CAN EELGRASS MITIGATE THE EFFECTS OF OCEAN ACIDIFICATION ON OYSTER HEALTH? A STABLE ISOTOPIC APPROACH.  

Ocean acidification is a term to describe a water quality condition that occurs as a result of excess atmospheric CO₂ absorbed into surface water at a rate beyond that which natural processes can buffer against increased carbonic acid. This condition varies temporally and spatially within estuaries and can result in a significant decrease in pH, which lowers the availability of carbonate ions to calcifying organisms. This acidification can increase the solubility of calcium carbonate, making the process of calcification difficult for marine species such as oysters. As climate models draw increasingly dire predictions for global pCO₂ over the next century, practices that can locally mitigate these impacts could play crucial role in the sustainability of global shellfisheries.

The focus of this study was to determine if the natural process of photosynthesis by resident eelgrass can locally decrease or "draw down" pCO₂ in the presence of oyster habitat. Six laboratory tank experiments were conducted, wherein juvenile oysters were kept under controlled pCO₂ (1600, 800, and 400 ppm) water conditions with and without eelgrass. An isotopically labeled CO₂ tracer was used as a carbon signature through the system; water δ¹³C_DIC was concurrently sampled and analyzed. Isotopic analysis of eelgrass tissues demonstrated that eelgrass preferentially incorporated the lighter isotope (δ¹³C_eelgrass = ~-23‰) resulting in an enrichment of C₁₃ in the remaining DIC. The measured difference in δ¹³C_DIC between tanks with and without eelgrass indicate an average 0.72% ± 0.19% uptake of carbon by photosynthesis. Implications are that co-occurring eelgrass habitat may mitigate the negative shell impacts on oyster growth. Isotopic analyses of juvenile shell carbon composition across growth bands showed minimal difference among treatments, indicating that at their life stage, shell carbonate may not derive carbon directly from ambient water DIC.