Survival and behaviour of juvenile unionid mussels exposed to thermal stress and dewatering in the presence of a sediment temperature gradient

Freshwater mussels (Unionidae) are a highly imperilled faunal group. One critical threat is thermal sensitivity, because global climate change and other anthropogenic activities contribute to increasing stream temperature and altered hydrologic flow that may be detrimental to freshwater mussels. We incorporated four benthic environmental components - temperature, sediment, water level (a surrogate for flow) and a vertical thermal gradient in the sediment column - in laboratory mesocosm experiments with juveniles of two species of freshwater mussels (Lampsilis abrupta and Lampsilis radiata) and tested their effects on survival, burrowing behaviour and byssus production. Increasing temperature diminished burrowing behaviour significantly in both species (P<0.01), and the dewatered treatment significantly reduced burrowing in L.radiata, compared with that in the watered treatment. Increasing temperature also significantly reduced byssus production in both species (P<0.01). Median lethal temperatures (LT50) ranged from 29.9 to 35.6 degrees C. Mussels did not burrow beneath the top stratum of sediment (0-2.5cm) and thus did not use the available thermal refuge. Our findings suggest that rising stream water temperature and dewatering may directly impact freshwater mussel abundance by causing mortality and may have indirect impacts via sublethal effects. Reduced burrowing capacity may hamper ability to escape predation or unfavourably high or low flows, and decreased byssus production may inhibit attachment and dispersal capabilities in juveniles.