Atlantic Trade Wind Biome
191
Response of the Pelagic Ecosystems
The chlorophyll images show several regions of permanently high pigment concentration,
probably due to benthic processes, that must be distinguished from features in the
chlorophyll field due to phytoplankton biomass. The largest of these permanent anomalies
represents the underwater platform of the Bahamas Bank, overlain with extremely clear
oceanic water, which appears as a high-pigment anomaly that exactly matches its outline.
This is surely due to symbiotic and benthic algal chlorophyll showing through a few
meters of the clearest ocean water. Similar patches of apparently benthic chlorophyll are
associated with the Florida Keys and several regions along the southwest coast of Cuba,
especially around the Isle of Pines.
The regional, seasonal cycle of productivity indicated by satellite data is of small
magnitude, but is not simple: there appear to be chlorophyll maxima in both winter
and summer: in each, the indicated biomass is between 0.4 and 06 mg chl m
−3 . Relative
productivity, integrated by various models, follows a simpler pattern following the cycle of
irradiance, ranging from 300 to 500 mgC m
−2 mo
−1 . The serial images enable a distinction
to be made between the two seasons of maximal surface chlorophyll biomass: in summer,
there is a very clear difference between oligotrophic regions having very clear surface
water and regions of chlorophyll enhancement, whereas in winter this distinction is more
obscure, because the entire CARB province then experiences uniform, but rather slight,
chlorophyll enhancement.
A constant feature of the chlorophyll field in the Gulf of Mexico reflects the offshore
entrainment of shelf water and the upwelling of nutrients induced by the anticyclonicity
of the Loop Current in the eastern Gulf (Paluskiewicz et al., 1983). Anticyclonic eddies
significantly modify the nutrient budget as they propagate into the western Gulf because
they have, of course, a bowl-shaped pycnocline topography so that nitrate isopleths
are brought up into the lighted zone around their perimeters (Walsh et al., 1989).
Nitracline doming in cyclonic eddies shed from the Loop Current into the eastern
Gulf also produces features in the chlorophyll field. Thus, both families of rings exhibit
appropriate chlorophyll enrichment, observable in satellite images (Yentsch, 1982; Salas
de Leon and Monreal-Gomez, 1986; Trees and El-Sayed, 1986).
Hobson and Lorenzen (1972) explored the deep oligotrophic chlorophyll profiles that
are characteristic of the Gulf of Mexico. They noted the significant nonuniformity of
the depth of the pycnocline, even at the same season, due to complex flow structure at
the mesoscale. This was one of the earliest observations of how maxima of chlorophyll,
phytoplankton carbon, and microzooplankton carbon coincide within the pycnocline
and above a nitracline. As has subsequently been found to be the general case, when the
pycnocline was deeper than about 100 m, the DCM was extremely weak.
Associated with the oligotrophic profiles, the vertical distribution of mesozooplankton
follows the usual pattern. The >150-m fraction is dominated by copepods (90% numerically and >50% of biomass) that are concentrated in the mixed layer; these are a typical
suite of warm-water genera (Clausocalanus, Euchaeta, Scolecithrix, and Nannocalanus).
Diel migrants (especially Pleuromamma and Sergestes) shuttle between daytime depths
of 300–400 m and the photic zone at night (Hopkins, 1982). Though this study is not
explicit in this regard, we may assume that the usual relationship exists between residence
depths of mesoplankton and the features of the phytoplankton profile.
At upwelling sites along the coast of northern Yucatan, the mesozooplankton respond
in a predictable manner to upwelling episodes, as they do at larger scale in eastern boundary current upwelling zones (Suárez-Morales, 1995). Bray-Curtis diversity is relatively low
and biomass relatively high in regions of strong upwelling and high chlorophyll, where
the dominant copepod (Temora stylifera) may reach abundances of >20000 ind m
−3 .
Beyond each upwelling cell, diversity increases and biomass decreases, and both neritic
and oceanic species mingle in this dynamic coastal region.
