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Physical
Oceanography
of
Massachusetts
and Cape Cod
Bays

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¢ A New View of The Bays

What Does the Bays’
Water Column Look Like?

Circulation Patterns of
The Bays

* Significance of Findings

The circulation patterns of the bays control the transport of pollutants throughout the bays
ecosystem. These patterns are influenced by a number of physical processes which occur on a
local and a regional scale, including the wind, the rain and temperature. This fact sheet is a brief
Summary of a number of research projects which provide insight into the physical processes that
influence the health of the entire bays ecosystem. It is one in a Series of fact sheets published by
the Massachusetts Bays Program. For more information, please call 1-800-447-BAYS.

A New View of
The Bays

Tumors in fish, advisories against consump-
tion of lobster tomalley, and questions relat-
ing to the extension of the Massachusetts

up by marine organisms. We are now seeing the
cumulative effects of centuries of nearshore dump-
ing and disposal, as well as contamination from the
atmosphere, and runoff from a concentrated popula-
tion along the coast.

Water Resources Authority (MWRA) outfall As technology has made more of the marine
pipe into Massachusetts Bay have made us environment accessible to us, we have learned that
begin to look at how pollutants are transport- even the deep ocean supports a lively ecosystem.

ed throughout the marine waters.

For most of human history, people
have used rivers, lakes and the ocean
to dispose of their waste. There
seemed to be no limit to the capacity
of the ocean, especially, to absorb
noxious materials that residents want-
ed to put out of sight and out of mind.

- People believed that currents carried

the wastes far out to sea where they
were diluted by seawater or sank to
the bottom of the deep ocean,
thought to be a silent, dark region that
supported no important life.

Of course, those living along
the shores of Massachusetts and
Cape Cod Bays are realizing that this
characterization of the ocean is
wrong. Currents do indeed bear some
of our pollutants into the open ocean,
but we've learned that these sub-
stances frequently affect the estuaries
through which they pass. We've
learned, too, that some contaminants
settle to the bottom but may be taken

Figure 1 — Circulation Patterns

Overall circulation.patterns in Massachusetts and Cape Cod Bays.
Note the small gyre (pronounced “ji-er’; a circular or spiral motion)
in eastern Cape Cod Bay which may be a factor in the unusually
intense phytoplankton blooms observed there.

U.S. Geological Survey, Woods Hole , MA

part of
Merrimack

Figure 2 — What is an Estuary?

Massachusetts and Cape Cod Bays form a large estuary, defined as a “semi-enclosed body

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Gulf of Maine

of water, which has a free connection with the open sea and within which seawater is
measurably diluted with freshwater from land drainage.” This means that freshwater
(primarily derived from rivers) mixes with seawater (from the Gulf of Maine) and that
circulation of water is somewhat restricted within the bays. Estuaries are among the most
productive ecosystems on earth because nutrients are retained and potentially recycled.
However, retention has its drawbacks. Contaminants dissolved or attached to particles
entering the bays tend to become trapped, resulting in accumulation of contaminated
sediments or uptake by aquatic organisms.

It has become clear that human activity is affecting
the health of both the coastal and estuarine waters
we enjoy and the less visible ecosystem further off-
shore. Tumors in fish and shellfish, and advisories
against consumption of lobster tomalley in Boston
Harbor, are the most visible consequences of this
stress. Sediments are contaminated not only in
Boston Harbor and Salem Sound, but also offshore
in Massachusetts Bay where contaminants have
been found in areas thought to be relatively unim-
pacted.

Preliminary work related to the clean-up of
Boston Harbor and extension of the outfall pipe into
Massachusetts Bay revealed how little we knew

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about the bays ecosystem and the effect this relo-
cated discharge (an average of 550 million gallons
per day) would have on the bays. With $1.6 million
in settlement funds from the lawsuit against the
Commonwealth over the pollution of Boston Harbor,
the Massachusetts Bays Program launched the first
integrated research program focused on the bays.
This was done in coordination with related projects
of the Massachusetts Water Resources Authority
(MWRA), the U.S. Geological Survey (USGS), and
the Massachusetts Institute of Technology Sea Grant
Program.

What Does The
Bays’ Water Column
Look Like?

