Microhabitat and Landscape Drivers of Richness and Abundance of Freshwater Mussels (Unionida: Unionidae) in a Coastal Plain River

Abstract

Although rivers support significant unionid mussel (Unionida: Unionidae) diversity, Gulf of Mexico tributary rivers have been subject to changes in water quality and habitat due to altered watershed land use. We quantified mussel species richness and relative abundance and environmental factors in small tributary streams of the Pearl River, Mississippi-Louisiana. Freshwater mussel and habitat surveys were conducted at 27 stream sampling sites over two summers (9 sites revisited), and coverage of seven land use categories and seven geological categories above each reach were calculated. Mussels were patchily distributed (53% of sites sampled yielded mussels) and typically not abundant (only 26% of sites yielded >10 mussels). Surveys revealed nine species, with total abundance ranging from 0–66 mussels and richness ranging from 0–5 species per site. Assemblages were driven by an upper to lower watershed gradient of decreasing CPUE and richness, with microhabitat and water quality, land cover, and geology locally modifying this gradient. Environmental variables did not seem of sufficient magnitude to account for the patchy distributions and low abundances of mussels at most study sites, and we hypothesize that high discharge events related to tropical storm passage may have exerted an overriding influence on mussel assemblages in these streams through direct mortality and/or altered availability of suitable glochidial hosts.