Physical oceanography of Massachusetts and Cape Cod Bays

8 Pages·1995·0.52 MB·English

by  Massachusetts Bays Program

act Sheet No
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Physical 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
Oceanography
local and a regional scale, including the wind, the rain and temperature. This fact sheet is a brief
of summary of a number of research projects which provide insight into the physical processes that
Massachusetts 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.
and Cape Cod
A
Bays New View of up by marine organisms. We are now seeing the
The Bays
cumulative effects of centuries of nearshore dump-
ing and disposal, as well as contamination from the
Tumors in fish, advisories against consump-
atmosphere, and runoff from a concentrated popula-
tion oflobster tomalley, andquestions relat-
tion along the coast.
ing to the extension ofthe Massachusetts
WaterResourcesAuthority (MWRA) outfall As technology has made more of the marine
pipe into Massachusetts Bayhave made us environment accessible to us, we have learned that
begin to lookathowpollutants are transport- even the deep ocean supports a lively ecosystem.
ed throughoutthe marine waters.
—
£38 For most of human history, people Figure 1 Circulation Patterns
Overall circulation patterns in Massachusetts and Cape CodBays.
have used rivers, lakes and the ocean
Note the smallgyre (pronounced "ji-er"; a circular orspiralmotion)
to dispose of their waste. There
in eastern Cape CodBay which maybe a factorin the unusually
seemed to be no limit to the capacity
r> intense phytoplankton blooms observedthere.
of the ocean, especially, to absorb
noxious materials that residents want-
ed to put out of sight and out of mind.
T& 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
• A New View of The Bays
the shores of Massachusetts and
Cape Cod Bays are realizing that this
• What Does the Bays' characterization of the ocean is
Water Column Look Like? wrong. Currents do indeed bear some
of our pollutants into the open ocean,
but we've learned that these sub-
• Circulation Patterns of
stances frequently affect the estuaries
The Bays
through which they pass. We've
learned, too, that some contaminants
• Significance of Findings settle to the bottom but may be taken
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
9
Look Like?
Heatedby the sun andmixed with freshwater
andthe air, the surface waters tendto be
warmer, less saltyandmore oxygen-rich than
the deep waters ofthe 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
Figure 2 What is an Estuary? temperature and salinity vary horizontally and verti-
Massachusetts and Cape CodBays form a large estuary, definedas a "semi-enclosedbody
cally. Freshwater input and solar heating stratify, or
ofwater, which has a free connection with the open sea and within which seawater is
separate, the water column into layers of varying
measurably diluted with freshwaterfrom landdrainage. " This means that freshwater
water properties. Rivers provide freshwater which
(primarily derivedfrom rivers) mixes with seawater (from the GulfofMaine) andthat
circulation of wateris somewhatrestricted within the bays. Estuaries are among the most can form a brackish (moderately salty), less dense
productive ecosystems on earth because nutrients are retainedandpotentiallyrecycled. layer over the deeper, saltier (and therefore denser)
However, retention has its drawbacks. Contaminants dissolvedorattachedtoparticles water, and sunlight warms the upper part of the
entering the bays tendto become trapped, resulting in accumulation ofcontaminated water column, making it less dense than the lower
sediments oruptake byaguatic organisms. part. The relatively sharp change in density between
layers is called a pycnocline. In Massachusetts
It has become clear that human activity is affecting Bays, the pycnocline is seasonal, lasting from
the health of both the coastal and estuarine waters roughly April to October, (see Figure 3).
we enjoy and the less visible ecosystem further off- Dissolved oxygen, important to the respira-
shore. Tumors in fish and shellfish, and advisories tion of marine animals, is generally more concen-
against consumption of lobster tomalley in Boston trated in the upper parts of the water column
Harbor, are the most visible consequences of this because of mixing with the atmosphere and because
stress. Sediments are contaminated not only in phytoplankton living near the surface produce oxy-
Boston Harbor and Salem Sound, but also offshore gen during photosynthesis. There is some dissolved
in Massachusetts Bay where contaminants have oxygen in deep waters, which is used by bottom-
been found in areas thought to be relatively unim- dwelling animals during respiration, and by decay-
pacted. ing organic matter. The pycnocline effectively
Preliminary work related to the clean-up of isolates the deeper waters in Stellwagen Basin (west
Boston Harbor and extension of the outfall pipe into of Stellwagen Bank) resulting in levels of dissolved
Massachusetts Bay revealed how little we knew
2
M
oxygen which are among the lowest in the bays, but Depending on the direction, winds can cause
still adequate to support marine life. "upwelling"and "downwelling" which can
The importance of each of these factors bring cold, deep waters up orpush the
warmersurface waters down.
