Ocean acidification due to increasing atmospheric CO2 concentrations results in a decrease in seawater pH and shifts in the carbonate chemistry that can negatively affect marine organisms. Marine bivalves such as the hard shell clams Mercenaria mercenaria serve as ecosystem engineers in estuaries and coastal zones of the western Atlantic and, as for many marine calcifiers, are sensitive to the impacts of ocean acidification. In estuaries, the effects of ocean acidification can be exacerbated by low buffering capacity of brackish waters, acidic inputs from freshwaters and land, and/or the negative effects of salinity on organisms’ physiology. We determined the interactive effects of 21 weeks of exposure to different levels of CO2 (~395, 800 and 1500 µatm corresponding to pH of 8.2, 8.1 and 7.7 respectively) and salinity (32 vs. 16) on biomineralization, shell properties and energy metabolism of juveniles of the hard shell clam M. mercenaria. Low salinity had profound effects on survival, energy metabolism and biomineralization of hard shell clams and modulated their responses to elevated PCO2. Negative effects of low salinity in juvenile clams were mostly due to the strongly elevated basal energy demand indicating energy deficiency that led to reduced growth, elevated mortality and impaired shell maintenance (evidenced by the extensive damage to the periostracum). The effects of elevated PCO2 on physiology and biomineralization of hard shell clams were more complex. Elevated PCO2 (~800-1500 µatm) had no significant effects on standard metabolic rates (indicative of the basal energy demand), but affected growth and shell mechanical properties in juvenile clams. Moderate hypercapnia (~800 µatm PCO2) increased shell and tissue growth and reduced mortality of juvenile clams in high salinity exposures; however, these effects were abolished under the low salinity conditions or at high PCO2 (~1500 µatm). Mechanical properties of the shell (measured as microhardness and fracture toughness of the shells) were negatively affected by elevated CO2 alone or in combination with low salinity, which may have important implications for protection against predators or environmental stressors. Our data indicate that environmental salinity can strongly modulate responses to ocean acidification in hard shell clams and thus should be taken into account when predicting the effects of ocean acidification on estuarine bivalves.
Dickinson G. H., Matoo O. B., Tourek R. T., Sokolova I. M. & Beniash E., in press. Environmental salinity modulates the effects of elevated CO2 levels on juvenile hard shell clams, Mercenaria mercenaria. Journal of Experimental Biology. Article (subscription required).