Coupling freshwater mussel ecology and river dynamics using a simplified dynamic interaction model

Freshwater faunal diversity and abundance have declined dramatically worldwide, concurrent with changes in streamflow and sediment loads in rivers. Cumulative effects and interdependencies of chronic covarying environmental stressors can obscure causal linkages that may be controlling the population dynamics of longer-lived freshwater fauna, such as mussels. To understand changes in long-term mussel population density we developed a dynamic, process-based interaction model that couples streamflow, suspended sediment phytoplankton, and mussel abundance under the hypothesis that chronic exposure to increased suspended sediment and food limitation are the primary factors controlling native mussel population density in a midwestern USA agricultural river basin. We calibrated and validated the model with extensive survey data from multiple time periods and used it to evaluate changes in mussel abundance at a subbasin scale over decades. We evaluated sensitivity of simulated mussel densities across a range of mortality rates and initial population densities. In scenarios representing altered sediment concentrations, such as might occur with climate or landuse-induced changes in streamflow or sediment generation rates, mussel population density showed critical threshold responses to long-term changes in suspended sediment concentration. This model of mussel population density can be used to test hypotheses about limiting factors, identify priority locations for restoration activities, and evaluate the effects of climate-or landuse-change scenarios.