Trend lines drawn freehand by us.
(^) (After Rowe 1983.)
Both greater numbers and greater biomass are found nearshore and at shoal depths,
decreasing seaward and down. That is because landmasses provide nutrient-rich run-
off and/or promote upwelling, both of which enhance surface productivity, and
because less of that production is eaten and metabolized before sinking organic matter
reaches the shallow bottom. Because trenches with hadal depths are almost entirely
found close to island arcs and continents, the trend levels off, even reverses at about
5000 m, just above the abyssal–hadal boundary (Fig. 13.13). Waterlogged organic
matter (coconuts appear in trench-floor photographs) supplements the food supply
there. There are also recurring sediment slumps and resulting turbidity currents that
carry shallow sediment with high carbon content down into trenches.
(^) Production consistently falls off seaward, and the opportunities for consumption on
the way to the seafloor are progressively greater. The fitted regressions (Fig. 13.12)
show a decrease of numbers by 50-fold from 0 to 5000 m, while biomass decreases by
∼224-fold. In alternate mathematical form, the reductions are exponential at ca. −0.8
(1000 m)−1 for numbers and ca. −1.1 (1000 m)−1 for biomass, respectively. The
difference is necessarily accounted for by a decrease in individual size. As we
discussed particularly for polychaetes, deep-sea benthonts are smaller than related
species on the continental slopes and shelves. If the dispersion around the general
trends is examined in detail, particular areas turn out to have higher or lower
abundance and biomass as a result of regional variations in conditions, most
importantly the supply of organic matter but also sediment type, currents, and
temperature. For example, the stocks of every faunal size group offshore from Cape
Hatteras, North Carolina, are substantially higher than the typical mid-slope (200–800
m) averages for the North Atlantic. According to Aller et al. (2002), “interplay of