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etween shelf and slope adjoined an area of tremendous change
downslope, while the rise was completely different.
On the abyssal plain of the western North Atlantic, at or below 4000
metres, very low figures had been found for standing stock, about 1 gram or
100 individuals per m 2 . A global analysis of metazoan fauna also showed a
decline in density with depth, to a particularly low level on the abyssal plain.
Such low-density levels made assessment difficult on the abyssal plain,
because the ability to replicate was low and it was hard to detect
differences from one place to the next, particularly when coupled with
reasonably high diversity.
Gordon Paterson, in a worldwide analysis of the numerically most
important group – polychaetes, or segmented worms – had also shown that
abundance declined exponentially with depth. The only places with
reasonably high density were locales where nutrient input was augmented
around seamounts or where reactive sediments were exposed and there
was more food.
Some of the most interesting figures in the deep-sea literature had
been published by Fred Grassle and Nancy Maciolek 37 , based on almost
170 box cores from an Atlantic continental slope and rise study, taken
along 2100 m isobaths. They showed a huge range of diversity, along with
relatively high abundance. The median diversity was high -- about 100
species for each of the nine central 10x10-centimetre sub-cores, or about 1
square foot. Data from the Echo-1 study in the Pacific Ocean showed much
lower density. The data were not perfectly comparable because the first set
covered all macrofauna while Echo-1 was limited to the principal groups –
isopods, bivalves and polychaetes -- and its samples were much larger, a
full 0.25m². Though both density and diversity were lower at Echo-1, the
finding of 40 species per 0.25m² was still notable.
Pointing to the difficulties in making projections about the number
of species in the deep sea, Rex said that Grassle had plotted the apparent
accumulation of species over a distance of about 176 km along the 2100
m isobath and had used that rate of accumulation to make an estimate for
the deep sea. The problem, however, laid in the number of singletons –
that is, the number of species and samples represented by a single, rare
individual. Data from Georges Bank, on the continental shelf, and Grassle’s
figures from the Atlantic continental slope and rise just south of there,
showed no significant difference in the proportion of singletons, which
seemed to be about 1:3 everywhere. In the deep sea, however, a huge
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