Macroinvertebrate diversity, community structure, and dispersal are affected by tributary identity and confluence conditions in a regulated river

Abstract

Tributaries are essential components of freshwater ecosystems, playing a crucial role in maintaining connectivity and providing habitat for a diverse array of aquatic organisms. The role of tributaries in creating heterogeneity in physical conditions and food resources for fishes could be critical, yet little is known about how variable conditions in different tributaries in regulated river systems influence the mainstem. Using field observations in five tributaries on the Madison River, Montana, we found that tributaries in the same network and within relatively short distances of 60km varied greatly in their environmental conditions, macroinvertebrate densities, and macroinvertebrate community structure. Downstream of confluences macroinvertebrate richness increased overall, and per capita weight of drifting macroinvertebrates decreased overall. These findings suggest that confluences may act as hotspots for biodiversity in regulated rivers and introduce smaller bodied macroinvertebrates to the drift. The amount that a tributary influenced benthic richness and mean per capita weight in the drift downstream of its confluence was related to land use and abiotic factors within that tributary; both macroinvertebrate metrics significantly increased in magnitude downstream of confluences with higher percentage of US Forest Service land, cooler temperatures, decreased discharge, and increased elevation loss (i.e. steeper watershed slope). In contrast, tributaries that had a larger proportion of agricultural land, warmer temperatures, and higher discharge more strongly influenced benthic macroinvertebrate metrics. These tributaries supported higher benthic density and biomass downstream of confluences. Our results offer insight into the ways that tributaries can create heterogeneous habitats that in turn structure macroinvertebrate communities in mainstem rivers and suggest that conservation and restoration of these essential components of freshwater ecosystems is a well-spent endeavor in rivers with regulated mainstems. Future research will need to test the ubiquity of the patterns we observed in other river networks and under other global changes such as pollution, invasive species, and drought. Continued understanding of the importance of heterogeneity imparted by tributaries and their confluences on diversity, availability, and quality of food for threaten fishes is needed to guide restoration efforts aimed at improving river condition and resilience.