COMMUNITY STRUCTURE OF LACUSTRINE MACROBENTHOS - DO TAXON-BASED AND SIZE-BASED APPROACHES YIELD SIMILAR INSIGHTS

We sampled the macrobenthos of three Laurentian Shield lakes to evaluate how the taxonomic structure and the size distribution (biomass spectra) of littoral communities varied (1) seasonally over the summer, (2) among lakes, and (3) in relation to substratum. Seasonal variation was limited: although total numbers differed significantly among months, the taxonomic structure (biomass) and the biomass size spectrum for each lake-year combination were relatively constant during the summer. Taxonomic structure differed significantly among lake-year combinations; substratum variables explained 55% of this variation. Biomass spectra were conspicuously bimodal for all lake-date combinations, with a main peak at 6.4-102.4 mg dry mass and a lesser peak at 0.1-0.4 mg dry mass. These spectra differ considerably from those reported previously for marine and lacustrine macrobenthos. The shape of the biomass spectrum was relatively invariant across months and lakes, which is remarkable in view of the differences in taxonomic structure, substratum, and fish species composition among lakes. This conservative pattern provides indirect evidence for structuring of littoral macrobenthic communities by size-dependent processes. The results illustrate that taxon-based and size-based approaches can emphasize strikingly different aspects of community structure.