varies over the course of the year and from place to
place. Nearshore regions, for instance, are more In order to better understand the rate of transport
influenced by tides and freshwater input than are and the fate and movement of waterborne pollutants
offshore waters. Though significant stratification in the bays, the Massachusetts Bays Program fund-
occurs in some areas of Boston Harbor, in much of ed a major study to gather information about the
the Harbor the freshwater flow from rivers plus tidal physical processes operating on the bays. The study
action, combined with the shallow depth of the addressed seasonal variations in circulation patterns
Harbor, result in a well-mixed water column. The and looked at how currents, tides, winds, and runoff
picture is disrupted in nearshore and offshore influence circulation. Scientists from the Woods
regions by large events, such as hurricanes and Hole Oceanographic Institution, the University of
other coastal storms. Massachusetts - Boston, the University of New
Circulation Patterns Hampshire and the USGS used information gathered
of The Bays from moored instrument arrays, satellite-tracked
surface drifters, shipboard surveys and satellite
Though surface water circulation in images to provide data on large-scale water move-
—
Massachusetts Bays is generallycounter- ments and properties the physical oceanogra-
—
clockwise itslows down through Cape Cod phy of the bays.
Bay, moves faster than deeper waters during
The study confirmed that surface water circu-
the summer, andslows during the winter
lation in Massachusetts Bays is generally counter-
when the well-mixed water column flows
clockwise, entering from the Gulf of Maine near
more uniformly throughout the bays.
FEBRUARY MARCH
Salinity (PSU) Temperature°C Salinity (PSU) Temperature (°C)
"
20 " 20
Pycnocline
40 " 40
60 - 60
1
1
1
80 1 80
1—1 +
1 1 -t-
31 31.5 32 32.5 33 12 16 20 31 31.5 32 32.5 33 12 16 20
—
Figure 3 Seasonal Variations in the Water Column The profile forMarch shows little change in temperature, butdoes show
Seasonalprofiles ofthe watercolumn in the offshoreparts of a decrease in the salinity (andconseguentlythe density) ofsurface water
Massachusetts Bays are depictedabove. The depths are shown in because ofincreasedfreshwaterinput from snowmeltandspring rains.
meters. The profile forFebruaryis typicalfor winter, with salinityand The part ofthe water column where the change in densityis most
temperature fairlyconstant from the surface to the bottom. These pronouncedis calledthe pycnocline.
characteristics allowmixing throughoutthe water column because there
is verylittle difference in water density from top to bottom.
—
Cape Ann and moving first west, then south along begin growing early in the year when the overall
the South Shore, travelling either directly east or light level is still low in the deeper waters.
through Cape Cod Bay and finally leaving the bays Generally, in large bodies of water, the deep-
north of Race Point at the tip of Cape Cod, (see er water is somewhat dependent on the surface
Figure 1). Contaminants may travel a considerable water for its movement. The surface water creates a
distance from their point of entry into the bays to shear that drives the movement of deeper waters
other areas where they may accumulate in sedi- and "drags" it along its path. In Massachusetts and
ments or be taken up by marine organisms. The Cape Cod Bays, from late spring through fall, verti-
waters are influenced by the larger counterclockwise cal variations in density tend to isolate the deeper
patterns in the Gulf of Maine, which are, in turn, dri- water from the surface. In fact, there may be very lit-
ven by rivers, by circulation in the North Atlantic, tle net flow of bottom waters (below about 60
and by large-scale atmospheric weather patterns. meters) through the relatively shallow northern con-
Surface water (down to about 18 meters) nection with the Gulf of Maine. However, in the win-
takes anywhere from 20 to 40 days to complete its ter the water column is well mixed and the flow
trip through the bays. A slow-moving eddy (a closed through the bays is more nearly uniform with depth.
