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

We present the first global-scale analysis of standing stock (abundance and biomass) for 4 major size classes of deep-sea biota: bacteria, metazoan meiofauna, macrofauna and megafauna. The community standing stock decreases with depth; this is a universal phenomenon that involves a complex 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 decreases in both abundance and biomass. The abundance of larger animals is significantly lower and decreases more rapidly than for smaller groups. The resulting drop in average body size with depth confirms 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 ecological and evolutionary opportunity in the form of energy availability for larger organisms, and consequently supports higher macrofaunal and megafaunal species diversity than the abyss (> 4000 m).