Juvenile Marine Bivalves within Corrosive Sediments: How Do (Or Don't) They Do It?

In coastal and estuarine settings a variety or biogeochemical processes alter the overlying water and sediment porewater carbonate chemistry. Furthermore, these habitats are inherently variable in carbonate chemistry due to diurnal changes in production-respiration, seasonal changes in freshwater input, and longer term changes in the earth’s carbon pools. The pCO₂ found in many of these habitats far exceeds predictions of future atmospheric carbon dioxide scenarios. Should organisms be nearing threshold values currently in systems, future increases in acidity may have disproportionate effects on organisms in these systems. Marine and estuarine sediments are sites of high rates of organic matter remineralization, leading to production of aqueous carbon dioxide. Shell forming animals that reside in these sedimentary habitats must overcome significant physiological stress to survive and flourish. Furthermore, infaunal shell forming bivalves typically build their shells of aragonite, a more soluble calcium carbonate mineral form than calcite which is often found in epi-benthic bivalves. The transition from a generally less corrosive water column to a generally more corrosive sediment environment is an energetically demanding metamorphosis that is a population bottleneck. However, the role of dissolution in structuring infaunal bivalve populations has not received much attention until recently. 

We have been conducting laboratory experiments, field manipulations, and modeling exercises to understand the dynamics of post-larval bivalve-sediment interactions of the hard clam, Mercenaria mercenaria. Increased mortality, decreased shell deposition, and poor recruitment all result from increased acidity at the sediment-water interface. However, survival and shell-growth effects are size-dependent, with larger sized organisms able to overcome acidification stress in our short-term experiments. Our work suggests that sediment carbonate chemistry may be an important driver of population dynamics in sediment-dwelling clams, and that a suite of current and future biogeochemical impacts on estuarine carbonate dynamics may have significant effects on hard clams, and other commercially important infaunal bivalves.

Employing a stage-based population model of M. mercenaria, parameterized by our studies and others, we will examine how sediment porewater carbonate chemistry may alter population dynamics by examining this bottleneck of recruitment. Our population model will be linked to a sediment carbonate diagenesis model that we can use to force various simulations such as changes organic matter deposition rates or changes in overlying water chemistry. These modeling exercises will permit a more realistic approach to understanding the impacts of coastal and estuarine acidification on commercially and ecologically important bivalves found in these systems.