
corresponding with the timing of the Tripos muelleri bloom (Figure 3-8). Particulate
organic carbon concentrations were elevated during the same period. Both
chlorophyll and particulate organic carbon concentrations dropped sharply from June
to July and remained at or below long-term median levels throughout Massachusetts
Bay for the rest of the survey season. Chlorophyll fluorescence data from satellite
imagery and the NERACOOS Buoy A01 showed a similar pattern.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
19 
Figure 3-8. Monthly areal chlorophyll concentrations at selected stations in 2023 compared to
prior years. Survey phytoplankton biomass is reported as vertically summed (areal) chlorophyll from
measurements collected throughout the water column from the surface to the sea floor, rather than as
an amount within a fixed volume. mg/m 2 = milligrams per square meter summed over depth. 
Dissolved Oxygen 
Typically, bottom-water dissolved oxygen levels (concentration and percent
saturation) begin the year relatively high and steadily decline throughout the summer.
Stratification prevents exchange between the atmosphere and bottom waters, so
dissolved oxygen levels naturally decline at depth, as animals and microbes respire.
Although that general pattern persisted in 2023, physical parameters (warm waters
and strong stratification) and the Tripos muelleri phytoplankton bloom affected the
timing and extent of the declines (Figure 3-9).

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
20 
Figure 3-9. Near-bottom water dissolved oxygen concentrations in 2023 compared to prior years. 
DO = dissolved oxygen; mg/L = milligrams per liter . 
Dissolved oxygen concentrations were already relatively low at the start of the survey
season in February 2023. These early low levels can be attributed to unusually warm
bottom waters (see Figure 3-3 on page 13). Physically, warmer water can dissolve
less oxygen than cooler water; warmer water also promotes greater consumption of
oxygen by bacteria and other marine organisms. From March to April, dissolved
oxygen levels rose in surface waters, remaining high in May and June. During the
same period, oxygen levels rapidly declined to well below historic minimums in
bottom waters. These results were consistent with what would be predicted for a large
phytoplankton bloom. Phytoplankton produce oxygen during photosynthesis, but
oxygen is consumed when phytoplankton die and decompose in the water column and
at the sea floor. The high chlorophyll and low dissolved oxygen levels observed in
2023 resulted in exceedances of several Contingency Plan thresholds (see page 28). 
Plankton Communities 
Marine plankton includes plants and animals that have limited mobility on their own
and that move through the water with winds, tides, and currents. MWRA monitors 
phytoplankton and zooplankton abundance and species composition to detect any
changes that might be associated with the outfall discharge (Cibik et al. 1998). A
focus on potentially toxic or nuisance phytoplankton species addresses concerns for
marine mammal and human health.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
21
Massachusetts Bay phytoplankton includes small algae and other plant-like
organisms, such as microflagellates, centric (rounded) and pennate (elongated)
diatoms, and dinoflagellates (Figure 3-10). Microflagellates are extremely small and
are typically the most abundant phytoplankton type in Massachusetts Bay. Centric
and pennate diatoms have silica-rich cell walls and often dominate spring blooms.
Dinoflagellates have two perpendicular whip-like flagella, which propel them up and
down in the water column. Dinoflagellates are grouped into the phytoplankton,
because many of them photosynthesize. Tripos muelleri , the species responsible for
the large 2023 phytoplankton bloom, is one common dinoflagellate in the bay.
Another common dinoflagellate, Alexandrium catenella , often blooms in
Massachusetts Bay and the wider Gulf of Maine region and is the species known in
New England for causing “red tide.” Other geographic areas designate different
species, usually other dinoflagellates, as causing red tides, and many types of
phytoplankton can, when abundant, color the water column red or brown. 
Figure 3-10. Typical phytoplankton from Massachusetts Bay. The long strings are diatoms, and the
large, hooked organism is the dinoflagellate Tripos muelleri . The boxy diatom chains are species in the
genus Skeletonema , and the very thin chain is a species of the diatom genus Pseudo-nitzschia . Photo
credit David Borkman, Pausacaco Plankton, Saunderstown, Rhode Island.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
22
Zooplankton communities in Massachusetts Bay include animal species that live their
entire lives as plankton, particularly shrimp-like crustaceans called copepods, and
those that live in the plankton for only a part of their life cycles, such as barnacle and
clam larvae. Most species are small, but larger animals, including jellyfish, are also
part of the zooplankton community. Zooplankton are food for many other species,
including other zooplankton, fishes, and the endangered North Atlantic right whale.
Zooplankton abundance and species composition are highly variable through the
seasons and between years. 
Phytoplankton Communities 
Total phytoplankton abundance was at or near the long-term mean at most stations
and during most surveys in 2023 (Figure 3-11; Libby et al. 2024). Microflagellates
were abundant throughout the year. Microflagellates have consistently remained the
most numerically abundant plankton group, and in 2023, they made up almost 80% of
the phytoplankton community. The diatom species in the genus Skeletonema and the
species Asterionellopsis glacialis were also abundant during the summer . 
Figure 3-11. Total phytoplankton abundance at selected stations in 2023 compared to prior
years . cells/L = cells per liter

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
23
Dinoflagellates were more abundant than the long-term average during most of 2023,
largely due to the Tripos muelleri bloom, which dominated the region from April
through July. Tripos muelleri abundance far exceeded past maxima throughout
Massachusetts Bay (Figure 3-12). The bloom is discussed in greater detail in Section
6, Special Topics. 
Figure 3-12. Tripos muelleri abundance at selected stations in 2023 compared to prior years .
Abundance in 2023 was so high that, in many cases, it is difficult to see the historic ranges (blue
shading). Photo credit David Borkman, Pausacaco Plankton, Saunderstown, Rhode Island. cells/L =
cells per liter 
The potentially toxic diatom genus Pseudo-nitzschia occurred at one of the lowest
abundances observed since monitoring began. Abundance was orders of magnitude
lower than the Contingency Plan thresholds. Some Pseudo-nitzschia species produce
domoic acid, a neurotoxin that causes amnesic shellfish poisoning (ASP), a life-
threatening condition in seabirds, marine mammals, and humans that consume
contaminated shellfish. Historically, ASP has not caused shellfish closures in
Massachusetts or Cape Cod Bays.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
24
Abundance of the potentially toxic dinoflagellate Alexandrium catenella was lower in
2023 than in recent years (Figure 3-13), with a maximum abundance of only 32 cells
per liter, observed at Station N01 to the north of the outfall in April. Abundances
remained well below the Contingency Plan threshold and also below the threshold for
conducting Alexandrium Rapid-Response Study surveys. The toxin produced by 
Alexandrium catenella causes paralytic shellfish poisoning (PSP), a potentially deadly
condition for people and marine mammals that consume shellfish. No PSP toxicity
was detected in Massachusetts Bay. Alexandrium catenella abundance and PSP
toxicity were low throughout the Gulf of Maine in 2023, possibly because of low
nutrient levels or other factors related to the Tripos muelleri bloom. 
Figure 3-13. A lexandrium catenella abundance in 2023 compared to prior years. Photo credit
Donald M. Anderson, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts. Cells/L =
cells per liter. 
Zooplankton Communities 
Total zooplankton abundance and seasonal patterns were largely typical in 2023
(Figure 3-14) not reflecting any response to the Tripos muelleri phytoplankton bloom.
Relatively few zooplankton species consume Tripos muelleri, which is spiny and has
some ability to move up and down in the water column to avoid predation.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
25
As in past years, copepod adults and younger life stages dominated in every survey.
Numerically, the small copepod species Oithona similis continued to be the most
abundant species in the zooplankton community, as it has since the beginning of the
monitoring program. The June and July peak abundance was driven by early life
stages of bivalve and gastropod mollusks (clams and snails). The somewhat unusual
October peak observed at some stations was largely attributed to early life stages of
copepods. The October peaks also reflected high numbers of radiolarians, single-
celled protozoans that build ornate silica-rich exoskeletons. Radiolarians are more
typically found in offshore waters, and their presence each year since 2020 may
reflect an influx of warm water masses from offshore. 
Figure 3-14. Total zooplankton abundance at selected stations in 2023 compared to prior years. 
The high historical values in July occurred in 2015, when sampling took place after a major bivalve
spawning event. Abundances in 2023 were close to average at most stations in most months. The
increases in October compared to other years were due to early life stages of copepods and to
radiolarians, which are usually found in more offshore waters. individuals/m 3 = individuals per cubic
meter

