Atlantic Trade Wind Biome
187
the feature is fully formed; at that time, retroflection off Demerara (at about 8
N) may
induce a major chlorophyll enhancement in the extreme southwestern corner of NTRA,
and it is such episodes that are probably responsible for the anomalous peaks in regional
chlorophyll biomass obtained within the “statutory” boundaries of this province.
It was in this region that the “Typical Tropical Structure” (TTS) for profiles of
phytoplankton and associated measures was explored and described by Herbland and
Voituriez (1979); the key characteristic of the TTS is, of course, the manner in which the
depth of the deep chlorophyll maximum is determined by pycnocline, nutricline, and
irradiance, and how the profiles of heterotrophic bacteria and of herbivores match that
of autotrophs in a manner suggestive of consumption and recycling in situ of organic
material. These relationships have been best worked out in the eastern Pacific and are
discussed later.
Apparently, we know almost nothing of the dynamics of herbivore consumption
and its seasonality in this province and can only speculate about the mechanism that
leads to the weak accumulation of chlorophyll in winter when the primary production
rate appears to be at a minimum. The NATR is the southern part of a region of very
sparse zooplankton that occupies the central part of the North Atlantic gyre; integrated
biomass from 0 to 100 m, indicated by profiles obtained with bioluminescence probes, is
significantly lower (always <100 mg m
−3 ) than in the WTRA to the south where a zone
of high values stretches across the ocean along the NEC/NECC ridge (Piontkowski et al.,
1997). Incidentally, these 2000-odd Soviet observations over the North Atlantic confirm
the basin-scale model of North Atlantic plankton dynamics offered by Wroblewski,
Sarmiento, and Flierl (1988); this model solution, based on mixing rates, nutrient profiles,
and mixed-layer depths, indicates a zone of very small phytoplankton and zooplankton
biomass in NAST and NATR, clearly differentiated from higher values lying across the
ocean to the south of NATR.
The French “Eumeli” oligotrophic station showed clearly that macrozooplankton and
nekton numbers in NATR were 5–10 times lower (263 ind m
−2 , 0–965 m) than at the
mesotrophic and eutrophic stations in the Canary Current (Andersen et al., 1997).
The size-selected assemblage used by these authors comprised siphonophores (Chelophyes), pteropods (Clio), chaetognaths, euphausiids (Stylocheiron, Euphausia), and fish
(Cyclothone). In NATR, compared with some other trade-wind provinces from which
we have zooplankton profiles, the pycnocline is relatively weak and, I think in consequence, the vertical distribution of individual organisms is rather dispersed. Daytime
residence depths of diel migrants were found to be more various than the depths entered
at night, which are consistently within the upper 50–75 m. As usual, the bathypelagic
fish Cyclothone remains at depth day and night and, also as usual, their species partition
space among themselves: C. braueri lies 200 m shoaler than C. acclidens. Euphausiids are
active diel migrants, with the exception of the genus Stylocheiron, which resides day and
night at species-specific depth ranges within the upper 200 m.
This province, together with the Gulf of Mexico and the Gulf Stream, is within the
ambit of the summer migrations of tropical yellowfin (Thunnus albacares) and skipjack
(Katsuwonus pelamis) tuna. That is to say, these large predators were abundant here in
the summer months until at least the 1950s and 1960s, but are now reduced to relatively
very small populations.
Synopsis
Case 4—Small-amplitude response to trade wind seasonality—Z m based on the density
criterion shows the effects of winter mixing in the northern part of province (regionally,
25 m June–October, 50 m in January–February). Z eu is consistently at between 50 and
60 m, so thermocline is permanently illuminated. Productivity has very weak seasonal
187
the feature is fully formed; at that time, retroflection off Demerara (at about 8
N) may
induce a major chlorophyll enhancement in the extreme southwestern corner of NTRA,
and it is such episodes that are probably responsible for the anomalous peaks in regional
chlorophyll biomass obtained within the “statutory” boundaries of this province.
It was in this region that the “Typical Tropical Structure” (TTS) for profiles of
phytoplankton and associated measures was explored and described by Herbland and
Voituriez (1979); the key characteristic of the TTS is, of course, the manner in which the
depth of the deep chlorophyll maximum is determined by pycnocline, nutricline, and
irradiance, and how the profiles of heterotrophic bacteria and of herbivores match that
of autotrophs in a manner suggestive of consumption and recycling in situ of organic
material. These relationships have been best worked out in the eastern Pacific and are
discussed later.
Apparently, we know almost nothing of the dynamics of herbivore consumption
and its seasonality in this province and can only speculate about the mechanism that
leads to the weak accumulation of chlorophyll in winter when the primary production
rate appears to be at a minimum. The NATR is the southern part of a region of very
sparse zooplankton that occupies the central part of the North Atlantic gyre; integrated
biomass from 0 to 100 m, indicated by profiles obtained with bioluminescence probes, is
significantly lower (always <100 mg m
−3 ) than in the WTRA to the south where a zone
of high values stretches across the ocean along the NEC/NECC ridge (Piontkowski et al.,
1997). Incidentally, these 2000-odd Soviet observations over the North Atlantic confirm
the basin-scale model of North Atlantic plankton dynamics offered by Wroblewski,
Sarmiento, and Flierl (1988); this model solution, based on mixing rates, nutrient profiles,
and mixed-layer depths, indicates a zone of very small phytoplankton and zooplankton
biomass in NAST and NATR, clearly differentiated from higher values lying across the
ocean to the south of NATR.
The French “Eumeli” oligotrophic station showed clearly that macrozooplankton and
nekton numbers in NATR were 5–10 times lower (263 ind m
−2 , 0–965 m) than at the
mesotrophic and eutrophic stations in the Canary Current (Andersen et al., 1997).
The size-selected assemblage used by these authors comprised siphonophores (Chelophyes), pteropods (Clio), chaetognaths, euphausiids (Stylocheiron, Euphausia), and fish
(Cyclothone). In NATR, compared with some other trade-wind provinces from which
we have zooplankton profiles, the pycnocline is relatively weak and, I think in consequence, the vertical distribution of individual organisms is rather dispersed. Daytime
residence depths of diel migrants were found to be more various than the depths entered
at night, which are consistently within the upper 50–75 m. As usual, the bathypelagic
fish Cyclothone remains at depth day and night and, also as usual, their species partition
space among themselves: C. braueri lies 200 m shoaler than C. acclidens. Euphausiids are
active diel migrants, with the exception of the genus Stylocheiron, which resides day and
night at species-specific depth ranges within the upper 200 m.
This province, together with the Gulf of Mexico and the Gulf Stream, is within the
ambit of the summer migrations of tropical yellowfin (Thunnus albacares) and skipjack
(Katsuwonus pelamis) tuna. That is to say, these large predators were abundant here in
the summer months until at least the 1950s and 1960s, but are now reduced to relatively
very small populations.
Synopsis
Case 4—Small-amplitude response to trade wind seasonality—Z m based on the density
criterion shows the effects of winter mixing in the northern part of province (regionally,
25 m June–October, 50 m in January–February). Z eu is consistently at between 50 and
60 m, so thermocline is permanently illuminated. Productivity has very weak seasonal