Unregulated tributaries are often assumed to provide ecological benefits to regulated rivers, yet empirical evidence remains inconsistent. We assessed whether inflows from unregulated tributaries influenced macroinvertebrate communities of a heavily regulated river.
We collected macroinvertebrate samples from three tributary locations on nine occasions over 24 months to determine the extent of community change caused by a tributary and the varying magnitude of inflows. Samples were collected from the regulated mainstem upstream and downstream of three tributary confluences, at two additional control sites on the mainstem with no intervening tributary, and three sites within each unregulated tributary.
Overall, we found little evidence to show that tributaries significantly influenced macroinvertebrate community composition within the regulated river. Macroinvertebrate community structure was scarcely different between regulated upstream and downstream sites separated by a tributary. In addition, upstream-downstream community dissimilarity was independent of the magnitude and variability of antecedent tributary discharge prior to each sampling occasion. Tributary influence on the density of functional feeding groups was also small and inconsistent. Our results suggest that persistent environmental conditions within the regulated mainstem may override any potential influence of tributaries on macroinvertebrate communities.