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
26 
Stellwagen Bank National Marine Sanctuary 
The NPDES permit requires MWRA to report on results relevant to Stellwagen Bank
National Marine Sanctuary. Water-column Station F22 is just north of Stellwagen
Basin, to the west of the sanctuary, and is representative of offshore conditions. The
instrumented NERACOOS Buoy A01 is located in the northwest corner of the
sanctuary.
Ammonium levels have not increased at Station F22 in the years since the outfall
began to discharge (see Figure 3-6, page 17). However, like elsewhere throughout the
region, the Tripos muelleri phytoplankton resulted in abnormally high chlorophyll
concentrations in 2023. Physical conditions, as well as the bloom, led to low
summertime oxygen levels and consequent Contingency Plan threshold exceedances.
Measurements at the NERACOOS Buoy A01 and at Station F22 documented the
rapid increase in chlorophyll concentrations in the spring, followed by a rapid decline
with the end of the Tripos muelleri bloom (Figure 3-15). Similar buoy and shipboard
measurements documented the decline in oxygen concentrations though the summer
months (Figure 3-16). 
Figure 3-15. Surface chlorophyll measured at the NERACOOS Buoy A01 and nearby MWRA station
F22 in 2023. Buoy values are daily means. μg/l = micrograms per liter.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
27 
Figure 3-16. Dissolved oxygen concentrations at the NERACOOS Buoy A01 and at nearby
deepwater MWRA survey Station F22 in 2023. Buoy values are daily means. The state water quality
standard for dissolved oxygen is 6.0 milligrams per liter. mg/l = milligrams per liter. 
The sanctuary’s most recent condition report rated water quality within its borders as
“good,” and its most recent management plan agrees with MWRA’s assessment that
the outfall is not adversely affecting its waters (NOAA Office of Marine Sanctuaries
2020, 2023). The management plan notes that “ongoing monitoring suggests that the
MWRA outfall is currently not adversely influencing monitored water quality
parameters, and no evidence suggests that eutrophication is occurring.” Sanctuary
staff and scientists will continue to collaborate with MWRA and other organizations,
viewing the sanctuary as a sentinel site for water quality monitoring in the Gulf of
Maine. 
Boston Harbor Water Quality 
Water quality in Boston Harbor greatly improved during the Boston Harbor Project,
and those advances remained evident in 2023. Perhaps the most dramatic
improvement in Boston Harbor was the decrease in ammonium levels, which dropped
precipitously in 2000 after the effluent discharge was diverted from the harbor to the
bay. Ammonium levels have remained low. The decreases in nutrient inputs have
been accompanied by decreases in primary production and phytoplankton biomass, an
abatement of the harbor’s historically high level of eutrophication (Werme et al.
2018, Taylor et al. 2020).

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
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Water-Column Contingency Plan Thresholds 
There were exceedances of five water-column thresholds in 2023, including caution-
level exceedances for annual and summer chlorophyll concentrations, a warning-level
exceedance for nearfield dissolved oxygen concentration, and caution- and warning-
level exceedances for Stellwagen Bank dissolved oxygen concentration and percent
saturation (Table 3-1). 
Table 3-1. Contingency Plan threshold values and 2023 results for water-column monitoring. 
Parameter Baseline Caution
Level
Warning
Level 2023 Results 
Chlorophyll 
Annual 72 mg/m 2 >108 mg/m 2 >144 mg/m 2 116 mg/m 2 
Winter/spring 50 mg/m 2 >199 mg/m 2 NA 96 mg/m 2 
Summer 51 mg/m 2 >89 mg/m 2 NA 163 mg/m 2 
Autumn 90 mg/m 2 >239 mg/m 2 NA 54 mg/m 2 
Dissolved oxygen* 
Nearfield
concentration 6.05 mg/L <6.5 mg/L <6.0 mg/L 5.96 mg/L
Nearfield percent
saturation 65.3% <80% <75% 68.0%
Stellwagen
concentration 6.23 mg/L <6.5 mg/L <6.0 mg/L 6.08 mg/L
Stellwagen
percent saturation 67.2% <80% <75% 62.7 %
Nearfield
depletion rate 0.024 mg/L/d >0.037 mg/L/d >0.049 mg/L/d 0.008 mg/L/d 
Nuisance algae nearfield Pseudo-nitzschia 
Winter/spring 6,735 cells/L >17,900 cells/L NA 13 cells/L
Summer 14,635 cells/L >43,100 cells/L NA 591 cells/L
Autumn 10,050 cells/L >27,500 cells/L NA 472 cells/L 
Nuisance algae nearfield Alexandrium catenella 
Any nearfield
sample
Baseline
maximum
163 cells/L
>100 cells/L NA 32 cells/L
PSP toxin extent NA New incidence NA No new
incidence 
“Nearfield” refers to the five stations closest to the outfall, where effects might be detected, if they
were to occur. Red shading shows a warning-level exceedance; orange shading shows a caution-level
exceedance. For dissolved oxygen parameters, “exceedances” refer to values less than the threshold.
*Dissolved oxygen caution and warning levels represent numerical criteria, with the caveat “unless
background conditions are lower.” Results are therefore compared to the baseline as well as the caution
and warning levels.
PSP = paralytic shellfish poisoning
NA = not applicable
mg/m 2 = milligrams per square meter
mg/L = milligrams per liter
mg/L/d = milligrams per liter per day
cells/L = cells per liter

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
29 
Chlorophyll Exceedances 
Nearfield summer chlorophyll levels were almost twice the caution level, and those
high summer values drove an exceedance of the annual chlorophyll concentration
threshold as well (Figure 3-17). Summer average chlorophyll concentrations in 2023
far exceeded those observed in any prior monitoring year, a result of the Tripos
muelleri bloom. To a lesser extent, summer chlorophyll levels were also elevated
during the baseline year 2000, in 2006 and in 2016. Annual chlorophyll levels were
higher in the baseline years 1999 and 2000 than in 2023. The somewhat high summer
chlorophyll level in 2006 resulted from a bloom of the diatom Dactyliosolen
fragilissimus , which was stimulated by an unusually high period of upwelling of
nutrients from deep waters. Elevated levels in 2000 and 2016 were attributed to multi-
species blooms. 
Figure 3-17. Summer (top) and annual (bottom) average chlorophyll concentrations, 1992–2023. 
No annual chlorophyll threshold could be calculated for 2020 due to COVID-19 restrictions on
sampling in March and April. mg/m 2 = milligrams per square meter. 
0
20
40
60
80
100
120
140
160
180 
Summer chlorophyll (mg/m 2 ) 
0
20
40
60
80
100
120
140
160
180
200 
Annual chlorophyll (mg/m 2 ) 
baseline outfall discharge caution threshold
warning threshold