Heated by the sun and mixed with freshwater

and the air, the surface waters tend to be

warmer, less salty and more oxygen-rich than

the deep waters of the bays. »

We generally think of the water in the ocean as
being uniformly cold and salty, not realizing that its
many characteristics vary throughout the water col-
umn (from the surface to the bottom.) Studies of
Massachusetts and Cape Cod Bays have found that
temperature and salinity vary horizontally and verti-
cally. Freshwater input and solar heating stratify, or
separate, the water column into layers of varying
water properties. Rivers provide freshwater which
can form a brackish (moderately salty), less dense
layer over the deeper, saltier (and therefore denser)
water, and sunlight warms the upper part of the
water column, making it less dense than the lower
part. The relatively sharp change in density between
layers is called a pycnocline. In Massachusetts
Bays, the pycnocline is seasonal, lasting from
roughly April to October, (See Figure 3).

Dissolved oxygen, important to the respira- |
tion of marine animals, is generally more concen-
trated in the upper parts of the water column
because of mixing with the atmosphere and because
phytoplankton living near the surface produce oxy-
gen during photosynthesis. There is some dissolved
oxygen in deep waters, which is used by bottom- |
dwelling animals during respiration, and by decay-
ing organic matter. The pycnocline effectively
isolates the deeper waters in Stellwagen Basin (west
of Stellwagen Bank) resulting in levels of dissolved

oxygen which are among the lowest in the bays, but
still adequate to support marine life.

The importance of each of these factors
varies over the course of the year and from place to
place. Nearshore regions, for instance, are more
influenced by tides and freshwater input than are
offshore waters. Though significant stratification
occurs in some areas of Boston Harbor, in much of
the Harbor the freshwater flow from rivers plus tidal
action, combined with the shallow depth of the
Harbor, result in a well-mixed water column. The
picture is disrupted in nearshore and offshore
regions by large events, such as hurricanes and
other coastal storms.

Circulation Patterns
of The Bays

Though surface water circulation in
Massachusetts Bays is generally counter-
clockwise it slows down through Cape Cod
Bay, moves faster than deeper waters during
the summer, and slows during the winter
when the well-mixed water column flows
more uniformly throughout the bays.

FEBRUARY

Salinity (PSU) Temperature °C

Sal Gils Sy eiay EE} (0) 4 8 12 16 20

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characteristics allow mixing throughout the water column because there
is very little difference in water density from top to bottom.

Depending on the direction, winds can cause
“upwelling” and “downwelling” which can
bring cold, deep waters up or push the
warmer surface waters down.

In order to better understand the rate of transport
and the fate and movement of waterborne pollutants
in the bays, the Massachusetts Bays Program fund-
ed a major study to gather information about the
physical processes operating on the bays. The study
addressed seasonal variations in circulation patterns
and looked at how currents, tides, winds, and runoff
influence circulation. Scientists from the Woods
Hole Oceanographic Institution, the University of
Massachusetts - Boston, the University of New
Hampshire and the USGS used information gathered
from moored instrument arrays, satellite-tracked
surface drifters, shipboard surveys and satellite
images to provide data on large-scale water move-
ments and properties—the physical oceanogra-
phy—of the bays.

The study confirmed that surface water circu-
lation in Massachusetts Bays is generally counter-
clockwise, entering from the Gulf of Maine near

MARCH

Salinity (PSU) Temperature (°C)

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Figure 3 — Seasonal Variations in the Water Column The profile for March shows little change in temperature, but does show
Seasonal profiles of the water column in the offshore parts of a decrease in the salinity (and consequently the density) of surface water
Massachusetts Bays are depicted above. The depths are shown in because of increased freshwater input from snowmelt and spring rains.
meters. The profile for February is typical for winter, with salinity and The part of the water column where the change in density is most
temperature fairly constant from the surface to the bottom. These pronounced Is called the pyenocline.

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Cape Ann and moving first west, then south along
the South Shore, travelling either directly east or
through Cape Cod Bay and finally leaving the bays
north of Race Point at the tip of Cape Cod, (see
Figure 1). Contaminants may travel a considerable
distance from their point of entry into the bays to
other areas where they may accumulate in sedi-
ments or be taken up by marine organisms. The
waters are influenced by the larger counterclockwise
patterns in the Gulf of Maine, which are, in turn, dri-
ven by rivers, by circulation in the North Atlantic,
and by large-scale atmospheric weather patterns.
Surface water (down to about 18 meters)
takes anywhere from 20 to 40 days to complete its
trip through the bays. A slow-moving eddy (a closed
loop of recirculating flow separate from the main
stream) in Cape Cod Bay sometimes results in the
waters remaining there longer than in
Massachusetts Bay. One drifter buoy released dur-
ing the study remained in Cape Cod Bay for more
than a month. This may help to explain the intense
phytoplankton blooms observed there in early to
mid-February. The longer residence time combined
with the shallow depth allows the phytoplankton to

JULY

Salinity (PSU) Temperature (°C)

eet

2.

begin growing early in the year when the overall
light level is still low in the deeper waters.
Generally, in large bodies of water, the deep-
er water is somewhat dependent on the surface
water for its movement. The surface water creates a
shear that drives the movement of deeper waters
and “drags” it along its path. In Massachusetts and

Cape Cod Bays, from late spring through fall, verti-

cal variations in density tend to isolate the deeper
water from the surface. In fact, there may be very lit-
tle net flow of bottom waters (below about 60 -
meters) through the relatively shallow northern con-
nection with the Gulf of Maine. However, in the win-
ter the water column is well mixed and the flow

through the bays is more nearly uniform with depth.

How fast does the water move
through the bays?

The study also found that current speed
varies with the season. During the winter, surface
currents proceed on their generally counterclock-
wise path through the bays more slowly than in the
summer due to a well-mixed water column. Surface
currents measured off the Scituate coast during the
winter averaged only about 4.6 cm/sec, whereas

OCTOBER

Salinity (PSU) Temperature (°C)

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By July, the pycnocline is stronger and higher in the water column
because of reduced freshwater input and because the surface waters are
warmed, and, hence, made less dense, by the sun. The warmer, less
saline water is less dense than in March and the stronger gradients
greatly reduce vertical mixing.

October’s profile shows a weakening of the pycnocline. Siriaas cooling |
due to shorter days and cooler air temperatures reduce the surface ieee ie 4
temperature and the lower gradients result in increased mixing. .

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during the summer currents averaged about 8.3
cm/sec. (These values reflect both the magnitude
and direction of the current.) Intermediate waters
(18 to 28 meters deep) flow more slowly, and, in
general, follow the course of the surface currents
throughout the year. Deep waters (below 30 meters)
are especially sluggish during the summer months
and are known to occasionally move in directions
opposite the surface waters.

What causes the vertical
movement of water in the
bays?

During the summer, strong southwesterly
winds move the surface water offshore bringing
colder, deeper water to the surface, and lifting or
elevating the pycnocline (see Figure 2). This phe-
nomenon, called upwelling, is most familiar to
beachgoers on the North and South shores, as they
are faced with cold water for swimming. It is impor-
tant because the deeper water also brings nutrients
(like nitrogen and phosphorus) into the upper part of
the water column where the light-dependent phyto-
plankton can grow. Winds blowing from an offshore
direction cause downwelling, forcing nearshore sur-
face water to sink as offshore surface water is
pushed inshore. Data suggest that during the three-
month summer stratified period about one-eighth of
the bottom water can be exchanged with the surface
layer.

Do the tides influence the
circulation patterns of the
bays?

Since tides move water back and forth with
no net direction, they do not have a strong effect on
the overall circulation patterns of the bays. However,
the tides in Massachusetts Bays, which range from
8 to 12 feet, can have strong local effects. Tides are
the major factor controlling the exchange of water
between Boston Harbor and Massachusetts Bay. In
shallow waters of the bays, tides create turbulence
near the bottom which can resuspend sediments or
disrupt stratification.

7

The movement of waters within the bays, such
as vertical mixing, circulation at various
depths, upwelling and the length of time
waters remain in a given area, will help
researchers predict the fate of the effluent
from outfall pipes, the distribution of large
concentrations (blooms) of phytoplankton,
and the movement of pollutants throughout
the bays ecosystem.

The findings of this project have significant implica-
tions for management strategies to reduce the
effects of pollution on Massachusetts and Cape Cod
Bays. They are especially important when one con-
siders that decisions regarding the disposal of
wastes from sewage treatment plants, industries,
and dredging projects are often based upon very
limited data. Data are usually limited to local circu-
lation patterns, which restricts the decision-makers’
ability to accurately predict the ultimate fate of pol-
lutants discharged into the larger bays system.

Particularly important is the new information
on vertical mixing of the water column, which pro-
vides Clues to the fate of the effluent from the
extended MWRA sewage outfall and how it could —
affect the production of phytoplankton. The study
suggests that the fate of the effluent plume will be
most affected by the circulation of mid-depth waters
(between 18 and 28 meters deep) in the lower por-
tion of the pycnocline, (see Figure 2), confirming
the assumption scientists made when they recom-
mended that site. Further measurements of currents
at this depth were made by scientists at the Woods
Hole Oceanographic Institution to determine the
extent of this mixing and to better estimate the
length of time the effluent will remain in the area of
the outfall.

