Part
2, Sec. 2B1
Sanctuary
Resources - Environmental Conditions
Stellwagen
Bank is a glacially-deposited, primarily sandy submerged feature
measuring nearly twenty miles in length, occurring in a southeast-to-northwest
direction between Cape Cod and Cape Ann, Massachusetts. Generally
counterclockwise surface currents flow over the Bank, where
waters depths range from 65 feet to over 300 feet. Bank waters
are characterized by two distinct productivity periods annually,
when overturn and mixing of coastal waters with nutrient-rich
waters from deeper strata produce a complex and rich system
of overlapping midwater and benthic habitats.
This
cyclic biological productivity supports a large variety of commercially
important fisheries, which have in turn supported generations
of fishermen. The Bank's resources are also important feeding
and nursery grounds for an abundance of endangered cetacean
species; and provide habitat for several additional marine mammal
species and associated coastal/pelagic seabirds. Because of
its proximity to land, Stellwagen Bank attracts an increasing
number of commercial, recreational and scientific users and
visitors.
Several
additional human activities occur over or near the Sanctuary,
including transit of commercial vessels and ocean disposal of
dredged materials.
1.
Environmental Conditions
a.
Geology
Like
Cape Cod and the islands of Martha's Vineyard and Nantucket,
Stellwagen Bank and other submerged banks and ledges off the
northeastern U.S. coast were created by the advance and retreat
of glaciers. The southward advance of massive ice sheets nearly
19,000 years ago was influenced by the existing topography;
and the ice was shaped into huge lobes. Two of these lobes created
the land masses identified above. One ice lobe was formed by
what is now Cape Cod Bay; the other by the present-day Great
South Channel, located to the southeast of Cape Cod. The advance
of ice over the continental land mass ground the land into fragments
and carried them along with the movement of the ice.
With
general climatic warming between 18,000 and 15,000 years ago,
the glaciers began to melt and retreat from their coverage.
The ice lobes became more pronounced, and retreated at differing
rates, depending on the depths of topographical depressions
within which they moved. During this process enormous amounts
of pulverized continental land were released from the melting
ice. These land fragments, or "outwash" from the two ice lobes
formed much of the present Cape Cod peninsula.
Retreat
of the ice lobe formed by the Great South Channel was sufficiently
slow that much of the land fragments it carried melted out and
were deposited on the sea floor. These materials formed the
submerged elevation now known as Stellwagen Bank. The Bank originally
was made up of sand, gravel, silt, and "rock flour" (ultra-finely
ground rock); but over time, most of the finer-grained materials
have been carried away by currents and deposited in basin areas
on either side of the Bank (Tucholke and Hollister, 1973; Hassol,
1987; and Campbell, 1987).
The
outer rim of the Gulf of Maine (including Nantucket Shoals,
Georges Bank, and the Nova Scotian Shelf) is floored primarily
with sand and gravel. There is a general tendency for grain
size to increase from southwest to northeast along this portion
of the Continental Shelf.
The
Gulf of Maine basin contains mostly silty-clay, or clayey-silt
sediments. Banks and ridges within the Gulf of Maine are floored
with gravel and boulders; gravel and sand are usual substrates
in nearshore areas.
Clayey-silt
also covers most of Stellwagen Basin and Cape Cod Bay, to the
west of Stellwagen Bank. Small hillocks of coarser, till-like
sediment are also generally found in both areas, and these areas
may act as local sources of detritus, in addition to the contiguous
Stellwagen Bank, Jeffreys Ledge, Tillies Bank, and the coastal
shelf.
Shallow
banks and ledges in this general area are veneered by sand and
mixtures of gravel and sand. Jeffreys Ledge, north of Stellwagen
Bank, is composed primarily of gravel or gravelly-sand, and
is flanked by a sandy apron to the southeast. Stellwagen Bank
is mainly sand or pebbly-sand, flanked to the east by gravel
or gravelly-sand. The broad area between Stellwagen Bank and
Jeffreys Ledge (and east of Tillies Bank) is also covered by
sand mixed with small amounts of gravel. The sand cover extends
from Stellwagen Bank southward into the current-swept channel
between the southern edge of Stellwagen Bank and the northern
tip of Cape Cod. From this channel, a cover of silty-clayey
sand extends westward and northward into the southern portion
of Stellwagen Basin.
Sand
is the predominant sediment for the inner shelf off Cape Cod.
