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

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
influence 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 relationship 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.