SIZE STRUCTURE OF THE METAZOAN COMMUNITY IN A PIEDMONT STREAM

We characterized the size structure of virtually the entire metazoan c ommunity in a fourth order, sandy-bottomed Piedmont stream during late summer. Our study, the first to sample across all habitat types and s izes of metazoans in an aquatic ecosystem, indicates that at the commu nity level, stream size spectra may be bimodal for the benthos or trim odal when fish are included. Animals spanning 10 orders of magnitude i n dry mass (from gastrotrichs to fish) were quantitatively collected f rom nine habitat types. The bimodal benthic size spectrum was characte rized by a meiofaunal component (mostly oligochaetes and micro-crustac ea) and a macrobenthic component (mostly the introduced asiatic clam, Corbicula fluminea). Insects contributed little to overall standing cr op. Size-specific contribution to whole-community metabolism was asses sed using allometric equations for respiration, and we found a distinc tly bimodal distribution across the entire metazoan size range, with p eaks in the meiofaunal and benthic macrofaunal size ranges. Our bimoda l benthic size spectrum is similar to that observed for marine benthos but not to other freshwater benthic systems, possibly because the ent ire range of habitat types and/or animal sizes were not sampled in the latter. Numerous factors may influence size spectra in stream ecosyst ems, including local geomorphic (habitat) conditions, water level fluc tuations, species introductions, and predation processes.