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 CO 2 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/CaCO 3 conversion factor, averaged 221 (±184) g CaCO 3 m −2 yr −1 . Net calcium carbonate production by this exotic bivalve releases CO 2 at a mean rate of 18 (±17) g C m −2 yr −1 . Respiration by P. amurensis , estimated from secondary production, releases additional CO 2 at a mean rate of 37 (±34) g C m −2 yr −1 . Therefore, total net CO 2 production by P. amurensis averages 55 (±51) g C m −2 yr −1 in an estuarine domain where net primary production consumes only 20 g inorganic C m −2 yr −1 . CO 2 production by P. amurensis in northern San Francisco Bay is an underestimate of the total CO 2 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 CO 2 production rate in the world's estuaries of 1 x 10 14 g C yr −1 , which suggests that calcified benthic organisms in estuaries generate CO 2 equal in magnitude to the CO 2 emissions from the world's lakes or from planetary volcanism (the net source is determined by the highly variable rate of CO 2 consumption by carbonate dissolution). This biogenic CO 2 source is increasing because of the continuing global translocation of mollusks and their successful colonization of new habitats.