Freshwater mussels are filter-feeders that link benthic and pelagic compartments, but the rates and magnitudes of contributed ecosystem services are context-dependent, varying with mussel abundance, community composition, and environmental conditions, all of which are impacted by human activities. The amount of material mussels remove from and contribute back to the water column and sediments depends on mussel biomass and flow regime (water volume and residence time). In addition, mussel species have different physiological optima that govern their performance, including filtration and excretion rates. Thus, within the biomass-flow regime constraint, species composition and environmental gradients interact to determine the magnitude of processes provided by mussels. For example, mussel species have temperature-dependent nitrogen excretion rates, so at warm temperatures a community dominated by thermally-tolerant species will make a different contribution to nitrogen cycling than a community dominated by thermally-sensitive species. Mussels are declining globally, reducing the biomass available to process stream water, most rivers have been manipulated so that flow and thermal regimes are altered, and mussel species dominance patterns are changing in response to both local landuse and regional climate change. All of these factors lower the tipping point beyond which processes performed by mussels in rivers may rapidly decline.