Physical Oceanography
Water
Properties
Physical Processes
Models
Summary
Physical
oceanographic characteristics in the Stellwagen Bank Marine Sanctuary
must be considered from the context of the oceanographic region in which
it is embedded. From an oceanographic perspective, the Stellwagen Bank
serves as a boundary between the Gulf of Maine to the east and Massachusetts
Bay to the west (see Figure 1). As such it is an important determinant
of the water properties within Massachusetts Bay. Oceanographic features
on the bank itself are primarily related to the strong tidal currents
driven by an approximately three meter tidal range. These tidal currents
interact directly with the sediments on the bank, limiting accumulation
of fine sediments. Additionally, during spring and summer, when there
is a significant density contrast between the surface and bottom layers,
large amplitude internal waves are generated by the tidal flow across
the shallow bank. These internal waves propagate across Massachusetts
Bay, potentially contributing to the flux of material across the density
interface within the bay or at the edges.
Physical
processes active at the bank are largely responsible for the ecological
significance of the bank. The high productivity is supported by the
resuspension of sediment (with associated nutrients) by currents and
wave action, both enhanced by the shallow depth. Additional nutrients
may be supplied to the surface layers in the vicinity of the bank by
the tidal flow of water up the slopes of the bank.
Water
Properties
The water
properties in the vicinity of Stellwagen Bank are determined in part
by the large scale circulation patterns and water types within the Gulf
of Maine. Bigelow (1927) presented a detailed summary of physical oceanographic
information for the Gulf available at the time. Based on drift bottle
returns and hydrographic data, a general counter-clockwise circulation
was described, with some evidence that currents from the north enter
Massachusetts Bay, at least episodically. This pattern has been confirmed
and considerably refined by more recent work. The circulation of surface
waters during spring was described in Brooks (1985). The large scale
counter-clockwise circulation pattern tends to separate from the coast
in central Maine. Flow in Wilkinson Basin, the region of the gulf adjacent
to Stellwagen Bank, is generally southward, associated more with a coastal
current driven by fresh water input from rivers and prevailing winds
than with the larger circulation pattern. The water in this current
is of lower salinity than that in the interior of the Gulf, due to the
influence of the rivers. While the properties of this water are related
to the flow of several rivers in Maine, the most proximate source of
fresh water is the Merrimack River, which enters the Gulf of Maine approximately
thirty kilometers north of Stellwagen Bank. The flow from this river
greatly exceeds the sum of all fresh water sources within Massachusetts
Bay.
The surface
salinity at Stellwagen Bank and within Massachusetts Bay is thus largely
determined by the magnitude of the inputs of fresh water from rivers
along the coast of the Gulf of Maine north of Cape Ann ( Butman, 1975).
This has been confirmed by more recent observations in the Bay. Gardner
et al. (1986) observed the Merrimack River plume in northern Massachusetts
Bay during fall, 1985 and Townsend et al. (1990) presented data indicating
the presence of the plume in the fall of 1990.
The Massachusetts
Bays Program funded a major study of physical oceanography in Massachusetts
and Cape Cod Bays from spring of 1990 through summer of 1991. The seasonal
variation in the distribution of water properties was determined by
a combination of moorings (providing time series of temperature, salinity
and velocity data) and hydrographic surveys. The results of this study
are reported in Geyer et al., 1992. Figures 2-4 show the seasonal variation
of temperature and salinity along a line from Boston Harbor to Stellwagen
Bank as presented in that report. Boston Harbor is at the left side
of these vertical contour plots and Stellwagen Bank is the shallow point
at the right side.
The series
of cruises began in mid April, 1990, when the temperature was very uniform
at 3.5-4.0. There was, however, a strong vertical salinity gradient
apparently related to the presence of the coastal current described
above. This salinity stratification facilitates warming of the surface
water by suppressing vertical mixing, so that by late April, there was
a well developed thermocline. The thermocline continues to strengthen
through the summer, with up to a 12( temperature difference between
surface and bottom waters. The salinity of the surface water varies
with the intensity and position of the coastal current, but remains
lower than that of the deep water. The latter salinity decreased from
about 33.0 PSU in April, 1990 to 32.4 in late September. This freshening
appeared to be largely due to vertical diffusion, though horizontal
advection may have played a role. A cruise in mid-October showed a diminishing
thermocline (and halocline) as the surface waters began to cool with
decreasing day-light. The next cruise was in early February, 1991. Winter
conditions were well mixed through most of the Bay, with temperature
and salinity both increasing with distance from shore. The coldest temperatures
were found in shallow areas, presumably due to more rapid cooling of
the shorter water column. The deep water in Massachusetts Bay in winter,
and with some warming as described above, for much of the year, is similar
in properties to the Gulf of Maine intermediate water described by Hopkins
and Garfield 1979). This water type is formed in coastal areas during
winter, and in fact Massachusetts Bay may be a source of Maine Intermediate
Water, though the 1990-91 winter was relatively mild and no evidence
of this formation was observed. The salinity gradient seemed to be related
to inflow from shore, with lowest salinities observed near Boston Harbor
and in eastern Cape Cod Bay. During this period, conditions in the vicinity
of Stellwagen Bank largely reflected Gulf of Maine water. Cruises in
March, April and June, 1991 repeated the general trend observed in 1990
of strong salinity stratification preceding development of the thermocline.
