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

Abstract. 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 μmol m⁻² s⁻¹ for influxes and 19 μmol 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 μmol m⁻² s⁻¹ at low tide during the day, and the efflux was 2.7 ± 3.7 μmol m⁻² s⁻¹ at low tide during the night. A gross primary production (GPP) of 4.4 ± 4.1 μmol 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 μmol m⁻² s⁻¹, respectively. Night time effluxes of CO₂ were 1.0 ± 0.9 and 0.2 ± 1.1 μmol 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 μmol m⁻² s⁻¹, respectively, attributed primarily to the seagrass community. At the same station in April 2009, before Zostera noltii started to grow, the CO₂ uptake at low tide during the day was the highest (−2.7 ± 2.0 μmol m⁻² s⁻¹). Influxes of CO₂ were also observed during immersion at the central station in spring and early autumn and were apparently related to phytoplankton blooms occurring at the mouth of the flat, followed by the advection of CO₂-depleted water with the flooding tide. Although winter data as well as water carbon measurements would be necessary to determine a precise CO₂ budget for the flat, our results suggest that tidal flat ecosystems are a modest contributor to the CO₂ budget of the coastal ocean.