SIZE SPECTRA OF BENTHIC COMMUNITIES IN LAURENTIAN STREAMS

The size spectra of benthic invertebrates in streams appear to differ from those reported in planktonic systems, but could reflect the narro w taxonomic scope of previous studies. When algae, protozoans, and inv ertebrates growing on rocks in three Laurentian streams (Quebec) were grouped in logarithmically increasing size classes, patterns in the di stribution of biomass were consistent over sites and dates, despite la rge differences in taxonomic composition. Peaks and troughs were obser ved in the size spectra, but the biomass of the various size classes u sually lay within an order of magnitude of the mean of all classes. Th e slopes of the normalized size spectra were always shallower than -1 (median = -0.91) indicating a slight biomass increase in the larger si ze classes. Strongly peaked spectra previously observed for stream inv ertebrates were smoothed by the inclusion of algae and protozoans. Com parison with size spectra described for benthic and planktonic communi ties in lakes and oceans, showed remarkable similarities among habitat s. These findings suggest that roughly similar biomass in logarithmic size classes is a general community pattern not restricted to the pecu liarities of plankton communities. A size-based approach could be usef ul to simplify the description of complex stream communities.