Spatial and temporal CO2 exchanges measured by Eddy Covariance over a temperate intertidal flat and their relationships to net ecosystem production

Measurements of carbon dioxide fluxes were performed over a temperate intertidal mudflat in southwestern France using the micrometeorological Eddy Covariance (EC) technique. EC measurements were carried out in two contrasting sites of the Arcachon flat during four periods and in three different seasons (autumn 2007, summer 2008, autumn 2008 and spring 2009). In addition, satellite images of the tidal flat at low tide were used to link the net ecosystem CO2 exchange (NEE) with the occupation of the mudflat by primary producers, particularly by Zostera noltii meadows. CO2 fluxes during the four deployments showed important spatial and temporal variations, with the flat rapidly shifting from sink to source with the tide. Absolute CO2 fluxes showed generally small negative (influx) and positive (efflux) values, with larger values up to -13 mu mol m(-2) s(-1) for influxes and 19 mu mol m(-2) s(-1) for effluxes. Low tide during the day was mostly associated with a net uptake of atmospheric CO2. In contrast, during immersion and during low tide at night, CO2 fluxes where positive, negative or close to zero, depending on the season and the site. During the autumn of 2007, at the innermost station with a patchy Zostera noltii bed (cover of 22 +/- 14% in the wind direction of measurements), CO2 influx was -1.7 +/- 1.7 mu mol m(-2) s(-1) at low tide during the day, and the efflux was 2.7 +/- 3.7 mu mol m(-2) s(-1) at low tide during the night. A gross primary production (GPP) of 4.4 +/- 4.1 mu mol m(-2) s(-1) during emersion could be attributed to microphytobenthic communities. During the summer and autumn of 2008, at the central station with a dense eelgrass bed (92 +/- 10%), CO2 uptakes at low tide during the day were -1.5 +/- 1.2 and -0.9 +/- 1.7 mu mol m(-2) s(-1), respectively. Night time effluxes of CO2 were 1.0 +/- 0.9 and 0.2 +/- 1.1 mu mol m(-2) s(-1) in summer and autumn, respectively, resulting in a GPP during emersion of 2.5 +/- 1.5 and 1.1 +/- 2.0 mu mol m(-2) s(