How fast does the water move
loop of recirculating flow separate from the main
through the bays?
stream) in Cape Cod Bay sometimes results in the
waters remaining there longer than in The study also found that current speed
Massachusetts Bay. One drifter buoy released dur- varies with the season. During the winter, surface
ing the study remained in Cape Cod Bay for more currents proceed on their generally counterclock-
than a month. This may help to explain the intense wise path through the bays more slowly than in the
phytoplankton blooms observed there in early to summer due to a well-mixed water column. Surface
mid-February. The longer residence time combined currents measured off the Scituate coast during the
with the shallow depth allows the phytoplankton to winter averaged only about 4.6 cm/sec, whereas
JULY OCTOBER
Salinity (PSU) Temperature (°C) Salinity (PSU) Temperature (°C)
—
+
I 1 H 31 31.5 32 32.5 33 12 16 20
31 31.5 32 32.5 33 12 16 20
ByJuly, thepycnocline is strongerandhigherin the water column October's profile shows a weakening ofthepycnocline. Surface cooling
because ofreduced freshwaterinputandbecause the surface waters are due to shorterdays andcoolerairtemperatures reduce the surface layer
warmed, and, hence, made less dense, bythe sun. The warmer, less temperature andthe lowergradients resultin increasedmixing.
saline wateris less dense than in March andthe strongergradients
greatlyreduce verticalmixing.

during the summer currents averaged about 8.3 Significance of These
Findings
cm/sec. (These values reflect both the magnitude
and direction of the current.) Intermediate waters
The movement ofwaters within the bays, such
(18 to 28 meters deep) flow more slowly, and, in
as verticalmixing, circulation at various
general, follow the course of the surface currents
depths, upwelling andthe length oftime
throughout the year. Deep waters (below 30 meters) waters remain in a given area, willhelp
are especially sluggish during the summer months researcherspredict the fate ofthe effluent
and are known to occasionally move in directions from outfallpipes, the distribution oflarge
opposite the surface waters. concentrations (blooms) ofphytoplankton,
What causes the vertical andthe movement ofpollutants throughout
movement of water in the the bays ecosystem.
bays?
The findings of this project have significant implica-
During the summer, strong southwesterly tions for management strategies to reduce the
winds move the surface water offshore bringing effects of pollution on Massachusetts and Cape Cod
colder, deeper water to the surface, and lifting or Bays. They are especially important when one con-
elevating the pycnocline (see Figure 2). This phe- siders that decisions regarding the disposal of
nomenon, called upwelling, is most familiar to wastes from sewage treatment plants, industries,
beachgoers on the North and South shores, as they and dredging projects are often based upon very
are faced with cold water for swimming. It is impor- limited data. Data are usually limited to local circu-
tant because the deeper water also brings nutrients lation patterns, which restricts the decision-makers'
(like nitrogen and phosphorus) into the upper part of ability to accurately predict the ultimate fate of pol-
the water column where the light-dependent phyto- lutants discharged into the larger bays system.
plankton can grow. Winds blowing from an offshore Particularly important is the new information
direction cause downwelling, forcing nearshore sur- on vertical mixing of the water column, which pro-
face water to sink as offshore surface water is vides clues to the fate of the effluent from the
pushed inshore. Data suggest that during the three- extended MWRA sewage outfall and how it could
month summer stratified period about one-eighth of affect the production of phytoplankton. The study
the bottom water can be exchanged with the surface suggests that the fate of the effluent plume will be
layer. most affected by the circulation of mid-depth waters
Do the tides influence the
(between 18 and 28 meters deep) in the lower por-
circulation patterns of the
tion of the pycnocline, (see Figure 2), confirming
bays?
