Combined Impact of Acute Exposure to Ammonia and Temperature Stress on the Freshwater Mussel Unio pictorum

Nitrogen compounds, particularly ammonia, and temperature are suspected major stressors for aquatic organisms, but little is known about their impact on globally declining freshwater mussels (Unionoida). In this study, we tested the combined effects of ammonia and temperature stress on painter's mussel (Unio pictorum) survival, filtration behavior, hemocyte abundance, hemocyte mortality and glycogen energy status, at concentrations ranging from 0.3 to 9.0 mg.L-1 total ammonia nitrogen (NH4-N) in 96 h acute exposures at two temperatures, 17 degrees C and 25 degrees C and a pH of 8.8. The results indicate a low sensitivity of U. pictorum to elevated ammonia concentrations after short-term exposure, although effects on cell morphology were evident and delayed mortality occurred at the highest test concentration. Most pronounced effects were observed for sublethal physiological endpoints due to elevated temperature, but no synergistic effects with ammonia were evident. Temperature increase resulted in significant effects on tissue glycogen, hyalinocyte mortality and clearance rates. Hemocyte mortalities showed a linear dependency on initial mussel activity as measured by their clearance rate. Since the main stressors tested in this study, ammonia and temperature, are predicted to increase in most freshwater ecosystems, their impact on other freshwater mussel species including different life-stages should be comprehensively assessed.