Abstract. Factors controlling biomass distributions in marine benthic organisms (meio- to macro-fauna, 1 μg–32 mg wet weight) were investigated through observations and allometric modelling. Biomass (and abundance) size spectra were measured at three locations: the Faroe-Shetland Channel in the north-east Atlantic (FSC, water depth 1600 m, September 2000); the Fladen Ground in the North Sea (FG, 150 m, September 2000); and the hypoxic Oman Margin (OM, 500 m, September 2002) in the Arabian Sea. Biomass increased with body size through a power law at FG (allometric exponent, b = 0.16) and at FSC (b = 0.32), but less convincingly at OM (b was not significantly different from −1/4 or 0). Our results question the assumption that metazoan biomass spectra are bimodal in marine sediments.

The model incorporated 16 metazoan size classes, as derived from the observed spectra, all reliant on a common detrital food pool. All physiological (ingestion, mortality, assimilation and respiration) parameters scaled to body size following optimisation to the data at each site, the resulting values being consistent within expectations from the literature. For all sites, body size related changes in mortality played the greatest role in determining the trend of the biomass size spectra. The body size trend in the respiration rate was most sensitive to allometry in both mortality and ingestion, and the trend in body size spectra of the production: biomass ratio was explained by the allometry in ingestion.

Our results suggest that size-scaling mortality and ingestion are important factors determining the distribution of biomass across the meiofauna to macrofauna size range in marine sedimentary communities, in agreement with the general observation that biomass tends to accumulates in larger rather than smaller size classes in these environments.