254
Chapter 9: The Atlantic Ocean
in the Gulf of Maine and on the eastern Scotian shelf. As in the northeastern Atlantic,
these are essentially deep-water expatriates—though they may still be the dominant
organisms of their trophic group—and the shelf basins are their deep-water refuges. In
winter, when seasonal migration would have carried them to depths of 1000 m in the
open ocean, extremely dense aggregations of oceanic copepods and euphausiids occur
close to the bottom in these basins, whose depths exceed 200 m.
The spring bloom over the deep water of the Gulf of Maine can precede water column
stratification (Townsend et al., 1992), and this may be a general but overlooked process
wherever spring blooms occur over deeper water. The principle involved is that deep
penetration of light in a clear, winter-mixed water column may support cell growth rates
sufficiently high as to overcome the vertical excursion rates induced by wind stress at the
surface, especially in calm weather. This process is assisted by the nature of the springbloom cells in some cases, including gelatinous colonies and diatom chains with very low
sinking rates. In such situations, it is also possible—as has been demonstrated for the
Arabian Sea—that the presence of a layer of light- and heat-absorbing chlorophyll may
be a contributing factor in the eventual establishment of stratification. This phenomenon
should be watched for in the open North Atlantic.
The winter-spring transition on Georges Bank was followed closely in the critical
period of 1997 during a JGOFS study of that habitat (Townsend and Thomas, 2001).
Initiation of the bloom occurs very early: in January, chlorophyll biomass was at a
seasonal low but already in February had risen to 2–3 g chl liter
−1 inside the 60-m
isobath, where nitrate and nitrite were together already about 4 M. The bloom survived
a sustained incursion of Scotian Shelf water and continued during the month of March
and spread over deeper parts of the Bank. Silicate appears to have been the overall
limiting molecule, although there was some recycling. In May and June, high near-surface
concentrations (>5 g chl liter
−1 ) of chlorophyll occurred around the deeper margin of
the Bank, although these were patchily distributed. As already noted, cross-frontal transfer
of nutrients plays a major role in supplying nitrate to support the summer bloom in
the tidally mixed parts of Georges Bank (Loder and Platt, 1985; Horne et al., 1989). A
leading candidate for cross-frontal transport is the residual circulation associated with
tidal current interactions. The computed rate of this transfer is sufficient to support
observed new production at the front itself (67% of total production is nitrate based)
and within the enclosed mixed area on the bank, where only 27% of production is fueled
by nitrate in summer.
On Georges Bank, and further south, C. finmarchicus is scarce and the dominant copepods, both numerically and by biomass, are Oithona spp. that are distributed throughout
the water column, even in stratified water. A conceptual model was developed to answer
the question, “Can Georges Bank larval cod survive on a calanoid diet?” (Lynch et al.,
2001), and this concluded that the smallest (4–6 mm) classes of larvae cannot do so,
although larger classes (>10–12 mm) should grow satisfactorily at indicated copepod
densities. On the other hand, the apparent abundance of Pseudocalanus on the Bank
should provide sufficient food for all larval size classes, but is appropriately distributed
in neither space nor time. Since it is known that Calanus undergo episodic starvation on
Georges Bank, it is not surprising that cod larval survival, and hence subsequent year-class
strength, should be as uncertain as it is known to be. At a larger scale, Calanus abundance
in the entire Gulf of Maine is also known to be variable over the long term, responding
to changes in the value of the NOA index and hence to changes in temperature within
the Gulf.
Most areas south of Cape Cod have a rather variable seasonal production cycle, with
highest rates tending to occur toward the end of summer, and only the slope water has
a classical seasonal cycle of productivity with a single spring peak. There are seasonal
changes in the vertical distribution of mesoplankton herbivores relative to the DCM:
Chapter 9: The Atlantic Ocean
in the Gulf of Maine and on the eastern Scotian shelf. As in the northeastern Atlantic,
these are essentially deep-water expatriates—though they may still be the dominant
organisms of their trophic group—and the shelf basins are their deep-water refuges. In
winter, when seasonal migration would have carried them to depths of 1000 m in the
open ocean, extremely dense aggregations of oceanic copepods and euphausiids occur
close to the bottom in these basins, whose depths exceed 200 m.
The spring bloom over the deep water of the Gulf of Maine can precede water column
stratification (Townsend et al., 1992), and this may be a general but overlooked process
wherever spring blooms occur over deeper water. The principle involved is that deep
penetration of light in a clear, winter-mixed water column may support cell growth rates
sufficiently high as to overcome the vertical excursion rates induced by wind stress at the
surface, especially in calm weather. This process is assisted by the nature of the springbloom cells in some cases, including gelatinous colonies and diatom chains with very low
sinking rates. In such situations, it is also possible—as has been demonstrated for the
Arabian Sea—that the presence of a layer of light- and heat-absorbing chlorophyll may
be a contributing factor in the eventual establishment of stratification. This phenomenon
should be watched for in the open North Atlantic.
The winter-spring transition on Georges Bank was followed closely in the critical
period of 1997 during a JGOFS study of that habitat (Townsend and Thomas, 2001).
Initiation of the bloom occurs very early: in January, chlorophyll biomass was at a
seasonal low but already in February had risen to 2–3 g chl liter
−1 inside the 60-m
isobath, where nitrate and nitrite were together already about 4 M. The bloom survived
a sustained incursion of Scotian Shelf water and continued during the month of March
and spread over deeper parts of the Bank. Silicate appears to have been the overall
limiting molecule, although there was some recycling. In May and June, high near-surface
concentrations (>5 g chl liter
−1 ) of chlorophyll occurred around the deeper margin of
the Bank, although these were patchily distributed. As already noted, cross-frontal transfer
of nutrients plays a major role in supplying nitrate to support the summer bloom in
the tidally mixed parts of Georges Bank (Loder and Platt, 1985; Horne et al., 1989). A
leading candidate for cross-frontal transport is the residual circulation associated with
tidal current interactions. The computed rate of this transfer is sufficient to support
observed new production at the front itself (67% of total production is nitrate based)
and within the enclosed mixed area on the bank, where only 27% of production is fueled
by nitrate in summer.
On Georges Bank, and further south, C. finmarchicus is scarce and the dominant copepods, both numerically and by biomass, are Oithona spp. that are distributed throughout
the water column, even in stratified water. A conceptual model was developed to answer
the question, “Can Georges Bank larval cod survive on a calanoid diet?” (Lynch et al.,
2001), and this concluded that the smallest (4–6 mm) classes of larvae cannot do so,
although larger classes (>10–12 mm) should grow satisfactorily at indicated copepod
densities. On the other hand, the apparent abundance of Pseudocalanus on the Bank
should provide sufficient food for all larval size classes, but is appropriately distributed
in neither space nor time. Since it is known that Calanus undergo episodic starvation on
Georges Bank, it is not surprising that cod larval survival, and hence subsequent year-class
strength, should be as uncertain as it is known to be. At a larger scale, Calanus abundance
in the entire Gulf of Maine is also known to be variable over the long term, responding
to changes in the value of the NOA index and hence to changes in temperature within
the Gulf.
Most areas south of Cape Cod have a rather variable seasonal production cycle, with
highest rates tending to occur toward the end of summer, and only the slope water has
a classical seasonal cycle of productivity with a single spring peak. There are seasonal
changes in the vertical distribution of mesoplankton herbivores relative to the DCM: