Statewide assessment of freshwater mussels (Bivalvia: Unionidae) in Iowa streams
This study examined freshwater mussel communities, population densities and recent impacts in Iowa, USA. Data collected from streams previously surveyed (1984-85) and from field surveys in this study (1998-99) were used to examine presence, absence, abundance and recent changes in presence/absence of mussel species at different spatial scales. Results indicate an association between mussel impacts and agricultural land use in separate analyses examining habitat characteristics at different spatial scales. Analysis of mussel species richness at 118 sites showed sharp declines in species richness over the past decade. Species richness declined most dramatically at sites having < 50% riparian woodland along the stream length surveyed. At the watershed scale, species richness declined in watersheds where agricultural land use accounted for > 25% of the total land area. Over 38 watersheds, we found that watershed mean mussel density and species richness were best correlated with average watershed slope (topographic relief) and presence of alluvial deposits. An analysis of the influence of riparian and instream characteristics on mussel species richness and population density at 200 sites surveyed in 1998-99 showed that stream shading (an effect of riparian woodland) had a significant positive effect on mussel density and mussel species richness. Mussel species richness was negatively correlated with agricultural nutrients, total nitrogen (TN) and total phosphorus (TP). The influence of landscape features on mussel communities is clear in each analysis: degradation of lands adjacent to freshwater ecosystems adversely impacts mussel habitat and the associated mussel communities. These results have important implications in the context of restoration and conservation efforts.