The details
of conditions in the vicinity of the Bank vary due to differences in
river flow and local effects of wind and cloud cover, but the pattern
presented above is a good summary of the general seasonal progression.
Additional observations of hydrographic conditions have been reported
by Townsend et al. (1990). Also a series of reports describing surveys
conducted by Battelle New England under contract with the Massachusetts
Water Resources Authority contain considerable data from within the
Sanctuary.
Physical
Processes at Stellwagen Bank
As previously
mentioned, physical processes at the bank have a significant impact
on the productivity of the adjacent waters. Phytoplankton growth is
limited, in part, by nutrient supply and light. Optimum growth is therefore
possible only when sufficient nutrients are available close enough to
the surface to provide adequate light. The shallow depths of Stellwagen
Bank, combined with waves and currents, provide these conditions for
a significant portion of the year. Several mechanisms for maintaining
nutrient supply to the upper layers are active at or near the bank.
The tidal
range in the Gulf of Maine is quite large due to the configuration of
the coastline and topography. The extreme ranges found in the Bay of
Fundy (at the northeastern end of the Gulf) are not found at Stellwagen
Bank, but the three meter range is still large enough to generate strong
currents at the bank. In conjunction with the shallow depth, these currents
may generate sufficient turbulence to mix nutrient rich water into the
surface layer. The water column is generally stratified during the period
from late spring through early fall, when light is maximum. This stratification
suppresses mixing, so that nutrients are depleted in the upper layer.
The energetic tidal currents at the bank may be sufficient to overcome
this limitation, and thus lead to enhanced productivity.
The effectiveness
of the tidal mixing is enhanced by the nature of the flow of the stratified
(light water over more dense water) water column. The interface between
upper and lower water types is depressed over the crest of the bank,
and the flow velocity in the lower layer is increased. Under appropriate
conditions, this depression and increased velocity extend down the 'down-stream'
(gulf side on ebb, bay side on flood) of the bank, leading to large
amplitude internal lee waves Haury et al.( 1979), Hibiya (1988) and
Chereskin, T.K.(1983). The flow associated with these waves, and with
the large bottom velocity, probably accounts for some of the flux of
nutrients into the upper layer in the vicinity of Stellwagen Bank.
When
the tide turns, e.g. from ebb to flood, the lee waves are no longer
supported by the tidal flow. The energy contained in the waves then
propagates across Stellwagen Bank into Massachusetts Bay as a train
of large amplitude internal waves (Haury et al. 1979, Haury et al.,
1983, Halpern 1971). These waves are regularly observed in the bay during
stratified conditions. The flows associated with the waves may be sufficient
to cause mixing within the bay, and probably result in considerable
mixing at the point where the interface intersects the bottom. Observations
made in during the Massachusetts Bays Program project in April, 1990
(Gardner, 1990) provided evidence that these waves may have considerable
impact on the productivity within the bay. Recent research by W.R. Geyer
and J. R. Ledwell (1995) examined the importance of internal wave induced
mixing in Massachusetts Bay. They found that as the internal waves (or
internal tide) propagated over shallow areas increased shear in the
pycnocline leads to enhanced vertical diffusion.
Models
While
the measurements made in the vicinity of Stellwagen Bank in recent years
have done much to improve understanding of physical processes and conditions
within the Sanctuary, interpretation of those data is complicated by
limited spatial and temporal resolution. An alternate approach is to
apply numerical models to the system, allowing the response to various
forcing conditions to be investigated. Blumberg, et al. (1993) and Signell
et al. (1994) describe the development of a three dimensional numerical
model of Massachusetts and Cape Cod Bays, including sufficient distance
offshore and north of Cape Ann to allow reasonable representation of
the effect of the coastal current. This model will be invaluable in
the future in studying the transports within and through the Sanctuary.
Summary
Stellwagen
Bank is a partial boundary between Massachusetts Bay and the Gulf of
Maine. As such it effects the conditions within Massachusetts Bay, and
to a lesser extent, conditions in adjacent waters in the Gulf of Maine.
Tidal flows over the shallow bank generate relatively large currents
and during stratified periods internal waves (both stationary lee waves
and propagating waves) result from these currents. The propagating internal
waves effect conditions in regions of Massachusetts Bay remote from
the Bank. All of these physical phenomena have the potential to enhance
the flux of nutrients into the upper layer during periods of stratification,
and thereby contribute to the high levels of productivity found in the
region of the bank.
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