The sand is likely derived from the reworked sandy deposits
of Cape Cod. In deeper waters, sandy deposits give way to silty-clayey
sand; in the center of Stellwagen Basin, sandy cover gives way
to sand-silt-clay bordering clayey silt.
Broad
bathymetric features such as Stellwagen, and other banks and
basins, relate to sediment type, whereas smaller topographic
features such as hillocks, knobs, and swales in rugged areas
bordering the Massachusetts coastline, have little relation
to sediment types. These latter types of areas exhibit a large
variety of sediment types, and lateral changes from one type
to another are rapid.
Sediment
types in basins are affected by nearby sources of coarse-grained
sediment. Tillies Basin, for example, is a small narrow depression
surrounded by shallow banks and ledges, which are covered with
coarse-grained sediment. This coarse "debris" is apparently
easily moved into the adjacent Tillies Basin, as evidenced by
the presence of sand in Basin floor sediments. In Stellwagen
Basin and Cape Cod Bay generally, it is also possible that nearby
coarse-grained glacial deposits provide a source for the coarse
sediments found in these areas.
The
highest concentrations of gravel in this general area are found
on Jeffreys Ledge; the inshore shelf between Cohasset and Plymouth;
and an area east of Stellwagen Bank. Minor amounts of gravel
are associated with sand on Stellwagen Bank, and also with till-like
deposits found at Fishing Ledge in Cape Cod Bay.
As
mentioned above, gravel deposits were most likely transported
to the Cape Cod-Cape Ann area by glaciers. Associated with many
sediment types, gravel occurs in different water current regimes.
It forms a lag veneer with sand, and marks a late stage of ice
deposition. Hence, gravel materials may provide a crude guide
for detecting the waning stages of ice retreat from the offshore
area. Assuming the basic theory of gravel's glacial deposition
and of gravel's indication of ice retreat, then both Stellwagen
Bank and Jeffreys Ledge may actually be offshore moraines and
outwash, which have been reworked during post-glacial rises
in sea level (Campbell, 1987).
Sand
dominates the inshore shelf, shallow banks (such as Stellwagen
and Jeffreys), and the deep water area east of Tillies Bank.
Sand forms an irregular belt of deposits stretching southward
from Jeffreys Ledge to Cape Cod. Although sand floors deep as
well as shallow areas, it is particularly abundant around the
periphery of Cape Cod Bay, and along parts of the Massachusetts
coastal shelf.
The
distribution of sand also provides a guide to water currents.
Currents are particularly strong on Stellwagen Bank and in the
channel between the Bank and the tip of Cape Cod. The inner
shelf also is an area of strong coastal currents and wave action;
and if sand is available as on Cape Cod, the contiguous Bay
sediments contain abundant sand. Areas of sand also are found
next to banks composed in part of glacial deposits, such as
Jeffreys Ledge. Sand deposited by currents apparently settles
the bottom of the inner shelf north of Cape Ann, where bathymetric
contours are widely spaced.
b.
Bathymetry
The
sea floor of the general area encompassing Cape Cod to Cape
Ann is dominated by two broad ridges, Stellwagen Bank and Jeffreys
Ledge, located to its north. Stellwagen Bank extends some 24.85
miles (40 km) in a northwest direction between Cape Cod and
Cape Ann, and occurs at depths of less than 50 meters (164 ft.).
Jeffreys Ledge extends northeast from Cape Ann at depths less
than 60 meters (196.8 ft.). A third, much smaller, and completely
dissected bank known as Tillies Bank, is located between these
two larger banks, and is oriented in roughly a north-south direction.
Tillies Bank rises to within 60 meters of the surface, and is
surrounded by a "moat" which reaches a maximum depth of 200
meters (656 ft.). There is also a subparallel ridge east of
Tillies Bank which rises abruptly to within approximately 65
meters of the surface. In general, most bottom areas west (or
shoreward) of this bank-ledge system are smooth and gently sloping.
East of the bank-ledge system, the bathymetry is more complex
and exhibits steeper gradients.
Together,
Stellwagen Bank, Jeffreys Ledge, and Tillies Bank partly isolate
three basin areas from the outer shelf. From north to south,
these areas are Scantum Basin, Tillies Basin, and Stellwagen
Basin. Stellwagen Basin is bordered by the Massachusetts coastline
on the south and west, and by Cape Cod and Stellwagen Bank on
the east and northeast. Like Stellwagen Bank lying along its
eastern and northeastern borders, the Stellwagen Basin is elliptical
in configuration, with a long axis trending in a northwest direction.
