Size Spectra of Benthic Communities in Laurentian Streams

Abstract

The size spectra of benthic invertebrates in streams appear to differ from those reported in planktonic systems, but could reflect the narrow taxonomic scope of previous studies. When algae, protozoans, and invertebrates growing on rocks in three Laurentian streams (Québec) were grouped in logarithmically increasing size classes, patterns in the distribution of biomass were consistent over sites and dates, despite large differences in taxonomic composition. Peaks and troughs were observed in the size spectra, but the biomass of the various size classes usually lay within an order of magnitude of the mean of all classes. The slopes of the normalized size spectra were always shallower than −1 (median = −0.91) indicating a slight biomass increase in the larger size classes. Strongly peaked spectra previously observed for stream invertebrates were smoothed by the inclusion of algae and protozoans. Comparison with size spectra described for benthic and pianktonic communities in lakes and oceans, showed remarkable similarities among habitats. These findings suggest that roughly similar biomass in logarithmic size classes is a general community pattern not restricted to the peculiarities of plankton communities. A size-based approach could be useful to simplify the description of complex stream communities.