Resource enhancement of grazing mayfly nymphs by retreat-building caddisfly larvae in a sandbed stream

Abstract

Over the summer-autumn low-flow period in the sandy upper and middle reaches of the Creightons Creek system of northern Victoria, Australia, several different macroscopically visible substrate patch types form on the streambed. Exploratory multivariate analyses revealed a distinct macroinvertebrate fauna associated with each patch type. The major patch types were the thalweg, or pathway of main flow, in which the substrate consisted of constantly shifting fine sand, and adjacent stable patches of coarser sand, covered with larval retreats of the hydropsychid caddisfly Cheurnatopsyche sp. (Trichoptera:Hydropsychidae). Although thalweg and Cheurnatopsyche patches contained a similar interstitial macroinvertebrate fauna, the former lacked a surface-dwelling, or epibenthic, fauna. The silken retreats of Cheurnatopsyche larvae entrained algal filaments and detritus, food of the grazing mayfly nymph Baetis sp. Baetis nymphs recolonized plots containing Cheurnatopsyche retreats in higher numbers than plots in which Cheurnatopsyche retreats and their entrained detritus had previously been scoured away. Given the widespread distribution of the Cheurnatopsyche patch type in the system during the low-flow period, it is hypothesized that the indirect interaction between these two species may constitute a significant pathway in the energy budget of the stream.