Physiological responses of three species of unionid mussels to intermittent exposure to elevated carbon dioxide.

Freshwater systems are at risk owing to increasing carbon dioxide (CO) levels, and one of the possible reasons for these elevations is the deployment of non-physical fish barriers to prevent invasive fish movements. Carbon dioxide barriers have the potential to create short, chronic and intermittent exposures of CO for surrounding freshwater biota. Although intermittent exposures to a stressor may be more ecologically relevant, the majority of laboratory tests use chronic or short-term time periods to determine how organisms will respond to an environmental stressor. Measurements of the physiological responses of three species of unionid mussel, giant floaters (), threeridge () and plain pocketbook (), exposed to control CO (~1000 µatm) or intermittent conditions of CO (ranging from ~1000 to ~55 000 µatm) 12 times per day over a 28 day period were gathered. There was no indication of recovery in the physiological responses of mussels between applications of CO, suggesting that the recovery time between CO pulses (1.5 h) was not sufficient for recovery from the CO exposure period (0.5 h). Observations of acid-base and stress responses were consistent with what has been observed in chronic studies of freshwater mussels exposed to elevated CO (i.e. elevations in HCO, Ca, Na and glucose, and decreases in Mg and Cl). However, species differences were observed across almost all variables measured, which emphasizes the need for multispecies studies.