Atlantic Trade Wind Biome
199
positive Ekman suction remains a prominent feature from northern Brazil to Senegal and
is the most prominent feature of Ekman vertical velocity in the North Atlantic Ocean at
any latitude (Isemer and Hasse, 1987).
Although subthermocline, high-nitrate water does not surface in the Guinea Dome,
as it does in the Costa Rica Dome (Voituriez and Herbland, 1982), this feature is observable in the chlorophyll imagery, although not consistently. At the eastern termination of
the NECC, large irregular patches of very high values that appear not to be plumes of
coastally upwelled water may occur to the southwest of Mauritania at 20–25
W, and this
appears to represent the region in which the Guinea Dome is most generally expressed
at the surface. Such features are most prominent during the boreal winter for a period of
4–7 months, and there is significant between-year variability in both duration and
strength.
The ecological profile observed in WTRA is usually characteristic of an oligotrophic
situation, resembling the “typical tropical situation” in the sense of Herbland (1983);
only within the equatorial divergence is there significant divergence from this pattern.
Depending on the depth of the mixed layer—from almost 100 m in the west to 20–30 m in
the Guinea Dome—the chlorophyll and productivity maxima and the subjacent nitracline
lie either at the top of the pycnocline or deep within it. As Voituriez and Herbland (1982)
note, the Guinea Dome exhibits a remarkable stratification of physical, chemical, and
biological parameters. Here, the situation is sufficiently stable that the nitrate-deficient
surface layer is significantly deeper than the wind-mixed surface layer. The nutricline,
then, is located at some depth within the pycnocline and is associated with very sharp
maxima of algal growth rate, of chlorophyll biomass, and of numbers of herbivorous
zooplankton. It is at this depth that optimal conditions of irradiance, nutrient supply,
and rate of vertical mixing occur. The general relationships between depths of maximum
grazing rate, of maximal algal growth rate, and of maximal algal biomass are complex
but the tropical ocean offers the best site for their investigation. It is found that depths
of highest plant biomass, of chlorophyll, and of growth rate become progressively more
separated in the vertical the deeper they lie in the tropical ocean. Thus, maximal growth
rate becomes progressively shallower than the chlorophyll maximum the deeper they lie.
Depths of maximal mesozooplankton are usually close to the depth of maximal algal
growth rate, implying a trophic link involving the regeneration of plant nutrients by
excretion of zooplankton (Longhurst and Harrison, 1989).
We have little information on the vertical distribution of individual biota here, but
there is no reason to believe that the eastern tropical Pacific profiles (see Chapter 11,
North Pacific Equatorial Countercurrent and North Pacific Tropical Gyre Provinces)
will not serve as relevant models for the tropical Atlantic. The progressive shoaling of horizons brings oxygen-deficient water (< 25 ml liter
−1 ) to within 75 m of the
surface in the Guinea Dome, and this has important effects on the distribution of
mesopelagic fish species. Myctophids dominate the biomass of this component and are
represented by species having northern, subtropical, and tropical distributions, whereas
other families of mesopelagic fish (Gonostomatidae, Sternoptychidae, and Scopeloarchidae), and some of the deeper-living myctophids are curiously absent. It has been
suggested that these anomalies may be accounted for by the unusual vertical distribution of temperature, and of other critical properties, in the doming region and
also by larval retention within the semiclosed circulation south of the Cape Verde
Frontal Zone.
For the horizontal distribution of biota, apart from bulk chlorophyll data, we have
recourse to rather few data: first, from the EQUALANT surveys of the 1960s, performed
in support of the U.S. tuna-fishing industry, and, second, from the accumulated Soviet
survey data (Piontkowski et al., 1997). Both these data sets emphasize the existence of
enhanced zooplankton biomass associated with the NECC and Guinea Dome, while the
199
positive Ekman suction remains a prominent feature from northern Brazil to Senegal and
is the most prominent feature of Ekman vertical velocity in the North Atlantic Ocean at
any latitude (Isemer and Hasse, 1987).
Although subthermocline, high-nitrate water does not surface in the Guinea Dome,
as it does in the Costa Rica Dome (Voituriez and Herbland, 1982), this feature is observable in the chlorophyll imagery, although not consistently. At the eastern termination of
the NECC, large irregular patches of very high values that appear not to be plumes of
coastally upwelled water may occur to the southwest of Mauritania at 20–25
W, and this
appears to represent the region in which the Guinea Dome is most generally expressed
at the surface. Such features are most prominent during the boreal winter for a period of
4–7 months, and there is significant between-year variability in both duration and
strength.
The ecological profile observed in WTRA is usually characteristic of an oligotrophic
situation, resembling the “typical tropical situation” in the sense of Herbland (1983);
only within the equatorial divergence is there significant divergence from this pattern.
Depending on the depth of the mixed layer—from almost 100 m in the west to 20–30 m in
the Guinea Dome—the chlorophyll and productivity maxima and the subjacent nitracline
lie either at the top of the pycnocline or deep within it. As Voituriez and Herbland (1982)
note, the Guinea Dome exhibits a remarkable stratification of physical, chemical, and
biological parameters. Here, the situation is sufficiently stable that the nitrate-deficient
surface layer is significantly deeper than the wind-mixed surface layer. The nutricline,
then, is located at some depth within the pycnocline and is associated with very sharp
maxima of algal growth rate, of chlorophyll biomass, and of numbers of herbivorous
zooplankton. It is at this depth that optimal conditions of irradiance, nutrient supply,
and rate of vertical mixing occur. The general relationships between depths of maximum
grazing rate, of maximal algal growth rate, and of maximal algal biomass are complex
but the tropical ocean offers the best site for their investigation. It is found that depths
of highest plant biomass, of chlorophyll, and of growth rate become progressively more
separated in the vertical the deeper they lie in the tropical ocean. Thus, maximal growth
rate becomes progressively shallower than the chlorophyll maximum the deeper they lie.
Depths of maximal mesozooplankton are usually close to the depth of maximal algal
growth rate, implying a trophic link involving the regeneration of plant nutrients by
excretion of zooplankton (Longhurst and Harrison, 1989).
We have little information on the vertical distribution of individual biota here, but
there is no reason to believe that the eastern tropical Pacific profiles (see Chapter 11,
North Pacific Equatorial Countercurrent and North Pacific Tropical Gyre Provinces)
will not serve as relevant models for the tropical Atlantic. The progressive shoaling of horizons brings oxygen-deficient water (< 25 ml liter
−1 ) to within 75 m of the
surface in the Guinea Dome, and this has important effects on the distribution of
mesopelagic fish species. Myctophids dominate the biomass of this component and are
represented by species having northern, subtropical, and tropical distributions, whereas
other families of mesopelagic fish (Gonostomatidae, Sternoptychidae, and Scopeloarchidae), and some of the deeper-living myctophids are curiously absent. It has been
suggested that these anomalies may be accounted for by the unusual vertical distribution of temperature, and of other critical properties, in the doming region and
also by larval retention within the semiclosed circulation south of the Cape Verde
Frontal Zone.
For the horizontal distribution of biota, apart from bulk chlorophyll data, we have
recourse to rather few data: first, from the EQUALANT surveys of the 1960s, performed
in support of the U.S. tuna-fishing industry, and, second, from the accumulated Soviet
survey data (Piontkowski et al., 1997). Both these data sets emphasize the existence of
enhanced zooplankton biomass associated with the NECC and Guinea Dome, while the