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
30 
Dissolved Oxygen Exceedances 
The Contingency Plan includes caution and warning thresholds for concentration,
percent saturation, and the rate of oxygen depletion during the summer months at the
nearfield stations close to the outfall and in Stellwagen Basin. The concentration is
the amount of oxygen dissolved per liter water, and the percent saturation is the
amount dissolved in the water compared to the maximum possible under the given
physical conditions (temperature, salinity, and depth). Those stations closest to the
outfall would be most likely to show an effect of the discharge, while the thresholds
in Stellwagen Basin provide information about the wider region. (Note that for the
dissolved oxygen parameters, the regulatory term “exceedance” refers to values that
are lower than the Contingency Plan thresholds.)
The Contingency Plan thresholds were developed in the mid-1990s and were based
on the established Massachusetts regulatory standards of the time. For some
parameters, such as dissolved oxygen concentrations in both the nearfield and
Stellwagen Basin, baseline levels were lower than those standards, so the thresholds
direct that monitoring results be compared to the baseline values. Since the 1990s,
New York and New England states other than Massachusetts have lowered their
regulatory standards to make them more representative of the levels now thought to
potentially harm marine life. Current standards for oxygen concentrations in those
states now range from 4.8 to 5.0 milligrams per liter.
Percent saturation thresholds were also based on the Massachusetts standards of the
1990s, but field measurements fell below the caution levels in seven of eight baseline
years, 1992–1999, and also in 2000, the year that the Massachusetts Bay outfall began
to discharge. No further excursions below the caution thresholds occurred until 2021.
Massachusetts removed percent oxygen saturation from its state standards in 2008,
after research showed that percent saturation was not strongly associated with
ecosystem health, but it remains a Contingency Plan parameter.
The first 2023 exceedances occurred in measurements made in July in Stellwagen
Basin. There were no exceedances in August, following a period of winds from the
south, which promoted upwelling and some mixing of the water column. In
September, there were exceedances in both the nearfield and Stellwagen Basin
(Figure 3-18). These exceedances have not been attributed to the outfall, but instead
appear to be the result of widely recognized regional physical conditions, particularly
warming waters, which have been broadly documented in the Gulf of Maine, and to
the aftermath or the Tripos muelleri phytoplankton bloom. Changing physical
conditions and the phytoplankton bloom are discussed further in Section 6, Special
Topics.