191
Response of the Pelagic Ecosystems
The chlorophyll images show several regions of permanently high pigment concentration,
probably due to benthic processes, that must be distinguished from features in the
chlorophyll field due to phytoplankton biomass. The largest of these permanent anomalies
represents the underwater platform of the Bahamas Bank, overlain with extremely clear
oceanic water, which appears as a high-pigment anomaly that exactly matches its outline.
This is surely due to symbiotic and benthic algal chlorophyll showing through a few
meters of the clearest ocean water. Similar patches of apparently benthic chlorophyll are
associated with the Florida Keys and several regions along the southwest coast of Cuba,
especially around the Isle of Pines.
The regional, seasonal cycle of productivity indicated by satellite data is of small
magnitude, but is not simple: there appear to be chlorophyll maxima in both winter
and summer: in each, the indicated biomass is between 0.4 and 06 mg chl m
−3 . Relative
productivity, integrated by various models, follows a simpler pattern following the cycle of
irradiance, ranging from 300 to 500 mgC m
−2 mo
−1 . The serial images enable a distinction
to be made between the two seasons of maximal surface chlorophyll biomass: in summer,
there is a very clear difference between oligotrophic regions having very clear surface
water and regions of chlorophyll enhancement, whereas in winter this distinction is more
obscure, because the entire CARB province then experiences uniform, but rather slight,
chlorophyll enhancement.
A constant feature of the chlorophyll field in the Gulf of Mexico reflects the offshore
entrainment of shelf water and the upwelling of nutrients induced by the anticyclonicity
of the Loop Current in the eastern Gulf (Paluskiewicz et al., 1983). Anticyclonic eddies
significantly modify the nutrient budget as they propagate into the western Gulf because
they have, of course, a bowl-shaped pycnocline topography so that nitrate isopleths
are brought up into the lighted zone around their perimeters (Walsh et al., 1989).
Nitracline doming in cyclonic eddies shed from the Loop Current into the eastern
Gulf also produces features in the chlorophyll field. Thus, both families of rings exhibit
appropriate chlorophyll enrichment, observable in satellite images (Yentsch, 1982; Salas
de Leon and Monreal-Gomez, 1986; Trees and El-Sayed, 1986).
Hobson and Lorenzen (1972) explored the deep oligotrophic chlorophyll profiles that
are characteristic of the Gulf of Mexico. They noted the significant nonuniformity of
the depth of the pycnocline, even at the same season, due to complex flow structure at
the mesoscale. This was one of the earliest observations of how maxima of chlorophyll,
phytoplankton carbon, and microzooplankton carbon coincide within the pycnocline
and above a nitracline. As has subsequently been found to be the general case, when the
pycnocline was deeper than about 100 m, the DCM was extremely weak.
Associated with the oligotrophic profiles, the vertical distribution of mesozooplankton
follows the usual pattern. The >150-m fraction is dominated by copepods (90% numerically and >50% of biomass) that are concentrated in the mixed layer; these are a typical
suite of warm-water genera (Clausocalanus, Euchaeta, Scolecithrix, and Nannocalanus).
Diel migrants (especially Pleuromamma and Sergestes) shuttle between daytime depths
of 300–400 m and the photic zone at night (Hopkins, 1982). Though this study is not
explicit in this regard, we may assume that the usual relationship exists between residence
depths of mesoplankton and the features of the phytoplankton profile.
At upwelling sites along the coast of northern Yucatan, the mesozooplankton respond
in a predictable manner to upwelling episodes, as they do at larger scale in eastern boundary current upwelling zones (Suárez-Morales, 1995). Bray-Curtis diversity is relatively low
and biomass relatively high in regions of strong upwelling and high chlorophyll, where
the dominant copepod (Temora stylifera) may reach abundances of >20000 ind m
−3 .
Beyond each upwelling cell, diversity increases and biomass decreases, and both neritic
and oceanic species mingle in this dynamic coastal region.