Clearing up cloudy waters: A review of sediment impacts to unionid freshwater mussels.

Freshwater unionid mussels are among the most imperiled fauna in North America, and their decline has been partially attributed to sediment from anthropogenic activities. However, there remains a debate regarding the role played by sediment in mussel declines due to a lack of field and laboratory evidence. If sediment is responsible for mussel declines, then a lack of information will likely impede efforts to mitigate species declines and protect remaining habitat. However, if the impacts of sediment are overstated, time and resources may be wasted on a threat that has little bearing on mussel declines or habitat loss. Given this knowledge gap, the purpose of this paper is to review the literature focused on the potential impact of suspended sediment and sedimentation on freshwater mussels. We focused our search on suspended sediment, expressed either as suspended sediment concentration (SSC) or total suspended solids (TSS), to describe the effects of suspended sediment, sediment deposition and scour. We found increases in suspended solids could impact mussels by decreasing food availability, physically interfering with filter feeding and respiration, and impeding various aspects of the mussel-host relationship. We also found mussel-sediment thresholds, wherein certain concentrations of sediment caused significant declines in population performance, which could serve as reference points for ecological research and management. Specifically, we found clearance rates (a measure of feeding) were negatively impacted by TSS concentrations > 8 mg/L, and respiratory stress occurred at ~600 mg/L. Declines in fertilization success and glochidial (i.e., mussel larvae) development were observed at TSS values of 15 mg/L, and reproductive failure occurred at 20 mg/L. Impacts on host fish attachment and glochidial encystment occurred at TSS concentrations of 1,250 – 5,000 mg/L. Impacts on fish varied depending on the biological endpoint but typically occurred at TSS values ranging from 20 – 5,000 mg/L. We also found mussels were sensitive to smothering and mortality occurred at depths as low as 0.6 – 2.5 cm of substrate. Finally, we found relative shear stress (RSS) values > 1, which is a measure of substrate stability in response to scour and entrainment, resulted in significant declines in mussel biodiversity.