Much of the Bank's southwest side slopes gently toward the deep
axis of the Basin at gradients of about 0.1 to 0.5 percent.
The northeast side of the Basin, however, dips steeply toward
the axis at gradients of up to 6 percent.
East,
or seaward of the Bank-Ledge system, the ocean bottom dips irregularly,
attaining a maximum depth of about 220 meters (722 feet) due
east of Boston (Schlee, Folger, and O'Hara, 1973).
c.
Oceanography
Stellwagen
Bank is subject to the same general surface circulation patterns
as the Gulf of Maine overall. In general, surface waters of
the Gulf exhibit a counterclockwise flow (or gyre), which moves
in a southwest direction along the coasts of Maine and New Hampshire
and into Massachusetts Bay. At Massachusetts Bay, the flow turns
gradually eastward, moving over the northern tip of Cape Cod
and toward the northern edge of Georges Bank. Continuing east
toward Nova Scotia, currents turn north toward the Maine and
New Brunswick coasts. Close to the coast, currents divide and
flow in different directions, with the major portion turning
westward toward Maine. The smaller remainder of the currents
flows north into the Bay of Fundy. In Massachusetts Bay, some
of the flow moves southward into Cape Cod Bay, moving along
the western edge. On the eastern side of Cape Cod, some of the
currents are directed southward and pass between Cape Cod and
Georges Bank, in the Great South Channel (Figure 4).
East
of Stellwagen Bank, net surface currents move to the southeast
at between 1.8 to 9.3 km/day (or 2 to 10 cm/sec). West of the
Bank, surface currents flow southerly in western Cape Cod Bay
and Massachusetts Bay, and northerly in eastern Cape Cod Bay,
forming the generally counter-clockwise movement discussed above.
Results of bottom drift testing indicate that residual bottom
water flow over Stellwagen Bank is southeasterly. Bottom flow
in much of the area west of Stellwagen Bank is usually southerly
into Cape Cod Bay.
Currents
on Stellwagen Bank move mostly east and west at maximum velocities
of 10 to 45 cm/second. Maximum bottom current velocities show
some relation to the bottom sediment type, and to the sea floor
bathymetry. Maximum bottom velocities measured on Stellwagen
Bank (45 cm/sec.) are adequate to move coarse sand. Similar
maximum velocities have been noted in the broad sandy-covered
channel separating Stellwagen Bank from the tip of Cape Cod.
Bottom current velocities are less strong in Stellwagen Basin,
and in the passage southeast of Cape Ann (where maximum bottom
current velocities usually do not exceed 18 cm/sec.).
Internal
waves are periodic phenomena occurring in all the world's oceans.
Investigations have indicated that tidally-generated internal
wave packets are common along the U.S. East Coast (Sawyer and
Apel, 1976), as well as other locations exhibiting the right
combination of bathymetry, tides, and stratification (Gregg
and Briscoe, 1979; and Haury, Wiebe, Orr, and Briscoe, 1983).
During
the late summer, internal wave packets occur twice daily throughout
Massachusetts Bay. These high-frequency, predictable wave packets
are formed over Stellwagen Bank and are transmitted into the
Bay at about 60 cm/sec., finally dissipating in the shallow
waters of the Bay's western edges. Dominant waves have been
measured at approximately 300 meters (984 ft.) in length, occurring
over 8 to 10 minutes, with amplitudes of up to 30 meters (98
ft.) being exhibited. Overturning of the waves has also been
acoustically recorded over Stellwagen Bank, in deep Bay center
waters (80 meters or 262.5 ft.), and during dissipation in shallow
western bay waters.
Phytoplankton
(chlorophyll) and zooplankton are carried up and down by the
overturning and mixing action of internal waves, causing the
vertical distributions of plankton to be altered. Within time
periods of approximately 10 minutes, displacement of plankton
by as much as 30 meters occurs twice daily during late summer
months in Massachusetts Bay. Thus, light levels experienced
by phytoplankton cells may vary from 0.1 to 26% of the ambient
surface illumination. Such rapid changes in light are thought
to alter fluorescent yields of plant cells, affecting in turn,
primary productivity of the Bay generally.
Water
temperature and salinity are seasonally variable in the Gulf
of Maine. During winter months, Gulf waters are coldest in shallow
areas, with little temperature differences exhibited in high
salinity waters along the eastern and western parts of the coast.
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