The American slipper limpet Crepidula fornicata L. has been an invasive species in European bays and estuaries since the 1950s and can reach densities up to several thousands of individuals m(-2). The objective of this study was to estimate the impact of C. fornicata on benthic community metabolism by comparing 2 contrasting sites with high (> 1000 ind. m(-2)) and low (< 200 ind. m(-2)) densities in the Bay of Brest (Brittany, France). Measurements of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) fluxes at the water-sediment interface were investigated using benthic chambers, Community respiration (CR) was 1.5- to 3-fold higher at the station with high densities, where it varied from 1.5 mmol C m(-2) h(-1) in winter to 5.9 mmol C m(-2) h(-1) in summer, and, considering C. fornicata biomass, from 0.7 mmol C g(-1) dry wt h(-1) in winter to 3.3 mmol C g(-1) dry wt h(-1) in summer. CR was mainly controlled by C. fornicata biomass, temperature, and chl a concentration in the water column. Community respiratory quotients (CRQ = vertical bar CRDIC/CRDO vertical bar) were 0.74 in winter-spring and 1.38 in summer-autumn. Annual CR averaged 440 g C m(-2) yr(-1) at the highly colonized station, and 180 g C m(-2) yrl at the station displaying low density of C. fornicata. The estimated annual CR for an average density of 260 ind. m(-2) in the whole Bay of Brest was 220 g C m(-2) yr(-1), which was higher than the carbon production of the overlying phytoplankton and microphyto-benthos in the bay. C. fornicata is thus considered a source of carbon influencing partial pressure of CO2 in seawater and favoring CO2 effluxes to the atmosphere.