Clams as CO 2 generators : The Potamocorbula amurensis example in San Francisco Bay

Respiration and calcium carbonate production by the invasive Asian clam, Potamocorbula amurensis, were calculated to assess their importance as CO2 sources in northern San Francisco Bay. Production, calculated using monthly population density and size structure measured at three sites over 7 yr and a shell length/CaCO3 conversion factor, averaged 221 (6184) g CaCO3 m22 yr21. Net calcium carbonate production by this exotic bivalve releases CO2 at a mean rate of 18 (617) g C m22 yr21. Respiration by P. amurensis, estimated from secondary production, releases additional CO2 at a mean rate of 37 (634) g C m22 yr21. Therefore, total net CO2 production by P. amurensis averages 55 (651) g C m22 yr21 in an estuarine domain where net primary production consumes only 20 g inorganic C m22 yr21. CO2 production by P. amurensis in northern San Francisco Bay is an underestimate of the total CO2 supply from the calcified zoobenthic communities of San Francisco Bay, and results from other studies have suggested that this rate is not unusual for temperate estuaries. Global extrapolation yields a gross CO2 production rate in the world’s estuaries of 1 3 1014 g C yr21, which suggests that calcified benthic organisms in estuaries generate CO2 equal in magnitude to the CO2 emissions from the world’s lakes or from planetary volcanism (the net source is determined by the highly variable rate of CO2 consumption by carbonate dissolution). This biogenic CO2 source is increasing because of the continuing global translocation of mollusks and their successful colonization of new habitats. The combination of a unique level of global warming in the late 20th century and narrowed constraints on the role of natural variability provide growing evidence that a greenhouse effect from anthropogenic CO2 emissions can be distinguished from natural variability in the climate system (Crowley 2000). However, there remain large uncertainties in the contemporary global carbon budget and even gaps in our knowledge of the processes contributing to planetary carbon cycling (Schlesinger 1997). We have not yet identified or measured all of the important sources and sinks of CO2, the greenhouse gas that contributes at least 50% to projected global warming (Lashof and Ahuja 1990). Carbon dioxide uptake by photosynthesis and its release by respiration are major processes by which marine organisms alter 1 Corresponding author ([email protected]).