TEMPORAL AND ENVIRONMENTAL VARIATION IN THE BIOMASS SPECTRUM OF BENTHIC INVERTEBRATES IN STREAMS - AN APPLICATION OF THIN-PLATE SPLINES AND RELATIVE WARP ANALYSIS

The size distribution of benthic invertebrates found on the surface of rocks was examined in 12 streams of the Ottawa Valley for 15 months. We used relative warp analysis and thin-plate splines to demonstrate that normalized biomass spectra were similar among sampling dates and among sites, although statistically significant seasonal patterns were detected. The relative abundance of the smallest organisms (mainly early instars of chironomid larvae between 0.1 and 1 mm long) varied the most and peaked in early summer in warm waters, leading to a reduction of the average size of invertebrates in early summer. Overall abundance also positively but weakly correlated with periphyton standing stock. Average size was larger in urban streams of Ottawa-Hull than in streams in cottage areas of the Outaouais or draining the protected forests of Gatineau Park, even after accounting for the effect of periphyton standing stock and concentration of total phosphorus. Total community production, estimated using the average size distribution in all sites and a published empirical model predicting instantaneous growth rate, peaked in early summer and reached a minimum in winter, following closely the seasonal water temperature pattern. The contribution of organisms smaller than I mm to total community production was only significant in early summer when it reached 20%.