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

peer reviewedMeasurements 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

Oxic/anoxic oscillations and organic carbon mineralization in an estuarine maximum turbidity zone (The Gironde, France)

The study of vertical particle dynamics in the highly turbid Gironde Estuary has shown intense cycles of sedimentation and resuspension at both diurnal and neap-spring time scales. Fluid mud, with suspended particulate matter (SPM) concentrations between 50 and 500 g liter-1, has been observed during neap tides. Vertical profiles of biogeochemical parameters have been measured in the fluid mud. Anoxic conditions have been detected when SPM concentration exceeded 50 g liter-1 in the upstream and 140 g liter-1 in the downstream parts of the maximum turbidity zone (MTZ). At the downstream part of the MTZ, anoxic fluid mud was partitioned into a denitrification layer (SPM = 140-250 g liter-1), intensively reworked at the tidal time scale, and by an Mn(IV)-reduction layer (SPM > 250 g liter-1) preferentially reworked at the neap-spring time scale. Due to the alternation of sedimentation and resuspension periods, most of the sediment experienced oxic/anoxic oscillations throughout the neap-spring cycle. Fluid mud resuspension occurred without any observable incidence on the surface-water oxygenation. An increase in total alkalinity was found in the fluid mud, due to both anaerobic respiration and a carbonate dissolution coupled to aerobic respiratory CO2 generation. This phenomenon significantly affected the inorganic carbon budget of the estuary, increasing the HCO3/- input to the coastal ocean and reducing the CO2 flux to the atmosphere. An accumulation of labile-dissolved organic carbon observed in the fluid mud suggests that these oscillations result in an acceleration of particulate organic matter (POM) decomposition. In the Gironde MTZ, a net loss of refractory land-derived POM occurs. This system acts as an efficient oxic/suboxic 'fluidized bed reactor', similar to mobile deltaic muds

Long term greenhouse gas emissions from the hydroelectric reservoir of Petit-Saut (French Guiana) and potential impacts

This paper summarizes, in a first part, results of greenhouse gas emissions from the hydroelectric reservoir of Petit Saut in French Guiana obtained during the three first years after impoundment (1994–1997). Results from three years of measurements have been extrapolated to estimate trends in methane emissions and the carbon budget of the reservoir over a 20-year period. Extrapolations were made using the global warming potential concept to calculate cumulative greenhouse gas emissions at a 100-year time horizon and to compare these emissions to potential emissions from thermal alternatives. In a second part, we analyze new data from long term continuous observations (1994–2003) of methane concentrations in the reservoir and flux data obtained during a recent campaign in May 2003. These data confirm predicted trends and show some suitable adjustments. They constitute a unique data base which is used for the development of a model to simulate both water quality and greenhouse gas emissions from tropical artificial reservoirs

Cycle du carbone et émissions de gaz à effet de serre (CO2 et CH4 dans la retenue de barrage de Petit-Saut et l'estuaire de Sinnamary (Guyane Française)

Rapport de restitution Comité ECC