Global bathymetric patterns of standing stock and body size in the deep-sea benthos

Abstract

We present the first global-scale analy- sis of standing stock (abundance and biomass) for 4 major size classes of deep-sea biota: bacteria, meta- zoan meiofauna, macrofauna and megafauna. The community standing stock decreases with depth; this is a universal phenomenon that involves a com- plex transition in the relative importance of the different size groups. Bacterial abundance and biomass show no decline with depth. All 3 animal size groups experience significant exponential de- creases in both abundance and biomass. The abun- dance of larger animals is significantly lower and decreases more rapidly than for smaller groups. The resulting drop in average body size with depth con- firms Thiel's size-structure hypothesis on very large spatial scales. In terms of their proportion of total community biomass, smaller size classes replace larger size classes. The upper continental slope is dominated by macrofaunal biomass, and the abyss by bacterial and meiofaunal biomass. The dramatic decrease in total community standing stock and the ascendancy of smaller organisms with depth has important implications for deep-sea biodiversity. The bathyal zone (200 to 4000 m) affords more eco- logical and evolutionary opportunity in the form of energy availability for larger organisms, and conse- quently supports higher macrofaunal and mega- faunal species diversity than the abyss (> 4000 m).