The effects of elevated carbon dioxide concentrations on the early life history of bivalve shellfish

Abstract

The combustion of fossil fuels has increased CO2 in the world's oceans and decreased pH. Ocean acidification may alter the growth, survival, and diversity of marine organisms that synthesize shells due to decreased carbonate ion availability. I examined the response of larvae and early juveniles from three species of commercially and ecologically valuable bivalve shellfish (Mercenaria mercenaria, Argopecten irradians, and Crassostrea virginica) exposed to past, pre-industrial levels (~250 ppm), present (~390 ppm), and future (~750, 1500 ppm) levels of CO2. Under higher CO2 concentrations, all three species experienced decreased growth, survival, and metamorphosis with C. virginica being the least affected. M. mercenaria and A. irradians larvae grown under past CO2 concentrations displayed significantly faster growth and metamorphosis as well as higher survival and lipid accumulation rates compared to individuals under current CO2 levels. Under pre-industrial CO2 levels, M. mercenaria and A. irradians larvae displayed thicker shells than individuals grown at present CO2 concentrations whereas bivalves exposed to high CO2 levels had shells that were malformed and eroded. This finding suggests ocean acidification from the past two centuries may be inhibiting the survival of larval shellfish and contributing to global declines of some bivalve populations. Short term physiological effects of higher CO2 on larval bivalves included decreased: growth rates, RNA:DNA ratios, calcification rates, and lipid content, all which would promote enhanced mortality in an ecosystem setting. Exposure of bivalve larvae to varying levels of CO2 identified threshold duration of exposure beyond which survival was compromised. Longer term experiments (8 months) demonstrated that individuals surviving high CO2 in the larval stage are capable of rapid, compensatory growth when deployed under normal CO2 levels as juveniles. Experiments combining higher CO2 and other environmental stressors such as increased temperature or the introduction of a harmful alga demonstrated synergistic negative effects. Collectively, this dissertation demonstrates that larval stage exposure to high CO2 concentrations has profound implications for the trajectories and restoration of coastal bivalve populations.