In situ recovery of bivalve shell characteristics after temporary exposure to elevated pCO₂

Ocean uptake of carbon dioxide (CO₂) is causing changes in carbonate chemistry that affect calcification in marine organisms. In coastal areas, this CO₂-enriched seawater mixes with waters affected by seasonal degradation of organic material loaded externally from watersheds or produced as a response to nutrient enrichment. As a result, coastal bivalves often experience strong seasonal changes in carbonate chemistry. In some cases, these changes may resemble those experienced by aquacultured bivalves during translocation activities. We mimicked these changes by exposing juvenile hard clams (500 microm, Mercenaria mercenaria) to pCO₂ in laboratory upwellers at levels resembling those already reported for northeastern US estuaries (mean upweller pCO₂ = 773, 1274, and 1838 microatm) and then transplanting to three grow-out sites along an expected nutrient gradient in Narragansett Bay, RI (154 bags of 100 clams). Prior to the field grow-out, clams exposed to elevated pCO₂ exhibited larger shells but lower dry weight per unit volume (dw/V). However, percent increase in dw/V was highest for this group during the 27-day field grow-out, suggesting that individuals with low dw/V after the laboratory treatment accelerated accumulation of dw/V when they were transferred to the bay. Treatments also appeared to affect shell mineral structure and condition of digestive diverticula. Although treatment effects diminished during the field grow-out, clams that were pre-exposed for several weeks to high pCO₂ would likely have been temporarily vulnerable to predation or other factors that interact with shell integrity. This would be expected to reduce population recovery from short-term exposures to high pCO₂.