A rapid method for estimating biomass size spectra of benthic metazoan communities

The construction of biomass size spectra in aquatic environments suffers from two main problems: (i) it is a labour-intensive process, and (ii) there is a lack of conformity in methodologies. This study evaluates a technique that produces biomass spectra for benthic metazoans from subsamples stratified using a geometric series of nested sieves. The technique was applied to metazoan communities from three sites, two freshwater and one brackish water, on a small river in southern Britain. Random subsamples of animals from each sieve size were measured using image analysis and showed significant differences in body size, as equivalent spherical diameter (ESD), among all but two sieves. There was no significant difference in ESD among sites. Despite the propensity for long thin animals to pass lengthwise through meshes, sieves contained a consistent range of individual body sizes. Mean individual body size and abundance data for each sieve were used to calculate total biomass. These size spectra were consistent with those from previous studies, and it is hoped that this method will encourage the wider application and more rigorous measurement of biomass size spectra in aquatic ecology.