Accounting for Spatial and Temporal Variation in Macroinvertebrate Community Abundances When Measuring the Food Supply of Stream Salmonids

The goal of salmonid habitat monitoring programs is to measure habitat attributes linked to salmonid productivity based on protocols that have sufficient precision to detect environmental variation at relevant spatial and temporal scales. Benthic macroinvertebrate community composition often is evaluated as part of habitat monitoring and assessment protocols, despite a lack of direct relationships between benthic composition and salmonid production. Macroinvertebrate drift provides a direct measure of the food resources available to stream salmonids, but drift is rarely evaluated as part of habitat monitoring protocols. This reluctance may stem from the complex spatial and temporal variability inherent in macroinvertebrate drift abundances and an assumed inability to obtain precise estimates of drift abundance at relevant spatial and temporal scales. We evaluated an extensive set of paired drift and benthic macroinvertebrate samples to characterize variation in the biomass and density (i.e., counts) of macroinvertebrate samples across a hierarchy of spatial and temporal scales. Results suggest that estimates of total drift biomass may offer the most precise approach for detecting differences in salmonid food availability among stream reaches and, thus, may be more appropriate than benthic sampling for incorporation into salmonid habitat monitoring programs.