Macrohabitat Factors Influencing the Distribution of Naiads in the St. Croix River, Minnesota and Wisconsin, USA

Abstract

Freshwater mussels (superfamily Unioniacea) are widely distributed throughout North America and are important components of the biodiversity of streams and large river systems. There are currently 297 species and subspecies of unionid mussels living in North America but, of those, 44 species are federally endangered and another 69 species may be endangered (Fish and Wildlife Service 1991; Bogan 1993; Neves 1993; Williams et al. 1993). Approximately 19 species of freshwater mussels in North America are presumed extinct. Unionid mussels are ecologically significant because of their diversity and, as filter-feeders, they are important in processing seston and as a trophic link to higher predators (Vannote et al. 1980). Mussels are one of the few organisms that can process the fine particulate organic matter that makes up a large portion of the energy basis in riverine systems (Wallace et al. 1977). Unionids are also considered important biological monitors (McMahon 1991).