2023 O UTFALL M ONITORING O VERVIEW : W ATER C OLUMN 
31 
Figure 3-18. Bottom dissolved oxygen concentrations and percent saturation in the nearfield and
Stellwagen Basin, 2023 . Nearfield = the five stations closest to the outfall; DO = dissolved oxygen;
mg/L = milligrams per liter.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
32 
4. Sea Floor 
The sea floor of Massachusetts Bay was shaped by glaciers, which scoured the
bottom and created Stellwagen Basin to the east of the outfall, deposited rocky debris
in elongated hills called drumlins, and left sand and gravel on what is now Stellwagen
Bank. Tides and currents continue to shape the bay, particularly during major storms
with waves large enough to resuspend and transport sediments. Seafloor habitats
range from mud in soft bottom depositional areas to rocky cobbles and boulders on
the hard-bottom tops and flanks of the drumlins.
Seafloor monitoring assesses potential outfall effects on the health of both the soft-
bottom areas and the hard-bottom drumlins. Most seafloor-monitoring studies focus
solely on soft-bottom sediments made up of sand, silt, and clay, but such depositional
sites are somewhat rare in the immediate vicinity of the outfall, so MWRA also
conducts hard-bottom surveys. Massachusetts Bay soft-bottom sampling surveys are
conducted annually, while photographic and video assessments of the hard-bottom
areas occur at three-year intervals, including 2023. A separate program in Boston
Harbor includes annual soft-bottom surveys and photographic sediment-profile
imagery.
In August 2023, MWRA sampled 14 soft-bottom stations for analysis of sediment
grain-size distribution, the wastewater tracer Clostridium perfringens spores, percent
total organic carbon, and infauna (animal) abundance and species composition
(Figure 4-1). The stations included four “inner nearfield” stations located within two
kilometers of the outfall; six “outer nearfield” stations in western Massachusetts Bay
somewhat farther from the outfall; one station in the “transition” area between Boston
Harbor and the nearfield; and three “farfield” reference stations in Massachusetts and
Cape Cod Bays. For the purposes of testing Contingency Plan thresholds for the sea
floor, “nearfield” includes both nearfield groups and the transition station, for a total
of 11 stations. Note that water-column and seafloor monitoring sample different
stations and have different definitions of nearfield and farfield.
Hard-bottom surveys of 23 stations were completed in mid-June 2023 (Figure 4-2).
The stations included one active and one inactive outfall diffuser cap. The surveys
yielded 487 minutes of analog video, which were analyzed for physical parameters
and species composition. 
Seafloor monitoring assesses the health of habitats and communities. 
MWRA measures sediment characteristics and assesses animal communities living
in soft-bottom and hard-bottom habitats. Because of consistently good results,
some sediment-contaminant monitoring has been reduced. Seafloor animal
communities vary within typical natural cycles, influenced by sediment types and
water depths.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
33 
Figure 4-1. Soft-bottom monitoring stations. Also shown are the instrumented buoys, the MWRA
outfall diffuser, and Stellwagen Bank National Marine Sanctuary. Nearfield stations include a
designation NF, while farfield stations and the transition station include a designation FF. The
transition station FF14 is included with the nearfield stations to calculate Contingency Plan thresholds.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
34 
Figure 4-2. Hard-bottom monitoring stations. Also shown are the instrumented buoys, the MWRA
outfall diffuser, and Stellwagen Bank National Marine Sanctuary.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
35 
Sediment Characteristics and Tracers 
As in past years, sediment grain-size distributions in 2023 varied broadly, ranging
from silt and clay at some stations (particularly Station FF04 within Stellwagen
Basin) to almost entirely sand at others (Nestler et al. 2024). Within individual
stations, sediment textures have remained generally stable over the years of
monitoring, with occasional changes following large storms. 
Clostridium perfringens is an anaerobic bacteria species, which is found in the
digestive tracts of humans and other mammals and is not completely removed by
wastewater treatment. It forms resting spores, and in sediments, those spores provide
a sensitive tracer of sewage effluent. Since spores can persist for decades, they do not
reflect immediate deposition onto the sea floor; instead, they integrate results over
years.
In 2023, as in other years, elevated abundance of Clostridium spores was detected at
some stations located closest to the outfall (Figure 4-2). In most years, Clostridium 
spore concentrations have been elevated at stations located within two kilometers of
the outfall. In 2022, some anomalously high concentrations were measured at two
sites farther from and out of the projected range of the outfall, but measurements from
those sites returned to more typical values in 2023. 
Figure 4-3. Concentrations of the effluent tracer Clostridium perfringens spores in 2023. 
Concentrations represent spore counts normalized to the percent of the sediment made up of fine silt
and clays, removing any effect of grain size from the analysis. As anticipated during outfall planning, 
Clostridium spore concentrations are elevated at stations closest to the outfall. cfu = colony forming
units, the measure used in spore counts.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
36
Percent total organic carbon content in the sediments was consistent with past results,
with no increased organic carbon from sewage effluent at stations near the outfall
(Figure 4-4). In general, stations with finer sediments, such as Station FF04, to the
east of the outfall in Stellwagen Basin, have higher mean total organic carbon
concentrations, while stations with coarser sediments, such as Station NF17 just to the
south of the outfall have lower concentrations. Overall, the total organic carbon data
continued to show no signs of organic enrichment from wastewater effluent,
consistent with predictions made before the outfall began to discharge and also
consistent with a special study of sediment metabolism, which was completed in 2010
(Tucker et al. 2010, Tucker et al. 2014). No sample showed increased organic carbon
concentrations following the large 2023 Tripos muelleri phytoplankton bloom, a
somewhat surprising result that remains under study. 
Figure 4-4. Total organic carbon at selected stations, 1992–2023. Nearfield 12 (NF12) is more than
two kilometers northwest of the outfall, and Nearfield 17 (NF17) is in closer proximity and southwest
of the outfall, where the effects of the discharge might be expected but have not been found. Farfield
01A (FF01A) is the northern reference station, and Farfield 04 (FF04) is in Stellwagen Basin; both
farfield stations are farther than 20 kilometers from the outfall. No effects of the discharge have been
observed at any station; total organic carbon content is correlated with sediment type rather than
distance from the outfall. (Note that 2023 data from Station FF04 are currently unavailable.)
.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
37 
Soft-bottom Communities 
A variety of small animals live in the sediments, including segmented polychaete
worms, mollusks, and crustaceans. MWRA monitors abundance and species
composition of animals living within soft-bottom communities to document any
changes that might be associated with the outfall discharge, such as altered species
composition, decreased biodiversity, or increased presence of the opportunistic
species typically found in disturbed habitats.
The 14 soft-bottom samples collected and analyzed in 2023 contained 27,099
organisms, classified into 174 species and 23 other discrete taxonomic groups of
animals that could not be identified to the species level (Nestler et al. 2024). Total
abundance of organisms was slightly higher in 2023 than in 2022 in all regional
groups, but remained within expected ranges (Figure 4-5). The several community
diversity measures calculated for nearfield samples remained at levels typical of past
years. 
Figure 4-5. Total abundance of soft-bottom organisms by region, 1992–2023 . Regions include the
inner nearfield within two kilometers of the outfall, the outer nearfield farther than two kilometers
from the outfall, the transition area between Boston Harbor and the outfall, and the farfield. 
Small polychaete worms (sometimes known as bristle worms, Figure 4-6), dominated
the samples at most stations, as they have for the duration of the monitoring program.
These segmented worms are among the most common marine organisms. The
polychaete Prionospio steenstrupi , which was the numerically dominant species in
1997–2005, has declined in abundance over recent years, but remained among a
group of dominant species. Other common polychaetes, including Aricidea
catherinae and Mediomastus californiensis , were present in numbers comparable to
other recent years.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
38 
Figure 4-6. Polychaete worms dominate the soft-bottom habitats in Massachusetts Bay. 
Polychaetes can range in size from less than a millimeter to several meters, but most are very small.
This species, Kirkegaardia baptistae , is about 14 millimeters long and frequently found in fine-
grained, offshore sediments. MWRA samples were used in its original description. Photo credits James
A. Blake, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
39
Community analyses continued to show no effects of the outfall on total abundance of
animals, relative abundance of species, or community composition. A series of
analyses assessed patterns in the soft-bottom communities over space and time and
found no particular species or type of community specifically associated with the
outfall.
One type of community analysis, called cluster analysis, groups stations by similarity
in species composition and abundance. Similar to prior years, cluster analysis of the
2023 samples identified two main groups (I and II) and one “outlier” group (III).
Group I was dominated by crustaceans and polychaete worms. Group II included a
greater abundance of polychaetes and also mollusks. Group II could be subdivided
into three subgroups (IIA, IIB, and IIC). The very small bivalve mollusk Crenella
decussata was a dominant species at Station FF09, the only station within Group IIA.
As in past years, the outlier Group III included only the offshore station located
within Stellwagen Basin, which has consistently supported a unique polychaete
community, typical of the other deepwater, offshore stations sampled in the past.
Further analysis, called ordination analysis, provides a picture of how closely the
bottom communities identified in the cluster analysis relate to each other. The 2023
ordination analysis continued to show no indication of any relation between
community composition and proximity to the outfall (Figure 4-7). Both main Groups
I and II included stations located at various distances and directions from the outfall.
Group II included stations from both nearfield station groups, the transition station
between the harbor and the outfall, and farfield stations to the northeast and southeast
of the outfall.
As in past years, further assessment of the ordination analysis demonstrated that
variations in species distributions largely followed differences in sediment grain size
(Figure 4-8). Water depth was another important factor in determining community
structure. Seafloor animal communities at stations with similar sediment textures and
water depths tended to be more closely related, as shown by closer proximity on the
ordination plots. These results continued to validate past findings that the
Massachusetts Bay seafloor communities are driven by physical factors rather than by
the discharge. The strong physical forces in the bay, as well as the low pollutant
levels in the effluent discharge, have helped to maintain healthy soft-bottom habitats
throughout Massachusetts Bay.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
40 
Figure 4-7. Ordination plot of 2023 Massachusetts Bay samples by location. The ordination
analysis shows that the distance from the outfall does not correspond with bottom community
composition. Each point on the plot represents one of 14 stations, and varying colors and shapes depict
locations. Placement of the points indicates similarity of the communities—the nearer two points are to
each other, the more similar their seafloor communities. NF<2k = stations within two kilometers of the
outfall; NF >2k = stations near but more than two kilometers from the outfall; Tran = the one station
between Boston Harbor and the outfall; FF = stations far from the outfall. (The 2D Stress value noted
in the upper right is a measure of good confidence in the analysis.) 
Figure 4-8. Percent fine sediments superimposed on the ordination plot of the 2023 samples. 
Combining the ordination analysis with sediment grain-size data shows that sediment texture is likely a
strong driver of community composition. Data are shown as in Figure 4-7, with the size of each station
marker indicating the percentage of fine sediments (percent fines). Stations with similar sediment grain
size tend to be represented in the same groups. Group I included only stations with coarser sediments,
and Group II subgroups reflected a range of finer-grained sediments. The offshore outlier Group III in
Stellwagen Basin has the finest-grained sediments (The 2D Stress value noted in the upper right is a
measure of good confidence in the analysis.)

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
41 
Hard-bottom Communities 
Past monitoring has shown that the hard-bottom habitats near the Massachusetts Bay
outfall are spatially diverse but relatively stable over time. The changes that have
occurred have not been associated with the outfall. For example, abundance of
encrusting coralline algae was once proposed as a potential indicator of outfall
effects, supposing that abundance could decline in response to increased deposition of
solid matter in the wastewater discharge. In fact, coralline algas abundance has
decreased throughout the area, but those decreases have been region-wide and first
occurred at northern reference locations, outside the projected influence of the
discharge.
Sporadic sets of juvenile barnacles, as they transition from the zooplankton to
immobile adults, have been common over time, but in 2023, for the first time, they
were observed at almost every station. At some locations, the barnacle sets were
followed by unprecedented numbers of the sea star Asterias rubens , feeding on the
young barnacles (Figure 4-9, top). These sorts of events likely occur more frequently
than can be detected by periodic monitoring and illustrate the natural variability of the
environment.
The active outfall diffuser head included as a monitoring station continued to support
a healthy seafloor community (Figure 4-9, bottom). Sea anemones have historically
been abundant on both active and inactive diffuser heads. The blue mussel Mytilus
edulis , which has traditionally been more common in shallower waters, was first seen
on the active diffuser head in 2005 and appears to be increasing in numbers
throughout the area. Blue mussels are now co-existing with the more typically
deepwater horse mussel Modiolus modiolus . Over the same years, intertidal and
shallow-water blue mussel populations have declined throughout the region. Some
researchers speculate that a shift to deeper water is a response to warming waters; the
Gulf of Maine Research Institute is currently mapping this change (Mussel Bed
Mapping - Gulf of Maine Research Institute).

