Assemblage structure of freshwater mussels (Bivalvia:Unionidae) in rivers with grassy and forested riparian zones

This study was designed to compare unionid assemblages in rivers with grass-dominated and densely treed riparian zones in southwestern Ontario. Environmental variables related to the riparian classes were measured at 24 sites (2 riparian classes x 3 river drainages per riparian class x 4 sites per drainage). We found 17 species in the study basins. Rivers with narrow, grassy riparian zones were characterized by Pyganodon grandis and Strophitus undulatus, whereas rivers with wider, forested riparian zones were characterised by Elliptio dilatata, Lampsilis radiata, Lampsilis cardium, and Fusconaia flava. Basin types did not differ in mean number of species per site ((x) over bar(grass) = 3.75 (SE = 0.708); (x) over bar(forested) = 4.00 (0.670)). However, a shift towards dominance by a single species was found in grassy rivers where over 60% of individuals in these rivers were P. grandis. Riparian classes differ in physical and chemical characteristics with grassy basins receiving greater amounts of solar radiation, having greater daily temperature fluctuations, and higher concentrations of ammonia and total kjeldahl nitrogen than forested basins. Some mussel species appear to be associated with particular riparian types.