Discrete longitudinal variation in freshwater mussel assemblages within two rivers of central Michigan, USA

A primary focus in the study of lotic ecosystems involves understanding the relationship between changes in the physical environment and changes in biota along a longitudinal gradient. Previous studies examining riverine organisms, primarily fish and aquatic insects, have noted two commonly occurring upstream-to-downstream patterns: either the restriction of species to distinct zones (i.e., discrete variation), or the gradual accumulation of species with increasing distance from the headwaters (i.e., continuous variation). Like other riverine organisms, freshwater mussels exhibit longitudinal patterns in distribution and abundance; however, few studies have quantified these patterns and the environmental variables that influence them. This study examined longitudinal patterns in the assemblage structure of mussels in the Chippewa and Pine rivers, Michigan, United States. A stratified random sampling design was used to sample mussels at 54 sites and timed searches allowed for the characterization of mussel assemblages. Multivariate regression tree models revealed differences in mussel assemblage structure that are consistent with upstream-to-downstream changes in surficial geology and wetland habitat. Certain species were restricted to specific geologies suggesting discrete variation with k-means partitioning and non-metric multidimensional scaling supporting these results. Because the results were consistent between rivers, we expect these patterns to prevail in similarly glaciated regions.