Potential effects of Asian clam (Corbicula fluminea) die-offs on native freshwater mussels (Unionidae) II: porewater ammonia

Abstract

The Asian clam (Corbicula fluminea) occurs in most of the southeastern US, often sharing habitat with native unionid mussels. Clam populations can reach high densities and, under conditions of low water flow and warm summer temperatures, may experience rapid die-offs. Clams are infaunal, so the interstitial zone may be subject to elevated levels of ammonia and reductions in dissolved oxygen (DO) that could affect organisms such as native mussels that also use this habitat. We conducted laboratory experiments to characterize concentrations of total ammonia and unionized ammonia (NH3-N) produced in the sediment pore water and in overlying water as a result of clam die-offs. Sediment porewater NH3-N concentrations ranged between 0.013 and 5.56 mg/L, levels that were consistently higher than NH3-N concentrations in the overlying water. Levels of NH3-N in both pore water and overlying water were positively correlated with temperature and density of clams involved in the die-offs. NH3-N concentrations in chambers maintained at 28°C were 5.56 mg/L, ∼20× levels in chambers maintained at 19°C. Increasing clam density from 200 to 1000 individuals/m2 resulted in an increase in porewater NH3-N from 0.17 to 0.55 mg/L. NH3-N concentrations in some tests exceeded acutely toxic levels for some species of unionid mussels (0.022 to 5.56 mg/L). DO was always lower in pore water (2.01 to 6.74 mg/L) than in overlying water (5.02 to 8.67 mg/L) in chambers containing dead Asian clams, and low DO could have further exacerbated stress associated with exposure to NH3-N. Overall, our results indicate that NH3-N production and DO reductions associated with Asian clam die-offs could pose a risk to unionid mussels, particularly during warm low-flow summer months.