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

[摘要] Measurements of carbon dioxide fluxes were performed over a temperateintertidal mudflat in southwestern France using the micrometeorological EddyCovariance (EC) technique. EC measurements were carried out in twocontrasting sites of the Arcachon flat during four periods and in threedifferent seasons (autumn 2007, summer 2008, autumn 2008 and spring 2009).In addition, satellite images of the tidal flat at low tide were used tolink the net ecosystem CO₂ exchange (NEE) with the occupation of themudflat by primary producers, particularly by Zostera noltii meadows. CO₂ fluxesduring the four deployments showed important spatial and temporalvariations, with the flat rapidly shifting from sink to source with thetide. Absolute CO₂ fluxes showed generally small negative (influx) andpositive (efflux) values, with larger values up to −13 μmol m⁻² s⁻¹for influxes and 19 μmol m⁻² s⁻¹ for effluxes. Lowtide during the day was mostly associated with a net uptake of atmosphericCO₂. In contrast, during immersion and during low tide at night, CO₂ fluxes where positive, negative or close to zero, depending on the seasonand the site. During the autumn of 2007, at the innermost station with apatchy Zostera noltii bed (cover of 22 ± 14% in the wind direction ofmeasurements), CO₂ influx was −1.7 ± 1.7 μmol m⁻² s⁻¹ at low tideduring the day, and the efflux was 2.7 ± 3.7 μmol m⁻² s⁻¹ at low tide during the night. A gross primaryproduction (GPP) of 4.4 ± 4.1 μmol m⁻² s⁻¹ duringemersion could be attributed to microphytobenthic communities. During thesummer 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⁻¹in summer and autumn, respectively, resulting in aGPP during emersion of 2.5 ± 1.5 and 1.1 ± 2.0 μmol m⁻² s⁻¹, respectively, attributed primarily to the seagrass community. Atthe same station in April 2009, before Zostera noltii started to grow, the CO₂ uptakeat low tide during the day was the highest (−2.7 ± 2.0 μmol m⁻² s⁻¹). Influxes of CO₂ were also observed during immersionat the central station in spring and early autumn and were apparentlyrelated to phytoplankton blooms occurring at the mouth of the flat, followedby the advection of CO₂-depleted water with the flooding tide.Althoughwinter data as well as water carbon measurements would be necessary todetermine a precise CO₂ budget for the flat, our results suggest thattidal flat ecosystems are a modest contributor to the CO₂ budget of thecoastal ocean.

[学科分类] 地球化学与岩石