The data gathered on upwelling and resi-
dence time, in conjunction with information on
nutrients (see Fact Sheet #7), are useful for pre-
dicting the distribution of phytoplankton blooms,
which form the base of the food chain for marine
animals. Circulation patterns may have an effect on
the shellfishery when “red tide” seed populations
are carried into the bays from waters north of
Massachusetts. Blooms of these toxic organisms
lead to extensive closures of shellfish beds. In addi-
tion, nutrient availability may indirectly affect the

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endangered whale populations that inhabit Cape
Cod Bay. The phytoplankton that serves as the pri-
mary food source for the tiny animals eaten by the
whales depend upon nutrient availability to grow.
The high residence time of water in Cape Cod Bay,
and its relationship to productivity, are being clari-
fied through further studies (Gardner & Villareal).

The value of this information is further

enhanced by the results of related studies. For ex-
ample, based upon sediment traps deployed near
the planned MWRA outfall, the USGS has found that
storms, especially during the fall and winter, peyiod-
ically resuspend material from the sea floor. When
combined with our new knowledge about the circu-
lation patterns, we can begin to predict the fate of
the contaminants in the bays. To that end, the USGS
is using the MBP data to calibrate a three-dimen-
sional model of circulation in Massachusetts and
Cape Cod Bays which will help improve our ability
to predict how particles, including those that re-
enter the water column during storms or other events,
become transported to other locations. Taken
together, this information provides a more complete
characterization of the processes at work in the
bays, and will greatly assist researchers in monitor-
ing the effects of pollutant discharges to the bays.

Resources

“Physical Oceanographic Investigation of
Massachusetts and Cape Cod Bays,” Geyer, et al,
1992: MBP-92-03.

“Biological and Physical Processes Controlling
Nutrient Dynamics and Primary Production in Cape
Cod Bay,” Gardner and Villareal, UMass-Boston;

The Estuary Program

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Loder, University of New Hampshire-Durham; in
prep.

Contacts

Massachusetts Bays Program, 100 Cambridge
Street, Room 2006, Boston, MA 02202; Diane
Gould, 617-727-9530.

Massachusetts Institute of Technology, Dept. of
Civil Engineering, 77 Massachusetts Avenue,
Cambridge, MA 02139; Eric Adams, 617-253-
6595.

Massachusetts Water Resources Authority,
Charlestown Navy Yard, 100 First Avenue, Boston,
MA 02129; Wendy Leo, 617-242-6000 x5501.

National Oceanic and Atmospheric Administration,
JFK Federal Building, HEE-CAN6, Boston, MA
02203; Kenneth Finkelstein, 617-223-5537.

U.S. Environmental Protection Agency, Region 1,
JFK Federal Building, WQE, Boston, MA 02203;
Matthew Liebman, 617-565-4866.

U.S. Geological Survey, 384 Woods Hole Road,
Quissett Campus, Woods Hole, MA 02543-1598;
Richard Signell, 508-457-2229.

University of Massachusetts-Boston, Environmental
Sciences Program, Harbor Campus, Boston, MA
02125; George B. Gardner, (617) 287-7454.

Woods Hole Oceanographic Institution, Applied
Ocean Physics, Woods Hole, MA 02543;
W. Rockwell Geyer, (508) 548-1400 x2868.

The Massachusetts Bays Program is a joint effort of local, state, and federal governments, as well as citizens, scientists, educators and
businesses to develop regional solutions to pollution problems in the Bays and their adjacent watersheds. The Program is funded under
the Clean Water Act through the U.S. Environmental Protection Agency (##CE001534-01-4), and administered by the Massachusetts —
Executive Office of Environmental Affairs’ Coastal Zone Management Office. In addition to developing a long-term plan to improve water
quality management, the Program offers information on and technical assistance for innovative, locally-based pollution prevention and
remediation projects, and sponsors a multi-faceted public outreach and education effort to heighten awareness of pollution problems and

to enlist support for and participation in bays protection.

Massachusetts Bays Program Fact Sheets —Editor: Elizabeth McEvoy. Contributing Writers: Maxine Schmidt and Matthew

Liebman. Design: Don Eunson.

For more information call 1-800-447-BAYS or write Massachusetts Bays Program, 100 Cambridge Street,

Room 2006, Boston, MA 02202.

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