Hot and bothered: effects of elevated Pco₂ and temperature on juvenile freshwater mussels

Multiple environmental stressors may interact in complex ways to exceed or diminish the impacts of individual stressors. In the present study, the interactive effects of two ecologically relevant stressors [increased temperature and partial pressure of carbon dioxide (Pco₂)] were assessed for freshwater mussels, a group of organisms that are among the most sensitive and rapidly declining worldwide. The individual and combined effects of elevated temperature (22°C-34°C) and Pco₂ (~230, 58,000 µatm) on juvenile Lampsilis siliquoidea were quantified over a 5- or 14-day period, during which physiological and whole animal responses were measured. Exposure to elevated temperature induced a series of physiological responses, including an increase in oxygen consumption rates following 5 days of exposure at 31°C and an increase in carbonic anhydrase ( ca) and heat shock protein 70 mRNA levels following 14 days of exposure at 28°C and 34°C, respectively. Treatment with elevated Pco₂ activated acid-base regulatory responses including increases in CA and Na⁺-K⁺-ATPase activity and a novel mechanism for acid-base regulation during Pco₂ exposure in freshwater mussels was proposed. Thermal and CO₂ stressors also interacted such that responses to the thermal stressor were diminished in mussels exposed to elevated Pco₂, resulting in the greatest level of mortality. Additionally, larger mussels were more likely to survive treatment with elevated Pco₂ and/or temperature. Together, exposure to elevated Pco₂ may compromise the ability of juvenile freshwater mussels to respond to additional stressors, such as increased temperatures, highlighting the importance of considering not only the individual but also the interactive effects of multiple environmental stressors.