252
Chapter 9: The Atlantic Ocean
In this province, the effect of changes in the North Atlantic Oscillation (NAO, see
Chapter 8) is as important as anywhere. The NAO Index characterizes the strength and
location of the westerly winds over the North Atlantic in winter and, consequently, the
meridional displacement of the Gulf Stream: during periods of low NAO Index, the flow
of the Labrador Current is increased and the area occupied by Labrador Slope Water
is enlarged as the North Wall of the Gulf Stream shifts southward and comes to lie
farther offshore. This is shown in Color plate 10, illustrating the differences between
early spring of 2000 when the NAO Index was high (+28), and spring of 2001 when it
was low (−189). Zwanenberg, et al. (2003) notes that annual changes in bottom water
temperature on the Scotian Shelf are among the most variable in the North Atlantic; a
50-year record shows periods of 5–10 years of sustained anomalies of both signs with very
rapid switching between cold and warm conditions. An influx of cold water into Emerald
Basin in 1998 dropped temperatures there by 3
C. To follow such events, it is very
helpful that a library of matched 2-km, 15-day AVHRR SST and SeaWiFS chlorophyll
images for the area from Cape Cod to northern Labrador is now readily accessible on-line
(http://www.mar.dfo-mpo.gc.ca/science/ocean/ias/seawifs).
Response of the Pelagic Ecosystems
In terms of primary production and (at the other end of the trophic chain) the potential
production of demersal and pelagic fish, this is one of the richest regions of the ocean;
unfortunately, it is also here that the failure of fishery science to provide adequate methods
for the management of stocks has been most acutely demonstrated.
Because of complexity of the geography of the NWCS it is difficult to generalize
the pelagic ecosystem and, without the satellite data now available, much would remain
obscure. At the regional scale, archived chlorophyll images show that the Grand Banks
and the Scotian Shelf, together with the Gulf of St. Lawrence and the Gulf of Maine,
are the principal centers of productivity, and also of seasonal change. Although there
are major differences between these regions, the seasonal productivity cycle in each
involves the occurrence of a spring bloom, which is observed in the images as a major
accumulation of chlorophyll biomass. In at least the northern regions, a second period
of higher chlorophyll biomass occurs in late autumn or early winter; Harrison et al.
(1999) detected bimodal seasonal cycles of primary production, modeled from surface
chlorophyll, on the southern Labrador shelf, the Newfoundland shelf, the Grand Banks,
the Scotian Shelf, and Georges Bank. The timing and magnitude of maxima at these
locations varied significantly: spring bloom is stronger than that of autumn off southern
Labrador, spring and autumn are similar off Newfoundland, and autumn is stronger than
spring at the more southerly locations.
Of course, chlorophyll enhancement does not occur uniformly across any of the
subregions listed earlier and, between years, the blooms change very significantly both as
to pattern and timing, although the perpetually high pigment signal observed in places
such as the Bay of Fundy, the inner Gulf of St. Lawrence, and Chesapeake Bay is assumed
to be at least in part the consequence of high loads of suspended sediments and of CDOM.
It will now be appropriate to review these regions individually and in greater detail.
A comparison of the pattern of SST and surface chlorophyll on the Grand Banks and
on the Scotian Shelf is very instructive. In the former region, the patches of strong
chlorophyll enhancement tend to be linear and change their location in response to the
distribution of Gulf Stream eddies and of cold Labrador Current water, so that there
is little commonality between years; on the Scotian shelf and in the Gulf of Maine, the
shallow and tidally mixed banks such as Georges Bank are routinely observed as areas
of enhanced chlorophyll where seasonality is weaker and chlorophyll remains relatively
high during winter months. Chlorophyll enhancement in the Gulf of St. Lawrence is also
Chapter 9: The Atlantic Ocean
In this province, the effect of changes in the North Atlantic Oscillation (NAO, see
Chapter 8) is as important as anywhere. The NAO Index characterizes the strength and
location of the westerly winds over the North Atlantic in winter and, consequently, the
meridional displacement of the Gulf Stream: during periods of low NAO Index, the flow
of the Labrador Current is increased and the area occupied by Labrador Slope Water
is enlarged as the North Wall of the Gulf Stream shifts southward and comes to lie
farther offshore. This is shown in Color plate 10, illustrating the differences between
early spring of 2000 when the NAO Index was high (+28), and spring of 2001 when it
was low (−189). Zwanenberg, et al. (2003) notes that annual changes in bottom water
temperature on the Scotian Shelf are among the most variable in the North Atlantic; a
50-year record shows periods of 5–10 years of sustained anomalies of both signs with very
rapid switching between cold and warm conditions. An influx of cold water into Emerald
Basin in 1998 dropped temperatures there by 3
C. To follow such events, it is very
helpful that a library of matched 2-km, 15-day AVHRR SST and SeaWiFS chlorophyll
images for the area from Cape Cod to northern Labrador is now readily accessible on-line
(http://www.mar.dfo-mpo.gc.ca/science/ocean/ias/seawifs).
Response of the Pelagic Ecosystems
In terms of primary production and (at the other end of the trophic chain) the potential
production of demersal and pelagic fish, this is one of the richest regions of the ocean;
unfortunately, it is also here that the failure of fishery science to provide adequate methods
for the management of stocks has been most acutely demonstrated.
Because of complexity of the geography of the NWCS it is difficult to generalize
the pelagic ecosystem and, without the satellite data now available, much would remain
obscure. At the regional scale, archived chlorophyll images show that the Grand Banks
and the Scotian Shelf, together with the Gulf of St. Lawrence and the Gulf of Maine,
are the principal centers of productivity, and also of seasonal change. Although there
are major differences between these regions, the seasonal productivity cycle in each
involves the occurrence of a spring bloom, which is observed in the images as a major
accumulation of chlorophyll biomass. In at least the northern regions, a second period
of higher chlorophyll biomass occurs in late autumn or early winter; Harrison et al.
(1999) detected bimodal seasonal cycles of primary production, modeled from surface
chlorophyll, on the southern Labrador shelf, the Newfoundland shelf, the Grand Banks,
the Scotian Shelf, and Georges Bank. The timing and magnitude of maxima at these
locations varied significantly: spring bloom is stronger than that of autumn off southern
Labrador, spring and autumn are similar off Newfoundland, and autumn is stronger than
spring at the more southerly locations.
Of course, chlorophyll enhancement does not occur uniformly across any of the
subregions listed earlier and, between years, the blooms change very significantly both as
to pattern and timing, although the perpetually high pigment signal observed in places
such as the Bay of Fundy, the inner Gulf of St. Lawrence, and Chesapeake Bay is assumed
to be at least in part the consequence of high loads of suspended sediments and of CDOM.
It will now be appropriate to review these regions individually and in greater detail.
A comparison of the pattern of SST and surface chlorophyll on the Grand Banks and
on the Scotian Shelf is very instructive. In the former region, the patches of strong
chlorophyll enhancement tend to be linear and change their location in response to the
distribution of Gulf Stream eddies and of cold Labrador Current water, so that there
is little commonality between years; on the Scotian shelf and in the Gulf of Maine, the
shallow and tidally mixed banks such as Georges Bank are routinely observed as areas
of enhanced chlorophyll where seasonality is weaker and chlorophyll remains relatively
high during winter months. Chlorophyll enhancement in the Gulf of St. Lawrence is also