Relationships between Size Structure of Invertebrate Assemblages and Trophy and Substrate Composition in Streams

Abstract

Nine stream sites, spanning the range of water total phosphorus concentrations ([TP]) found in Eastern Ontario and Western Québec, were sampled in June 1992 to describe how the size distribution of invertebrates varied along a nutrient gradient and among substrate categories ranging from sand to small boulders. Despite clear differences in the taxonomic composition among sites, the shape of the size distribution was remarkably similar in all sites. Substrate composition affected the overall abundance of invertebrates more than their size distribution. Overall density peaked on fine and coarse gravel and was lower on sand and boulders. Total abundance of invertebrates was highest in eutrophic sites ([TP] > 40 μg/L), although only organisms larger than 1 mm (approximately 1 μg dry mass) showed higher abundances in richer sites. The increase in total invertebrate abundance with increasing total phosphorus concentration suggests that these streams are phosphorus limited. Animals smaller than 1 mm long account for less than 3% of the respiration of the benthic assemblage on all substrates in our streams. Size distribution of invertebrates, although affected by substrate composition and trophy, is remarkably constant and predictable in our streams. Most of the biomass, and consequently the consumption, respiration, and secondary production, is found in relatively large invertebrates. Meiofauna is relatively rare and consequently has little direct influence on community metabolism in the streams we studied.