Macroinvertebrate Diversity, Community Structure, and Dispersal in a Regulated River Are Affected by Tributary Identity and Confluence Conditions

ABSTRACT Tributaries are essential components of river ecosystems that provide habitat and resources for many species, especially during ontogenetic shifts. The role of tributaries in creating heterogeneous physical conditions and resource supply for riverine macroinvertebrates and fishes could be critical, especially given homogenization by mainstem dams, yet little is known about how variable conditions in different tributaries influence mainstems. Using field observations in five tributaries on the Madison River, Montana, USA, we found that tributaries within 61 river km of each other varied greatly in environmental conditions, macroinvertebrate densities, and macroinvertebrate community structure. Downstream of confluences, macroinvertebrate richness in the mainstem increased, drift density increased, and average mass of a drifting individual decreased, suggesting that confluences may act as biodiversity and resource supply hotspots in regulated rivers. The magnitude of a tributary's influence on richness and body size of drifting macroinvertebrates was positively related to greater US Forest Service land, steeper slope in the watershed, cooler water temperature, and lower discharge. In contrast, a tributary's influence on benthic density and biomass was negatively related to these same drivers. The opposing effects on different metrics that we uncovered suggest that a suite of drivers influences physical and resource heterogeneity imparted by tributaries in complex ways. Future research will need to test the ubiquity of the patterns we observed in other river networks and in relation to other global changes such as pollution or drought. Continued understanding of the importance of tributaries for diversity, density, and biomass of freshwater species may help guide restoration efforts aimed at improving river resilience.