Atlantic Coastal Biome
255
during summer, there is a coincidence in depth between chlorophyll and larger herbivore
abundance, whereas at the end of summer the copepods begin to descend into the lower
layer below the pycnocline.
In the Mid-Atlantic Bight the winter-mixed region is linear and neritic so that chlorophyll increases regularly seaward: at the same time, the percentage of total production
contributed by nanoplankton also increases seaward. Consistently higher rates of primary
production occur in the New York Bight, associated with discharge of urban sewage into
an area of strong tidal fronts and mixing, and on the northern part of Georges Bank.
Productivity, and primary production in these two areas is 25% higher than in any other
part of the Mid-Atlantic Bight (O’Reilly and Busch, 1984). Below the warm, clear surface
water over the shelf in summer there may be a cold, well-lit, chlorophyll-rich layer of
bottom water; this originates in the Gulf of Maine and on the Nova Scotia shelf when
shallow summer stratification isolates the cooler bottom water. Advected off-shelf at
Cape Hatteras and vertically mixed in the Gulf Stream flow, this water body contributes
significantly to the nutrient budget of the Gulf Stream (Wood et al., 1996).
Off Cape Hatteras, recent studies (e.g., Lohrenz et al., 2002) have confirmed that
maximum chlorophyll biomass, measured in situ, occurs in March and is associated with
a strong DCM in which highest values (>10 mg chl m
−3 ) occur near the shelf edge front,
associated with nutrient flux there; this feature is dominated by large cells (>8 m) and,
once again, by July the lower biomass is dominated by small cells. Anomalously high
chlorophyll values in midsummer are associated with influxes across the shelf of highnutrient slope water and are located close to mesoscale meander features associated with
the Gulf Stream flow in the same way as this occurs in the South Atlantic Bight (see
later discussion). Herbivores have been found to remove ∼40% and ∼60% of large- and
small-fraction autotrophic cells, respectively, during the spring shelf edge bloom, so that
small autotrophs (which contribute ∼50% of total primary production) contribute very
significantly to overall production and are significantly incorporated into the microbial
food web (Verity et al., 2002).
In the South Atlantic Bight (Carolina Capes region) the western, cyclonic edge of the
Gulf Stream (as noted earlier) generates wavelike perturbations that propagate northward
along the continental edge and so induce upwelling of cool nutrient-rich Atlantic Central
water at topographic features, especially during summer (Blanton et al., 1981). Because
the upwelled water mass has great clarity, subsurface algal blooms may occur across
the shelf in the bottom water, and this process supports dense concentrations of small
copepods. Thus, abundance of most mesoplankton species on the Florida shelf increases
progressively toward deeper water, which has high particulate load (Paffenhofer, 1983).
As Schollaert et al. (2004) discuss, the actual track taken by the bistable Gulf Stream
(see earlier discussion) determines the relative strength of seasonal chlorophyll bloom
in the slope water area beyond the very narrow shelf of the South Atlantic Bight. Over
a 4-year period, the SeaWiFS images showed a consistent strengthening of this bloom,
with which was associated a progressive northward shift of the axis of the offshore Gulf
Stream. Analysis of this observation shows that the nutrients on the South Atlantic Bight
shelf are not delivered from a distant source by the extension of Labrador Current water
southward, as had previously been assumed, but rather that they are delivered into slope
water from subsurface Gulf Stream water. This mechanism is more effective when the
Gulf Stream takes a more northerly track.
Regional Benthic and Demersal Ecology
The community structure of the invertebrate benthos here is not as well described as on
comparable European continental shelves, and on trawlable grounds we must assume that
it has been highly modified; however, experimental trawling on grounds on the Grand
255
during summer, there is a coincidence in depth between chlorophyll and larger herbivore
abundance, whereas at the end of summer the copepods begin to descend into the lower
layer below the pycnocline.
In the Mid-Atlantic Bight the winter-mixed region is linear and neritic so that chlorophyll increases regularly seaward: at the same time, the percentage of total production
contributed by nanoplankton also increases seaward. Consistently higher rates of primary
production occur in the New York Bight, associated with discharge of urban sewage into
an area of strong tidal fronts and mixing, and on the northern part of Georges Bank.
Productivity, and primary production in these two areas is 25% higher than in any other
part of the Mid-Atlantic Bight (O’Reilly and Busch, 1984). Below the warm, clear surface
water over the shelf in summer there may be a cold, well-lit, chlorophyll-rich layer of
bottom water; this originates in the Gulf of Maine and on the Nova Scotia shelf when
shallow summer stratification isolates the cooler bottom water. Advected off-shelf at
Cape Hatteras and vertically mixed in the Gulf Stream flow, this water body contributes
significantly to the nutrient budget of the Gulf Stream (Wood et al., 1996).
Off Cape Hatteras, recent studies (e.g., Lohrenz et al., 2002) have confirmed that
maximum chlorophyll biomass, measured in situ, occurs in March and is associated with
a strong DCM in which highest values (>10 mg chl m
−3 ) occur near the shelf edge front,
associated with nutrient flux there; this feature is dominated by large cells (>8 m) and,
once again, by July the lower biomass is dominated by small cells. Anomalously high
chlorophyll values in midsummer are associated with influxes across the shelf of highnutrient slope water and are located close to mesoscale meander features associated with
the Gulf Stream flow in the same way as this occurs in the South Atlantic Bight (see
later discussion). Herbivores have been found to remove ∼40% and ∼60% of large- and
small-fraction autotrophic cells, respectively, during the spring shelf edge bloom, so that
small autotrophs (which contribute ∼50% of total primary production) contribute very
significantly to overall production and are significantly incorporated into the microbial
food web (Verity et al., 2002).
In the South Atlantic Bight (Carolina Capes region) the western, cyclonic edge of the
Gulf Stream (as noted earlier) generates wavelike perturbations that propagate northward
along the continental edge and so induce upwelling of cool nutrient-rich Atlantic Central
water at topographic features, especially during summer (Blanton et al., 1981). Because
the upwelled water mass has great clarity, subsurface algal blooms may occur across
the shelf in the bottom water, and this process supports dense concentrations of small
copepods. Thus, abundance of most mesoplankton species on the Florida shelf increases
progressively toward deeper water, which has high particulate load (Paffenhofer, 1983).
As Schollaert et al. (2004) discuss, the actual track taken by the bistable Gulf Stream
(see earlier discussion) determines the relative strength of seasonal chlorophyll bloom
in the slope water area beyond the very narrow shelf of the South Atlantic Bight. Over
a 4-year period, the SeaWiFS images showed a consistent strengthening of this bloom,
with which was associated a progressive northward shift of the axis of the offshore Gulf
Stream. Analysis of this observation shows that the nutrients on the South Atlantic Bight
shelf are not delivered from a distant source by the extension of Labrador Current water
southward, as had previously been assumed, but rather that they are delivered into slope
water from subsurface Gulf Stream water. This mechanism is more effective when the
Gulf Stream takes a more northerly track.
Regional Benthic and Demersal Ecology
The community structure of the invertebrate benthos here is not as well described as on
comparable European continental shelves, and on trawlable grounds we must assume that
it has been highly modified; however, experimental trawling on grounds on the Grand