Spatial and temporal CO2 exchanges measured by Eddy Correlation over a temperate intertidal flat and their relationships to net ecosystem production
Authors: Polsenaere P. , Lamaud E. , Lafon V. , Bonnefond J.-M. , Bretel P. , Delille B. , Deborde J. , Loustau D. , Abril G. 
Published: June 2011

Abstract. Measurements of carbon dioxide fluxes were performed over a temperate intertidal mudflat in southwestern France using the micrometeorological Eddy Correlation (EC) technique. EC measurements were carried out in two contrasting sites of the Arcachon lagoon during four periods and in three different seasons (autumn 2007, summer 2008, autumn 2008 and spring 2009). In this paper, spatial and temporal variations in vertical CO2 exchanges at the diurnal, tidal and seasonal scales are presented and discussed. In addition, satellite images of the tidal flat at low tide were used to link the net ecosystem 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 lagoon rapidly shifting from a sink to a source of CO2. CO2 fluxes showed generally low negative (influx) and positive (efflux) values and ranged from −13 to 19 μmol m−2 s−1 at maximum. Low tide and daytime conditions were always characterised by an 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 μmol m−2 s−1 during emersion could be attributed mostly to microphytobenthic communities. During immersion, the water was a source of CO2 to the atmosphere, suggesting strong heterotrophy or resuspension of microphytobenthic cells. 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. Nighttime 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 and 1.1 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) and could be attributed to microphytobenthos dominance on NEP in this case. NEE versus PAR relationships for data ranked by wind directions were generally negative where and when Zostera noltii was dominant and positive when this community was minor. The latter relationship suggests important processes of photo-acclimatisation by the microphytobenthos, such as migration through the sediment. 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 lagoon, followed by the advection of CO2-depleted water with the tide. Although winter data would be necessary to determine a precise CO2 budget for the lagoon, our results suggest that tidal flat ecosystems are a modest contributor to the CO2 budget of the coastal ocean.