Effects of Anthropogenic Disturbance on Freshwater Mussel Populations
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Freshwater mussels (Family Unionidae) are among the most imperiled group of organisms in the world, with nearly 70% of North American species considered threatened. Anthropogenic disturbances, including altered flow regimes, habitat alteration, and pollution, are considered to be the major drivers of this group's decline. We investigated the effects of municipal wastewater effluent on survivorship, growth, and physiological status of mussels in experimental cages in a small Central Texas stream. We tested effluent effects by measuring basic physical and physiological parameters of native threeridge mussels (Amblema plicata), and basic physical parameters of non-native Asian clams (Corbicula fluminea), before and after 72 days exposure at four sites above and below the effluent discharge. Survivorship and growth of the Corbicula and growth and condition indices of the Amblema were significantly higher at the reference site above the discharge than in downstream sites. We attribute this reduction in fitness below the discharge to elevated nutrient and heavy metal concentrations, and possibly the presence of other untested-for compounds commonly found in municipal effluent. These results, along with an absence of native mussels below the discharge, indicate a significant negative impact of wastewater effluent on both native and non-native mussels. We also assessed the impact of two major flood events on mussel abundance and richness in the Rio Grande River. We investigated the effects of the 2008 and 2010 floods in the Rio Grande on the mussels by re-surveying seven sites previously surveyed before the flood of 2008. We found significant differences in abundance only at the location impacted by the 2008 flood, which was of unusually long duration due to emergency releases from upstream dams. The lack of impact caused by the higher magnitude but shorter duration 2010 flood suggests the relative tolerance of mussels to natural flood events compared to anthropogenically-enhanced ones.