Summary 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 °C. Mussels did not burrow beneath the top stratum of sediment (0–2.5 cm) 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.

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Median lethal temperature (LT50) data from water-only exposures with the early life stages of freshwater mussels suggest that some species may be living near their upper thermal tolerances. However, evaluation of thermal sensitivity has never been conducted in sediment. Mussels live most of their lives burrowed in sediment, so understanding the effect of sediment on thermal sensitivity is a necessary step in evaluating the effectiveness of the water-only standard method, on which the regulatory framework for potential thermal criteria currently is based, as a test of thermal sensitivity. We developed a method for testing thermal sensitivity of juvenile mussels in sediment and used the method to assess thermal tolerance of 4 species across a range of temperatures common during summer. Stream beds may provide a thermal refuge in the wild, but we hypothesized that the presence of sediment alone does not alter thermal sensitivity. We also evaluated the effects of 2 temperature acclimation levels (22 and 27°C) and 2 water levels (watered and dewatered treatments). We then compared results from the sediment tests to those conducted using the water-only standard methods. We also conducted water-only LT tests with mussel larvae (glochidia) for comparison with the juvenile life stage. We found few consistent differences in thermal tolerance between sediment and water-only treatments, between acclimation temperatures, between waterlevel treatments, among species, or between juvenile and glochidial life stages (LT50 range = 33.3-37.2°C; mean = 35.6°C), supporting our hypothesis that the presence of sediment alone does not alter thermal sensitivity. The method we developed has potential for evaluating the role of other stressors (e.g., contaminants) in a more natural and complex environment.

Hydrilla (Hydrilla verticillata) is an invasive aquatic weed that has spread rapidly throughout the USA, especially in the southeast. A common control method is the application of aquatic herbicides, such as fluridone and endothall. However, there is limited documentation on the effects of herbicides commonly used to control hydrilla and other aquatic weeds on many non-target freshwater species and no published information exists on the toxicity of these herbicides to freshwater molluscs. We exposed juveniles (96 h) and glochidia (48 h) of the unionid mussel Lampsilis siliquoidea and adults (28 d) of Lampsilis fullerkati to a formulation of fluridone (Sonar À PR Ò ) in laboratory toxicity tests. The early life stages of L. siliquoidea were also exposed to a formulation of the dipotassium salt of endothall (Aquathol À K Ò ) in separate tests. Juveniles of the freshwater gastropod snail, Somatogyrus viriginicus (Lithoglyphidae), were exposed (96 h) to the Sonar À Genesis Ò fluridone formulation. Endpoints were survival (all species and life stages) as well as siphoning behavior and foot protrusion (adult mussels). Median lethal fluridone concentrations (LC50s) were 865 mg/L (95% CI, 729À1,026 mg/L) for glochidia (24 h), 511 mg/L (309À843 mg/L) for juvenile L. siliquoidea (96 h), and 500 mg/L (452À553 mg/L) for juvenile S. viriginicus (96 h). No mortality occurred in the 28-d exposure of adult L. fullerkati and we found no statistically significant effect of fluridone concentration on foot protrusion (p D 0.06) or siphoning behavior (p D 0.08). The 24-h LC50 for glochidia exposed to the dipotassium salt of endothall was 31.2 mg/L (30.3À32.2 mg/L) and the 96-h LC50 for juvenile mussels was 34.4 mg/L (29.3À40.5 mg/L). Freshwater molluscs were more sensitive to fluridone and endothall than most other species previously tested. Fluridone and endothall concentrations typically recommended for hydrilla treatment (5À15 mg/L and 1À5 mg/L, respectively) were not acutely toxic to the molluscs we tested and a 28-d exposure to fluridone was not lethal to adult mussels even at the highest concentration (300 mg/L), indicating minimal risk of short-term exposure effects.

The Clinch River in Virginia and Tennessee, USA, is well known for its diverse native freshwater mussel assemblages; however, notable declines in mussel populations in recent decades have prompted much concern and subsequent research. We examined the toxicity of recently-deposited sediments on juveniles of the freshwater mussel Epioblasma brevidens, by collecting time-integrated sediment samples from the water column with sediment traps from 11 sites in the Clinch River Basin, including 6 sites within an 88-km reach deemed a ‘mussel zone of decline’. Mussels were exposed to the riverine sediments and to 3 control sediments for 28 d; survival, shell length, and biomass were then assessed. Sediment treatment (i.e., river location) had a significant effect on mussel survival (p < 0.01) and biomass (p = 0.02), but did not affect length (p = 0.37), and sediments from 2 of the tributaries were the most toxic. Inorganic and organic analyses of sediments indicated the presence of metals and polycyclic aromatic hydrocarbons at all sites. Manganese was negatively correlated with mussel survival and biomass, as was ammonia with survival, and total organic carbon with biomass. Current land uses in the watershed indicate that fossil fuel mining and agriculture may be associated with elevated manganese and ammonia, respectively. We found that sediments collected with sediment traps over relatively short deployment durations can help elucidate recent contaminant influx and its potential for inducing toxicity in benthic organisms.