Atlantic Coastal Biome
271
extended to the shelf break at the head of the deep water SW of the Falklands (see Color
plate 20). The development of major blooms is subsequently induced in early November
across the width of the southern shelf regions down to at least 50
S. The final progression
in midsummer, in January, takes the strong midshelf blooms down to the shelf east of
Tierra del Fuego.
Already by the end of February, regression toward the north is well underway, and
by early April no strong blooms exceeding 2–3 mg chl m
−3 remain and the effects of the
shelf-break frontal upwelling are minimal. The winter period of low chlorophyll overall
is thus relatively brief and, in any case, difficult to observe because of heavy seasonal
cloud cover.
The tidally mixed area along the inner half of the shelf also appears as a consistent highchlorophyll feature (sometimes, off the Gulf of San Jorge, with clearer water inshore), as
does the shoal water around the Falkland Islands, where tidal mixing presumably also
occurs. It has been demonstrated that SeaWiFS-derived surface chlorophyll concentrations
are too high relative to in situ observations made at sea, and once again, the CDOM
problem is invoked. I have been unable to locate any detailed studies of phytoplankton
processes on the shelf to the south of the transition zone.
Mesozooplankton variability was studied in a four-season, multistation survey of the
entire shelf south of 44
S and was found to respond closely to the seasonal and spatial
pattern of productivity discussed earlier (Sabatini and Colombo, 2001). The changes
in numerical abundance of copepods followed a very significant pattern: in the winter
months, biomass at all stations was <100 mg m
−3 ; in the spring trimester, biomass
exceeded 1000 mg m
−3 in the north-central shelf regions and in coastal embayments; in
the summer trimester, similarly high biomass was concentrated in a linear zone above the
shelf break from 45
S to 50
S, with remnant high biomass in southern neritic regions.
Finally, in autumn, very high biomass occurred in neritic regions, but unfortunately the
middle and outer shelf regions were not surveyed. For neither euphausiid nor amphipod
biomass were high concentrations observed over the shelf break.
The dominant large copepod in the shelf regions that are most liable to strong summer
stratification is Calanus australis (Sabatini et al., 2000). This is a circumpolar austral
species that is often associated with upwelling fronts that here appears to produce a
single generation annually, so that by March—toward the end of austral summer—the
population is heavily biased to C5s that are then entering diapause. Off Patagonia, it
occurs neither in coastal, tidally mixed water, nor offshore beyond midshelf where it is
replaced by C. simillimus and Neocalanus tonsus. The former contributes very heavily
to a linear zone of high copepod biomass along the shelf break in spring and summer
months.
Regional Benthic and Demersal Ecology
This is an anomalous shelf region because, although it lies between latitudes that are
equivalent to those of southern Labrador, it is nevertheless equatorward of the oceanic
Subtropical Front that passes around almost the entire margin of the shelf topography.
It is also by far the largest high-latitude shelf region in the austral oceans.
The studies of the benthic ecology of this shelf that I have been able to locate are
not very coherent, so that it is difficult to present a unified account of the benthic and
demersal ecosystem. What must be said at once is that its isolation, far from global
centers of human population, has not protected it from the pressure of industrial fishing,
so that what is observed today must differ very significantly from the pristine state.
Catches increased to a maximum removal rate of about 20 × 10
6 t y
−1 during the latter
half of the 20th century, after which a progressive decline set in; stratified trawl surveys
suggested that longtail hake (Macronurus magellanicus) at one time represented ∼60%
271
extended to the shelf break at the head of the deep water SW of the Falklands (see Color
plate 20). The development of major blooms is subsequently induced in early November
across the width of the southern shelf regions down to at least 50
S. The final progression
in midsummer, in January, takes the strong midshelf blooms down to the shelf east of
Tierra del Fuego.
Already by the end of February, regression toward the north is well underway, and
by early April no strong blooms exceeding 2–3 mg chl m
−3 remain and the effects of the
shelf-break frontal upwelling are minimal. The winter period of low chlorophyll overall
is thus relatively brief and, in any case, difficult to observe because of heavy seasonal
cloud cover.
The tidally mixed area along the inner half of the shelf also appears as a consistent highchlorophyll feature (sometimes, off the Gulf of San Jorge, with clearer water inshore), as
does the shoal water around the Falkland Islands, where tidal mixing presumably also
occurs. It has been demonstrated that SeaWiFS-derived surface chlorophyll concentrations
are too high relative to in situ observations made at sea, and once again, the CDOM
problem is invoked. I have been unable to locate any detailed studies of phytoplankton
processes on the shelf to the south of the transition zone.
Mesozooplankton variability was studied in a four-season, multistation survey of the
entire shelf south of 44
S and was found to respond closely to the seasonal and spatial
pattern of productivity discussed earlier (Sabatini and Colombo, 2001). The changes
in numerical abundance of copepods followed a very significant pattern: in the winter
months, biomass at all stations was <100 mg m
−3 ; in the spring trimester, biomass
exceeded 1000 mg m
−3 in the north-central shelf regions and in coastal embayments; in
the summer trimester, similarly high biomass was concentrated in a linear zone above the
shelf break from 45
S to 50
S, with remnant high biomass in southern neritic regions.
Finally, in autumn, very high biomass occurred in neritic regions, but unfortunately the
middle and outer shelf regions were not surveyed. For neither euphausiid nor amphipod
biomass were high concentrations observed over the shelf break.
The dominant large copepod in the shelf regions that are most liable to strong summer
stratification is Calanus australis (Sabatini et al., 2000). This is a circumpolar austral
species that is often associated with upwelling fronts that here appears to produce a
single generation annually, so that by March—toward the end of austral summer—the
population is heavily biased to C5s that are then entering diapause. Off Patagonia, it
occurs neither in coastal, tidally mixed water, nor offshore beyond midshelf where it is
replaced by C. simillimus and Neocalanus tonsus. The former contributes very heavily
to a linear zone of high copepod biomass along the shelf break in spring and summer
months.
Regional Benthic and Demersal Ecology
This is an anomalous shelf region because, although it lies between latitudes that are
equivalent to those of southern Labrador, it is nevertheless equatorward of the oceanic
Subtropical Front that passes around almost the entire margin of the shelf topography.
It is also by far the largest high-latitude shelf region in the austral oceans.
The studies of the benthic ecology of this shelf that I have been able to locate are
not very coherent, so that it is difficult to present a unified account of the benthic and
demersal ecosystem. What must be said at once is that its isolation, far from global
centers of human population, has not protected it from the pressure of industrial fishing,
so that what is observed today must differ very significantly from the pristine state.
Catches increased to a maximum removal rate of about 20 × 10
6 t y
−1 during the latter
half of the 20th century, after which a progressive decline set in; stratified trawl surveys
suggested that longtail hake (Macronurus magellanicus) at one time represented ∼60%