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
42 
Figure 4-9. Top: Large numbers of sea stars feeding on barnacles in 2023. Bottom: Blue mussels
and sea anemones growing on an active outfall diffuser. Large barnacle sets occur periodically
throughout the study area, including 2023. Blue mussels, which have been disappearing from
shallower habitats throughout New England, were first detected on the diffuser in 2005 and have been
found on large boulders throughout the area since 2011.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
43 
Stellwagen Bank National Marine Sanctuary 
The NPDES permit to discharge from Deer Island Treatment Plant into
Massachusetts Bay requires annual reporting on results relevant to Stellwagen Bank
National Marine Sanctuary. MWRA’s seafloor deepwater reference Station FF04 lies
within the depositional part of the sanctuary, Stellwagen Basin, where long-term
accumulation of pollutants could feasibly occur.
Station FF04 is typical of deepwater, offshore habitats and is representative of a
larger group of stations that was monitored in earlier years of the program. It
continues to support a soft-bottom community typical of what had been found at the
larger suite of deepwater stations. Communities at those stations have always shown
distinct differences from those found in shallower areas, probably due to the distance
from shore, as well as greater depth and fine-grained sediments (Figures 4-7 and 4-8,
above). Superimposing percent grain size on the ordination plot for 2023 samples
continued to show those natural differences. Analysis of data from 1992–2023 has
demonstrated the temporal consistency of the seafloor community at that station.
The most recent management plan for the sanctuary (NOAA Office of Marine
Sanctuaries 2023) includes an action plan to assess all the major habitats within its
borders, including sand, boulder, gravel, mud, rocky areas, and shipwrecks. Their
most recent environmental assessment (NOAA Office of Marine Sanctuaries 2020)
found that changes to seafloor habitats and animal communities came mostly from
fishing. Fishing gear physically scrapes the sea floor, and fishing pressures can
change the species makeup of both seafloor and fish communities. 
Boston Harbor Seafloor Monitoring 
Seafloor conditions are much better in Boston Harbor than they were in the 1970s and
1980s. Boston Harbor received sewage sludge discharge until 1991 and effluent
discharge until September 2000. MWRA has conducted ongoing seafloor monitoring
in Boston Harbor since 1991. Each year, sediment samples are taken from nine
stations, and sediment-profile images are taken at 61 stations throughout the harbor.
Over time, sediment textures within individual harbor stations have become stable,
ranging from mostly sand at some stations to silt and clay at others. Percent total
organic carbon in harbor sediments has declined over time, reflecting the lowered
wastewater inputs.
Over the past decade, the total abundances of organisms and number of species per
sample have also stabilized (Figure 4-10), a sign of sustained improvement in
seafloor habitat conditions. In 2023, samples contained 19,686 specimens, classified
into 122 species and 17 other discrete taxonomic groups.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
44 
Figure 4-10. Total abundance of soft-bottom animals (top) and number of species (bottom) per
sample in Boston Harbor samples, 1991–2023. Both total abundance and numbers of species have
stabilized as the harbor has recovered from past inputs of sewage sludge and effluent. Data are from
eight harbor stations that have been consistently monitored since 1991. “Outfall startup” refers to the
beginning of effluent discharge from the Massachusetts Bay outfall and the end of discharge to the
harbor. 
0
2000
4000
6000
8000
10000 Outfall Startup
1991 1995 1999 2003 2007 2011 2015 2019 2023
Mean +/- 1StdErr

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
45
As in past years, the communities in 2023 varied along an outer-to-inner harbor
gradient, reflecting the greater stream and river inputs to the inner harbor and greater
tidal flushing at the outer-harbor stations. Sediment-profile imaging has confirmed
the gradient and clearly documented sustained improvements to habitat conditions
throughout the harbor over time.
Sediment-profile images from 2023 (Figure 4-11) depicted the continuing healthy
conditions. Some locations in 2023 had been disturbed by recent dredging activity,
but in general, sediments continued to be well-oxidized, with no evidence of excess
organic matter. 
Figure 4-11. Example of a sediment-profile image, showing a large mantis shrimp burrow at
Station R07, at Deer Island Flats, near the location of the former Boston Harbor effluent outfall. 
Scales at the sides of the image are in centimeters.

2023 O UTFALL M ONITORING O VERVIEW : S EA F LOOR 
46 
Seafloor Contingency Plan Thresholds 
There were no threshold exceedances for the seafloor Contingency Plan thresholds in
2023 (Table 4-1). For the purposes of threshold testing, “nearfield” includes stations
from both nearfield groups (both nearer and farther than two kilometers from the
outfall) and the station in the transition area between Boston Harbor and the
Massachusetts Bay outfall.
All threshold parameters continued to confirm that Massachusetts Bay maintains
healthy seafloor habitats, unaffected by the wastewater discharge. Number of species
per sample and the several diversity and evenness indices used to evaluate community
structure remained higher than Contingency Plan limits, while the percent
opportunistic species, a concern for disturbed or polluted habitats, remained far below
any level of concern, only 0.21% compared to caution and warning thresholds of 10%
and 25%. 
Table 4-1. Contingency Plan threshold values and 2023 results for seafloor monitoring.* 
Parameter Baseline Caution
Level
Warning
Level 2023 Results 
Species per
sample NA <42.99 NA 60.8
Fisher’s log-series
alpha NA <9.42 NA 12.4
Shannon diversity NA <3.37 NA 3.94
Pielou’s evenness NA <0.57 NA 0.67
% opportunists NA >10% >25% 0.21% 
NA = not applicable 
* Number of species per sample is often referred to as species richness. Fisher’s log-series alpha and
Shannon diversity are indicators of diversity, calculated from both numbers of species and numbers of
individuals per species. Pielou’s evenness assesses how evenly numbers of individuals are distributed
among the species in the sample. Percent opportunists is the percentage of the total number of animals
that are indicators of disturbed habitat. The percent opportunist threshold is a maximum; all other
thresholds are minimum values.

2023 O UTFALL M ONITORING O VERVIEW : F ISH AND S HELLFISH 
47 
5. Fish and Shellfish 
Fish and shellfish are important to the regional identity and economy of
Massachusetts and can be good indicators of overall environmental health. The
monitoring program focuses on winter flounder, lobster, and cage-deployed blue
mussels. Flounder and lobster are important commercial and recreational species that
live on the sea floor and feed on bottom-dwelling animals, likely sources for
contaminant exposure. Mussels filter large volumes of water and can concentrate
toxic metals and organic compounds in their tissues. Sampling and deployment
locations vary by species (Figure 5-1). Sampling locations for winter flounder and
lobster were selected to be far enough away from each other to maximize the
likelihood of sampling separate populations. Winter flounder are unlikely to range
farther than about two kilometers in areas with ample food; lobster migration is less
well understood.
Timing and parameters measured vary by species. MWRA monitors winter flounder
health each year and conducts chemical contaminant analyses of flounder, lobster,
and mussel tissues every three years, most recently in 2021. Flounder health
assessment was the only monitoring activity in 2023. It measured external conditions
and liver disease, including precancerous conditions and tumors.
In 2023, flounder monitoring was conducted at two locations, the Massachusetts Bay
outfall site and Deer Island Flats, near the location where wastewater effluent was
once discharged into Boston Harbor. Through 2021, fish were also collected from
eastern Cape Cod Bay, which was considered a relatively pristine control site. In
earlier years, other locations provided greater geographic representation. That past
monitoring concluded that there had been no adverse effects of the Massachusetts
Bay outfall and that instead, conditions have improved throughout the region, a result
of improved contaminant source reduction and better sewage treatment.
Consequently, a formal study with a control site was deemed no longer necessary.
However, the regulatory agencies maintain a continued interest in documenting
flounder health in Massachusetts Bay and sustained improvements in Boston Harbor,
so MWRA continues to sample sites at those two locations. 
Fish and shellfish are healthy. 
MWRA monitors winter flounder health and fish and shellfish tissues for chemical
pollutants that occur in wastewater effluent. Over time, flounder health has
improved, and contaminant concentrations have declined in flounder, lobster, and
mussel tissues from both Boston Harbor and Massachusetts Bay.

2023 O UTFALL M ONITORING O VERVIEW : F ISH AND S HELLFISH 
48 
Figure 5-1. Fish-and-shellfish monitoring stations. IH = Inner Harbor; DIF = Deer Island Flats; DIL
= Deer Island Light; OS = Outfall Site; OS-M = Outfall Site, Mussels; LNB = Large Navigation Buoy
(buoy south of the outfall); ECCB = Eastern Cape Cod Bay. Also shown are the outfall, the
instrumented buoys, and the Stellwagen Bank Marine Sanctuary.

2023 O UTFALL M ONITORING O VERVIEW : F ISH AND S HELLFISH 
49
Commercial fishing boats conducted flounder sampling over several days in April
and May 2023. Sampling was hampered by entangled lobster traps and other “ghost”
fishing gear, an abundance of seaweed (kelp), and a torn net (Moore et al. 2024;
Figure 5-2). Even accounting for entangled fishing gear and abundant algal growth,
which have regularly stalled fishing efforts, catches have been relatively low
throughout the region in the past several years. Winter flounder populations
throughout southern New England and the Mid-Atlantic have declined since the
1980s, probably a result of overfishing rather than pollution or warming waters (Frisk
et al. 2018). It has also become increasingly difficult to engage the fishing boats that
MWRA uses for sampling, particularly during time periods when commercial catches
may be more lucrative for the boat owners. Following a back-up plan, the 2023
MWRA catch was supplemented by fish collected by the Massachusetts Department
of Marine Fisheries.
Despite the extra effort, only 29 fish were taken from near the Massachusetts Bay
outfall and only 15 fish from Deer Island Flats, marking the first year that the full
complement of 50 fish was not reached in Massachusetts Bay. Catches from Deer
Island Flats have fallen short in four of the past five years, but the 2023 catches were
the lowest in more than three decades of monitoring. The low catch numbers, while
still valuable, make comparisons with prior years more challenging. 
Flounder Characteristics 
The mean age of fish in 2023 was about 5.7 years at both stations, and standard length
(length from the tip of the head to the base of the tail fin) ranged from 281 to 299
millimeters, within the historical ranges. Weights were also within the typical range
for the program. As is common throughout northeast coastal populations (Moore et al.
2016), the catches were dominated by females.

2023 O UTFALL M ONITORING O VERVIEW : F ISH AND S HELLFISH 
50 
Figure 5-2. Top: Lost lobster traps and other “ghost” fishing gear interfere with flounder
collection. Bottom: Excess kelp also interferes with sampling. Photos taken at Deer Island Flats,
near the former harbor outfall, in 2023. Photo credits Michael Moore, Woods Hole Oceanographic
Institution, Woods Hole, Massachusetts.

2023 O UTFALL M ONITORING O VERVIEW : F ISH AND S HELLFISH 
51 
Flounder Health 
Measures of external condition, such as bent
fin rays and occurrence and severity of fin
erosion, remained variable, but continued to
reflect improved conditions since the 1980s
and 1990s. Incidence of fin erosion, which
has been associated with excess ammonium
and other pollutants, remained low at both the
Massachusetts Bay outfall site and Deer
Island Flats. Ulcers, which are observed on the bottom or “blind side” of the fish in
some years, were not present in 2023.
Incidence of centrotubular hydropic vacuolation (CHV), a key tumor precursor and
mild liver condition associated with exposure to contaminants, was only 6.9% at the
Massachusetts Bay outfall and completely absent within the small catch at Deer
Island Flats (Figure 5-3). Although the low catch makes comparisons difficult, the
continued decrease in CHV incidence at Deer Island Flats is a major environmental
success. More than 75% of Boston Harbor flounder livers in the late 1980s showed at
least some level of CHV. Incidence has decreased across all age groups in both the
harbor and the bay, alleviating the concern prior to the Massachusetts Bay outfall
startup that there could be increased liver disease in the bay. In the few fish where
CHV has been found, severity of the condition has also declined over time. 
Figure 5-3. Annual prevalence of the tumor precursor centrotubular hydropic vacuolation
(CHV) in winter flounder from the Massachusetts Bay outfall site and Deer Island Flats, 1991–
2023. Nantasket Beach and East Cape Cod Bay are no longer sampled. CHV incidence and severity
have declined throughout the region. 
In 2023, no CHV liver disease
was found in the 15 flounder
from Deer Island Flats in
Boston Harbor. In contrast, in
the late 1980s, more than 75%
of Boston Harbor flounder had
CHV.

2023 O UTFALL M ONITORING O VERVIEW : F ISH AND S HELLFISH 
52
Cancerous liver tumors, also called neoplasia, which were present in up to 10% of
flounder from Boston Harbor in the 1980s, remained absent, as they have since 2004.
Tumors have never been found in fish taken from near the Massachusetts Bay outfall
site (Figure 5-4; see also Moore et al. 2018). 
Figure 5-4. Annual prevalence of liver tumors (neoplasia) in winter flounder, 1991–2023. No
tumors have ever been recorded in fish taken from near the Massachusetts Bay outfall. 
Winter Flounder Contingency Plan Thresholds 
There was no Contingency Plan threshold exceedance for the only parameter
measured in 2023, flounder liver disease, measured as percent fish caught near the
Massachusetts Bay outfall with CHV (Table 5-1). CHV occurred in only 6.9% of the
fish, compared to the 24.4% baseline and a caution threshold of 44.9%. 
Table 5-1. Contingency Plan threshold values and 2023 results for winter flounder monitoring. 
Parameter Baseline Caution Level Warning Level 2023 Results 
Flounder disease 
Liver disease
(CHV) 24.4% >44.9% NA 6.9% 
CHV = centrotubular hydropic vacuolation, a liver tumor precursor associated with degraded habitats

