Freshwater mussel abundance and species richness: GIS relationships with watershed land use and geology

We tested the hypotheses that mussel species richness and density are related to landscape features of watersheds. Measures of species richness and mussel density were estimated at 118 sites in 36 watersheds in the state of Iowa, U.S.A., a landscape characterized by >90% agricultural development. Geographical Information Systems (GIS) and regression analyses examined seven land use categories and nine geological descriptors, determining that both mean density and species richness were best correlated with mean watershed slope and the prevalence of alluvial deposits. Our analyses imply that agricultural watersheds with high slopes impact mussel abundance and richness through siltation and destabilization of stream substrate. Because alluvial deposits improve groundwater flux to streams, results suggest that relatively stable stream flows in alluvial watersheds improve mussel persistence. A second set of 82 observations on 38 independent watersheds corroborates the analyses, although historical and local impacts cause correlations between new observations and predictions to be weak.