192
Chapter 9: The Atlantic Ocean
Several recent investigations have demonstrated the effects of cyclonic and anticyclonic
eddy features on pelagic ecology in this province. The distribution of zooplankton and
micronekton biomass reflect nutrient enhancement, so that relatively strong algal growth
occurs in cyclonic eddies and in the confluences of flow within eddy pairs. This phenomenon translates to the highest trophic levels, so that whales and other cetaceans are
not only concentrated along the shelf break (where we would otherwise expect them to
be most abundant) but also associated with cyclones and flow confluence between eddy
pairs. Such a preferential distribution has also been related to an observed relationship
between the distribution of zooplankton biomass and of cephalopod paralarval numbers.
Cephalopods are, of course, a mainstay of the diet of the sperm whales, pygmy sperm
whales, pilot whales, and other cetaceans.
Regional Benthic and Demersal Ecology
Although coral reefs will not directly concern us, it must be noted in passing that the
coasts of the Bahamas and of the Antilles islands are largely characterized by this benthic
habitat. Throughout the Gulf and Caribbean, wherever the organic content of deposits is
low, patches and banks of deeper-water corals occur down almost to the shelf break, as
well as in the more familiar shallow-water reefs. This is quite different from the situation
in the eastern tropical Atlantic, as we shall see. The Atlantic reef fauna, which has its most
generous expression here in CARB, is still relatively depauperate: there are only 35 coral
species in 26 genera in the Atlantic, whereas in the Indo-Pacific there are 700 species in 80
genera. There is significant endemism at the generic level in the Atlantic and significant
regional differences between the reef fauna of the Caribbean and of isolated reefs along
the coast of Brazil. In passing, however, it should be noted that the Caribbean reef systems
are under severe pressure from exotic marine organisms, including pathogens, perhaps
introduced in ballast water of ships passing the Panama Canal. The now-classical case
is that of the die-off of the previously abundant sea-urchin Diadema spp., associated
with the identification of a lethal pathogenic bacterium (Clostridium). These sea urchins
were the dominant algal grazers on reef systems and, in their absence, macroalgae have
increased enormously in coverage and size, smothering corals to depths of 10–15 m
(Richards and Bohnsack, 1990). It is perhaps not accidental that this unusual mortality
of urchins was first noticed in the vicinity of the canal itself.
Walsh (1988) reviewed the role of the nutrient-laden effluent of the Mississippi River
in the productivity of the northern Gulf of Mexico. The Mississippi water has a nitrate
content of <150 g-at NO 3 liter
−1 during the spring floods. He concluded that only
about 21% of the 250–350 gC m
−2 y
−1 phytoplankton biomass produced over the TexasLouisiana continental shelf is taken up into the pelagic food chain. Of the remaining
79% of phytoplankton biomass that sinks into the benthic environment, more than half
goes directly to burial or to export into deep water by slumping at the shelf edge, and
only about 80 gC m
−2 y
−1 enters the benthic food chain. We shall find that a similar
conclusion was reached, on a much smaller scale, for the fate of the phytoplankton
biomass produced in the discharge of a West African river. Despite this, there is a very
direct relationship between river discharge and biological productivity on the adjacent
continental shelf, and it is in the Gulf of Mexico and Caribbean that this relationship
has been as well studied as anywhere else. Not only does a spatial correlation exist, but
there is also a positive correlation between annual discharge rate and the productivity of
the associated neritic fish stocks whose biomass is dominated by species of rapid growth
(Deegan et al., 1986).
Two features appear to be important in forcing the ecology of benthic communities
and those of demersal fish: the depth of the thermocline that underlies the tropical surface
water mass, which here lies near the break of slope, and also the consequence of river
Chapter 9: The Atlantic Ocean
Several recent investigations have demonstrated the effects of cyclonic and anticyclonic
eddy features on pelagic ecology in this province. The distribution of zooplankton and
micronekton biomass reflect nutrient enhancement, so that relatively strong algal growth
occurs in cyclonic eddies and in the confluences of flow within eddy pairs. This phenomenon translates to the highest trophic levels, so that whales and other cetaceans are
not only concentrated along the shelf break (where we would otherwise expect them to
be most abundant) but also associated with cyclones and flow confluence between eddy
pairs. Such a preferential distribution has also been related to an observed relationship
between the distribution of zooplankton biomass and of cephalopod paralarval numbers.
Cephalopods are, of course, a mainstay of the diet of the sperm whales, pygmy sperm
whales, pilot whales, and other cetaceans.
Regional Benthic and Demersal Ecology
Although coral reefs will not directly concern us, it must be noted in passing that the
coasts of the Bahamas and of the Antilles islands are largely characterized by this benthic
habitat. Throughout the Gulf and Caribbean, wherever the organic content of deposits is
low, patches and banks of deeper-water corals occur down almost to the shelf break, as
well as in the more familiar shallow-water reefs. This is quite different from the situation
in the eastern tropical Atlantic, as we shall see. The Atlantic reef fauna, which has its most
generous expression here in CARB, is still relatively depauperate: there are only 35 coral
species in 26 genera in the Atlantic, whereas in the Indo-Pacific there are 700 species in 80
genera. There is significant endemism at the generic level in the Atlantic and significant
regional differences between the reef fauna of the Caribbean and of isolated reefs along
the coast of Brazil. In passing, however, it should be noted that the Caribbean reef systems
are under severe pressure from exotic marine organisms, including pathogens, perhaps
introduced in ballast water of ships passing the Panama Canal. The now-classical case
is that of the die-off of the previously abundant sea-urchin Diadema spp., associated
with the identification of a lethal pathogenic bacterium (Clostridium). These sea urchins
were the dominant algal grazers on reef systems and, in their absence, macroalgae have
increased enormously in coverage and size, smothering corals to depths of 10–15 m
(Richards and Bohnsack, 1990). It is perhaps not accidental that this unusual mortality
of urchins was first noticed in the vicinity of the canal itself.
Walsh (1988) reviewed the role of the nutrient-laden effluent of the Mississippi River
in the productivity of the northern Gulf of Mexico. The Mississippi water has a nitrate
content of <150 g-at NO 3 liter
−1 during the spring floods. He concluded that only
about 21% of the 250–350 gC m
−2 y
−1 phytoplankton biomass produced over the TexasLouisiana continental shelf is taken up into the pelagic food chain. Of the remaining
79% of phytoplankton biomass that sinks into the benthic environment, more than half
goes directly to burial or to export into deep water by slumping at the shelf edge, and
only about 80 gC m
−2 y
−1 enters the benthic food chain. We shall find that a similar
conclusion was reached, on a much smaller scale, for the fate of the phytoplankton
biomass produced in the discharge of a West African river. Despite this, there is a very
direct relationship between river discharge and biological productivity on the adjacent
continental shelf, and it is in the Gulf of Mexico and Caribbean that this relationship
has been as well studied as anywhere else. Not only does a spatial correlation exist, but
there is also a positive correlation between annual discharge rate and the productivity of
the associated neritic fish stocks whose biomass is dominated by species of rapid growth
(Deegan et al., 1986).
Two features appear to be important in forcing the ecology of benthic communities
and those of demersal fish: the depth of the thermocline that underlies the tropical surface
water mass, which here lies near the break of slope, and also the consequence of river