continental shelf and slope topography with interacting circulation patterns of South
Atlantic Bight, Mid-Atlantic Bight, Slope bottom [currents], and Gulf Stream waters
near Cape Hatteras, NC, clearly promotes high primary productivity and makes the
Hatteras region a likely site for extensive modern carbon export to the neighboring
slope.” There are other special situations such as off the Peruvian coast: an anoxic
zone quite near the shore has almost no macrofaunal biomass, but there are biomass
peaks farther offshore where some oxygen is available and food is supplied from
nutrient-rich upwelled waters. Rex and Etter’s standardization to a fictional fixed
latitude removes an important regional distinction. High-latitude seas (save for the
Arctic Ocean) tend to have larger standing stocks of benthos than tropical ones. This
is at least partly because the large seasonality of primary production (blooms) does
not allow it to be so efficiently captured by the pelagic communities as in the more
consistent tropics.
(^) Only the decrease in macrofauna is illustrated here, but there are also decreases in
all the other categories, with greater rates per meter of added depth for megafauna and
progressively smaller rates for meiofauna and bacteria.
Deep-Sea Species Diversity
(^) It has provided a source of endless fascination, not to mention the pleasures of
taxonomy, for benthic biologists that species diversity, the number of kinds of animal
forms, does not decrease dramatically down to considerable depth; the fauna of the
upper abyssal floor is about as diverse as that of shallow water, in most comparisons
more so. Species diversity is considered by ecologists to have two aspects. An
assemblage of organisms, say the macrofauna sieved from an anchor-dredge load of
mud, is more diverse if it has a larger total number of species, S. It is also said to be
more diverse if the individuals are more equally distributed among the species. Thus,
it is more diverse if, as individuals are randomly selected and added to an identified
list, the number of species rises rapidly toward S. This aspect of diversity is termed
equitability. The significance of this is that an animal, say a predator, moving through
a low-equitability community, will mostly encounter the same few species over and
over. Perhaps a relatively small repertoire of behaviors for interactions will suffice for
it. It could afford to specialize on one or very few prey types. A predator moving
through a high-equitability community, in contrast, would seldom encounter
immediately other animals of the same kind as the one most recently met. It might
need a larger repertoire of behaviors. The two aspects of diversity are somewhat tied
together, since at least some individuals of every species must be present if S is to be
large, creating some equitability, but they are not tightly coupled.
(^) A measure of diversity emphasizing equitability is Simpson’s diversity index, L,
which is the probability (estimated from a sample of N individuals) that two