2023 O UTFALL M ONITORING O VERVIEW : S PECIAL T OPICS 
53 
6. Special Topics 
In addition to the permit-required monitoring of the effluent and the water column,
sea floor, and fish and shellfish in Massachusetts Bay, MWRA also initiates or
collaborates on studies of Massachusetts Bay and the wider Gulf of Maine. These
studies assess the physical conditions, biology, and ecology of the region. They also
better inform interpretations of MWRA monitoring results. MWRA evaluates a range
of topics in response to information from its routine monitoring programs, its directed
studies, and research conducted by other institutions. This year’s report presents
information about long-term environmental trends detected by MWRA and other
institutions and the large regional Tripos muelleri dinoflagellate bloom that occurred
in the spring and summer of 2023. 
Regional Environmental Trends 
Some components of MWRA’s monitoring program in Massachusetts Bay began in
1991, and the full program launched in 1992. As planned from the outset, the
monitoring program has evolved over time, but the same suite of parameters has been
measured at many of the same locations for more than 30 years. Such long-term
programs are rare, and MWRA’s monitoring program has yielded decades of
information about physical conditions and environmental trends in Massachusetts and
Cape Cod Bays.
For example, MWRA’s long-term monitoring and studies by other agencies and
scientists have documented increasing water temperatures in Massachusetts Bay and
the broader Gulf of Maine. Increased water temperatures can lead to lower dissolved
oxygen concentrations, both locally and regionally. Within the MWRA monitoring
area, increased water temperatures may be one factor leading to decreases in
dissolved oxygen levels and subsequent Contingency Plan threshold exceedances in
recent years. Some scientists believe that increasing temperatures and changing ocean
currents may be affecting the North Atlantic right whale population, which was
already imperiled by historic whaling, ship collisions, and entanglement (for example,
Meyer-Gutbrod et al. 2021).
In 2022, following two years of dissolved oxygen Contingency Plan exceedances,
MWRA began an internal review of water temperature and dissolved oxygen
concentrations over its 1992–2022 monitoring period (Codiga and Wu 2022). The
analyses focused on eight stations in Massachusetts Bay near and far from the outfall,
one station at the mouth of Boston Harbor, and one offshore station near Stellwagen
Basin. Each of these stations had been monitored during six surveys each year, all
completed during April through October, over 31 years. All measurements were made
at five depths and occurred within the same calendar weeks per year.

2023 O UTFALL M ONITORING O VERVIEW : S PECIAL T OPICS 
54
MWRA’s analyses showed clear and consistent increasing temperatures and declining
dissolved oxygen concentrations across all stations and all water depths. Average
April–October warming rates were about 0.4–0.6°C per decade. Dissolved oxygen
concentrations declined about 0.17–0.23 milligrams per liter per decade.
At the NERACOOS Buoy A01, located at the northern limit of Massachusetts Bay
(see Figure 3-1 on page 10), mean surface-water temperatures in July and August
have also risen steadily for more than 20 years (Figure 6-1). Maximum, mean, and
minimum temperatures increased by 0.7°C, 0.6°C, and 0.8°C per decade,
respectively. Similar data from the NDBC Buoy in central Massachusetts Bay show
the same pattern over an even longer time period. 
Figure 6-1. Increases in summertime (July and August) water temperatures in Massachusetts
Bay, 2001-2023. Data are from the NERACOOS Buoy A01, located in northern Massachusetts Bay.
The dashed lines show the linear trends: Maximum (slope = 0.6⁰C or 1.3⁰F/decade, P<0.01), mean
(slope = 0.7⁰C or 1.1⁰F/decade, P<0.05), minimum (slope = 0.8⁰C or 1.4⁰F/decade, P<0.05). The
shaded area shows the 95% confidence interval surrounding the trend lines. P = probability. P<0.01
and P<0.05 indicated that the measured changes are statistically significant. 
Water temperatures have increased more quickly than air temperatures, possibly due
to changes in wind conditions, which can bring warmer, saltier, and less oxygenated,
water masses into Massachusetts Bay from offshore. In recent years, summertime
winds have increasingly come from the southeast rather than the southwest (Figure 6-
2). Preliminary analysis by Malcolm Scully at the Woods Hole Oceanographic
Institution in Woods Hole, Massachusetts suggests that these changes in wind
direction may be influencing long-term warming of the water column. The changes in
wind directions and subsequent influx of water from offshore have been reflected in
MWRA zooplankton samples, which have documented increasing presence of
offshore zooplankton species since 2020 (see Section 3, Water Column).

2023 O UTFALL M ONITORING O VERVIEW : S PECIAL T OPICS 
55 
Figure 6-2. Wind stress, 1989-2023 . Summertime winds increasingly come from the southeast; in
past years, winds more typically came from the southwest. E-W = east to west, Pa = Pascal, a unit of
pressure used to quantify wind stress and other pressure-related measurements. 
MWRA colleagues from Bowdoin College in Brunswick, Maine have been studying
trends in chlorophyll fluorescence and turbidity at the NERACOOS Buoy A01, off
Cape Ann at the northern boundary of Massachusetts Bay (Roesler et al. 2024).
Bowdoin College, along with the University of Maine, helps to maintain the buoy and
has, over the years, added increasingly sophisticated sensors. Their recent
measurements in 2022–2023, combined with data collected since 2005, found that
spring and fall phytoplankton blooms are beginning earlier and lasting longer than
they did in the past. To date, these changes have not affected total annual
phytoplankton biomass, but the peak chlorophyll levels in the fall blooms appear to
be increasing.
MWRA will continue to track these region-wide physical trends, particularly as they
affect Boston Harbor and Massachusetts and Cape Cod Bays. Some of the changes
could alter the assumptions made during planning for the Massachusetts Bay outfall
or change data interpretation. Those assumptions and interpretations may have to be
re-evaluated in the face of the changing environment. 
2023
Tripos muelleri Bloom 
The biggest story of 2023 was the Tripos muelleri phytoplankton bloom, which
occurred in the spring and summer throughout much of New England (Figure 6-3). 
Tripos muelleri , formerly known as Ceratium tripos , is a frequently abundant
dinoflagellate species found around the world and is not considered a nuisance
species. Tripos muelleri abundance dominated the April–July MWRA phytoplankton
samples in 2023 and led to Contingency Plan threshold exceedances for chlorophyll
and dissolved oxygen. Other phytoplankton species, including other dinoflagellates in
the genus Tripos were not included in the bloom and were present in numbers typical
of other years of monitoring. The bloom was not repeated in 2024.

2023 O UTFALL M ONITORING O VERVIEW : S PECIAL T OPICS 
56 
Figure 6-3. A plankton tow near the mouth of the North River, Marshfield, Massachusetts,
during the 2023 Tripos muelleri bloom. Photo credits Joseph Vallino, The Ecosystems Center,
Marine Biological Laboratory, Woods Hole, Massachusetts, who described the sample as being “like
peanut butter.” 
Tripos species blooms have occurred around the world, from Europe and Africa, to
the Americas and Asia. Tripos muelleri blooms were documented in the Gulf of
Maine in the 1910s and 1920s. Some MWRA team members were working marine
scientists in 1976, when a Tripos muelleri bloom occurred in the New York Bight
(Malone 1978, Mahoney 1978, Falkowski et al. 1980). That bloom has been
attributed to a year with warm winter water temperatures, large river inputs, few
storms, a deep summer pycnocline, persistent winds from the south, inputs of carbon
from offshore waters, and low grazing pressure by zooplankton (Falkowski et al.
1980). It resulted in widespread anoxia and a $60 million loss to the shellfish industry
along the New Jersey coast. The bloom may have impacted a wider geographic area
than was studied at the time. 
Tripos muelleri abundance was higher than the average during every MWRA survey
in 2023, but was extremely high in the spring and summer months (Figure 6-4). In
April through June, abundance was 13–20 times the long-term average. In May and
June, peak abundance reached 100,000 cells per liter.