the assumption scientists made when they recom-
Since tides move water back and forth with mended that site. Further measurements of currents
no net direction, they do not have a strong effect on at this depth were made by scientists at the Woods
the overall circulation patterns of the bays. However, Hole Oceanographic Institution to determine the
the tides in Massachusetts Bays, which range from extent of this mixing and to better estimate the
8 to 12 feet, can have strong local effects. Tides are length of time the effluent will remain in the area of
the major factor controlling the exchange of water the outfall.
between Boston Harbor and Massachusetts Bay. In The data gathered on upwelling and resi-
shallow waters of the bays, tides create turbulence dence time, in conjunction with information on
near the bottom which can resuspend sediments or nutrients (see Fact Sheet #7), are useful for pre-
disrupt stratification. 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

. ,
endangered whale populations that inhabit Cape Loder, University of New Hampshire-Durham; in
Cod Bay. The phytoplankton that serves as the pri- prep.
mary food source for the tiny animals eaten by the
whales depend upon nutrient availability to grow. Contacts
The high residence time of water in Cape Cod Bay,
and its relationship to productivity, are being clari- Massachusetts Bays Program, 100 Cambridge
fied through further studies (Gardner & Villareal). Street, Room 2006, Boston, MA 02202; Diane
The value of this information is further Gould, 617-727-9530.
enhanced by the results of related studies. For ex- Massachusetts Institute of Technology, Dept. of
ample, based upon sediment traps deployed near Civil Engineering, 77 Massachusetts Avenue,
the planned MWRA outfall, the USGS has found that Cambridge, MA 02139; Eric Adams, 617-253-
storms, especially during the fall and winter, pe/iod- 6595.
ically resuspend material from the sea floor. When
combined with our new knowledge about the circu- Massachusetts Water Resources Authority,
lation patterns, we can begin to predict the fate of Charlestown Navy Yard, 100 First Avenue, Boston,
the contaminants in the bays. To that end, the USGS MA 02129; Wendy Leo, 617-242-6000 x5501
is using the MBP data to calibrate a three-dimen- National Oceanic and Atmospheric Administration,
sional model of circulation in Massachusetts and JFK Federal Building, HEE-CAN6, Boston, MA
Cape Cod Bays which will help improve our ability 02203; Kenneth Finkelstein, 617-223-5537.
to predict how particles, including those that re-
U.S. Environmental Protection Agency, Region
enter the water column during storms or other events, 1
JFK Federal Building, WQE, Boston, MA 02203;
become transported to other locations. Taken
Matthew Liebman, 617-565-4866.
together, this information provides a more complete
characterization of the processes at work in the U.S. Geological Survey, 384 Woods Hole Road,
bays, and will greatly assist researchers in monitor- Quissett Campus, Woods Hole, MA 02543-1598;
ing the effects of pollutant discharges to the bays. Richard Signell, 508-457-2229.
University of Massachusetts-Boston, Environmental
Resources Sciences Program, Harbor Campus, Boston, MA
02125; George B. Gardner, [phone].
"Physical Oceanographic Investigation of
Woods Hole Oceanographic Institution, Applied
Massachusetts and Cape Cod Bays," Geyer, et al,
Ocean Physics, Woods Hole, MA 02543;
1992; MBP-92-03.
W. Rockwell Geyer, [phone]x2868.
"Biological and Physical Processes Controlling
Nutrient Dynamics and Primary Production in Cape
Cod Bay," Gardner and Villareal, UMass-Boston;
The Estuary Program
The Massachusetts Bays Program isajoint effortof local, state, and federal governments, aswell as citizens, scientists, educatorsand
businesses to develop regional solutions to pollution problems in the Baysand theiradjacentwatersheds. The Program isfunded under
the Clean WaterActthrough the U.S. Environmental Protection Agency (#CE001534-01-4), and administered bythe 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 sponsorsa multi-faceted public outreach and education effortto heighten awareness ofpollution problemsand
to enlistsupportforand participation in bays protection.
Massachusetts Bays Program Fact Sheets—Editor: Elizabeth McEvoy. Contributing Writers: Maxine Schmidtand Matthew
Liebman. Design: Don Eunson.
®
For more information call 1-800-447-BAYS orwrite Massachusetts Bays Program, 100 Cambridge Street,
Room 2006, Boston, MA 02202.
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