Effects of severe drought on freshwater mussel assemblages

We examined changes in freshwater mussel abundance and species composition at eight sites in Alabama and Mississippi in response to a severe drought in 2000. Five small-stream sites in Bankhead National Forest were heavily impacted by drought; one site dried almost completely, and four sites experienced total or near cessation of flow but retained water in their channels to a large extent. In contrast, three large-stream sites retained flow and experienced only minor streambed exposure, primarily along the stream margins. In small streams, overall mussel density before and after the drought declined by 65-83%, and the magnitude of the decline did not differ among streams regardless of whether the channel dried or remained wetted. Introduced Corbicula fluminea (Asian clam) experienced near total mortality and declined to a greater extent than native unionids. The magnitude of decline was similar among unionid species, and the likelihood of surviving the drought was mostly a function of predrought abundance; differences in drought tolerance among species were not evident. Consequently, assemblage composition changed primarily because of the loss of rare species, resulting in drainagewide homogenization and convergence on a shrinking species pool. In contrast, we found no evidence for changes in the total abundance or composition of mussel assemblages in large streams that continued to flow during the drought. Our results show that mussels are highly sensitive to the secondary effects of drought-most likely the low levels of dissolved oxygen caused by low flow, warm temperatures, and high biological oxygen demand-in addition to the direct drying of their habitat. The postdrought abundances of some species in Bankhead National Forest may now be below the minimum necessary for successful reproduction. These populations, which are isolated by reservoirs, may be in a downward spiral from which they will have difficulty recovering in the absence of immigration.