2023 O UTFALL M ONITORING O VERVIEW : S PECIAL T OPICS 
57 
Figure 6-4. Average abundance of Tripos muelleri in 2023 MWRA samples compared to 1992–
2022. Note log scale on the y-axis. AVG = average. 
Tripos muelleri is larger than most other phytoplankton species, and it grows more
slowly (Figure 6-5). Microflagellates, which are the most abundant phytoplankton in
Massachusetts Bay, are about three micrometers long and wide, while Tripos muelleri 
is at least 20 times wider, about 100 times longer, and contains about 1,000 times the
amount of carbon (one measure of biomass) in each cell. It grows more slowly than
other types of phytoplankton, although there is evidence that growth rates in the
region were higher than usual during the spring of 2023, perhaps a response to
warming waters. 
Figure 6-5. Tripos muelleri is much larger, contains more biomass, and grows more slowly than
other types of phytoplankton . Photo credits David Borkman, Pausacaco Plankton, Saunderstown,
Rhode Island. μm = micrometer; pg = picogram. 
1
10
100
1,000
10,000
100,000
J F M A M J J A S O N D
Abundance (cells/L)
Month
2023 1992-2022 AVG

2023 O UTFALL M ONITORING O VERVIEW : S PECIAL T OPICS 
58
Strong and persistent stratification of the water column is correlated with Tripos
muelleri abundance in Massachusetts Bay, because stable conditions are best for
slow-growing species (Figure 6-6 and Hunt et al. 2010; see also Figure 3-4 on page
14, which shows stratification in 2023 compared to past years). More turbulent
conditions make it difficult for Tripos species to maneuver in the water column,
impeding their typical daily upward and downward migrations, which maximize light
and nutrient acquisition and increase predator avoidance. Early and persistent
stratification, driven by warm temperatures and low salinities in the surface waters,
may have been the main factor driving Tripos muelleri growth in 2023. 
Figure 6-6. Tripos muelleri abundance in June compared to water-column stratification during
May, 1992–2023. Greater stratification of the Massachusetts Bay water column in May is correlated
with Tripos muelleri abundance in June. cells/L = cells per liter. kg/m 3 = kilogram per cubic meter. 
Massive phytoplankton blooms, such as the Tripos muelleri event of 2023 are rare. It
is hard to determine not only why they occur, but also why they are rare. Ecologists
who study phytoplankton communities have long tried to understand how such
blooms develop and why they are not more common, as individual species should
regularly outcompete others for sunlight, nutrients, and other necessary resources.
MWRA participates in workshops and shares data with agencies and research
organizations that continue to assess the 2023 bloom. 
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014 2015
2016
2017
2018
2019
2023
R² = 0.1224
1
10
100
1,000
10,000
100,000
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
June Tripos (cells/L)
May stratification index (kg/m 3 )
P = 0.04

2023 O UTFALL M ONITORING O VERVIEW : P ERMIT U PDATE 
59 
7. Update on the NPDES Permit 
MWRA’s permit to discharge treated sewage effluent into Massachusetts Bay became
effective in August 2000. EPA issued the permit for a period of just five years, but it
has been administratively continued ever since.
EPA issued a new draft permit, NPDES Permit No. MA0103284, in May 2023 and
received public comments through November 28, 2023 (EPA's Permit for the
Massachusetts Water Resources Authority (MWRA) Deer Island Treatment Plant
Outfall and CSOs | US EPA). The Massachusetts Department of Environmental
Protection issued a companion Surface Water Discharge permit which, by reference,
incorporated the EPA NPDES permit requirements. After the close of the public
comment period, EPA began its review and response, required steps prior to issuing a
final permit. The issuance of the final NPDES permit by EPA is still pending.

2023 O UTFALL M ONITORING O VERVIEW : R EFERENCES 
60 
References 
Cibik SJ, Lemieux KB, Davis CS, Anderson DM. 1998. Massachusetts Bay plankton
communities: characterization and discussion of issues relative to MWRA’s outfall
relocation. Boston: Massachusetts Water Resources Authority. Report 1998-08.
140 p.
Codiga D, Wu D. 2022. Information briefing memorandum to the Outfall Science
Advisory Panel. Delivered by B Reilly, MWRA Environmental Quality. December 2,
2022.
Davis S. 2023. NPEDES compliance summary report, Fiscal Year 2023, Deer Island
Treatment Plan and CSOs. Boston: Massachusetts Water Resources Authority. Report
2024-05. 37 p.
Davis S, Hunt C, Wladkowski J. 2024. 2023 technical survey of nitrogen removal
alternatives for the Deer Island Treatment Plant. Boston: Massachusetts Water
Resources Authority. Report 2024-04. 51 p.
Deltares. 2022. Massachusetts Bay outfall treated effluent discharge plume
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Falkowski PG, TS Hopkins, JJ Walsh. 1980. Analysis of factors affecting oxygen
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Libby PS, Rex AC, Keay KE, Mickelson MJ. 2013. Alexandrium Rapid Response
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Malone TC. 1978. The 1976 Ceratium tripos bloom in the New York Bight: causes
and consequences. NOAA Technical Report NMFS Circular 410. 14 p.
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of the North Atlantic right whale. Oceanography 34:22–31.
Moore MJ, McElroy AE, Geoghegan P, Siskey MR, Pembroke A. 2016. Flounder
monitoring report: 2015 results. Boston: Massachusetts Water Resources Authority.
Report 2016-05. 40 p.
Moore, M, Pembroke A, Nestler E, Hall M, Lefkovitz L, Lambert M, Keay K. 2018.
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Moore MJ, Madray ME, Rutecki DA. 2024. Flounder monitoring report: 2023 results.
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of status and trends for 2007–2018. Silver Spring, MD. 263 p.
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Tucker J, Kelsey S, and Giblin A. 2010. 2009 Benthic nutrient flux annual report.
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2023 O UTFALL M ONITORING O VERVIEW : L IST OF A CRONYMS 
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List of Acronyms 
ASP Amnesic shellfish poisoning
AVG Average
BOD Biochemical oxygen demand
cBOD Carbonaceous biochemical oxygen demand
cells/L Cells per liter
cfu Colony forming units
CHV Centrotubular hydropic vacuolation
DDT Dichlorodiphenyltrichloroethane
DIF Deer Island Flats
DO Dissolved oxygen
ECCB Eastern Cape Cod Bay
EPA U.S. Environmental Protection Agency
E-W East to west
kg Kilograms
kg/m 3 Kilograms per cubic meter
IAAC Inter-Agency Advisory Committee
LC50 50% mortality concentration
LNB Large navigation buoy
m Meters
m 3 /s Cubic meters per second
mg/L Milligrams per liter
mg/m 3 Milligrams per cubic meter
mL Milliliters
MGD Million gallons per day
mtons Metric tons
MWRA Massachusetts Water Resources Authority
μg/L Micrograms per liter
μm Micrometer
μM Micromoles
NA Not analyzed/not applicable
NDBC National Data Buoy Center
NERACOOS Northeastern Regional Association of Coastal and Ocean Observing Systems
ng/L Nanograms per liter
NOAA National Oceanic and Atmospheric Administration
NOEC No observed effects concentration
NPDES National Pollutant Discharge Elimination System
OS Outfall site
P Probability
Pa Pascal
PAH Polycyclic aromatic hydrocarbon
PCB Polychlorinated biphenyl
Pg Picogram
PSP Paralytic shellfish poisoning
PSU Practical salinity units
TN Total nitrogen
TSS Total suspended solids

Massachusetts Water Resources Authority
Deer Island
33 Tafts Avenue • Boston, MA 02128
www.mwra.com
617-242-6000