Spatial and temporal CO<sub>2</sub> exchanges measured by Eddy Covariance over a temperate intertidal flat and their relationships to net ecosystem production

Abstract

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 CO 2 exchange (NEE) with the occupation of the mudflat by primary producers, particularly by Zostera noltii meadows. CO 2 fluxes during the four deployments showed important spatial and temporal variations, with the flat rapidly shifting from sink to source with the tide. Absolute CO 2 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 CO 2 . In contrast, during immersion and during low tide at night, CO 2 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), CO 2 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 %), CO 2 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 CO 2 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 CO 2 uptake at low tide during the day was the highest (-2.7 ± 2.0 µmol m −2 s −1 ). Influxes of CO 2 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 CO 2 -depleted water with the flooding tide.Published by Copernicus Publications on behalf of the European Geosciences Union. P. Polsenaere et al.: Spatial and temporal CO 2 exchanges measured by Eddy CovarianceAlthough winter data as well as water carbon measurements would be necessary to determine a precise CO 2 budget for the flat, our results suggest that tidal flat ecosystems are a modest contributor to the CO 2 budget of the coastal ocean.

On biotic and abiotic drivers of the microphytobenthos seasonal cycle in a temperate intertidal mudflat: a modelling study

“…Efforts are also focused in using remote sensing reflectances from airbone hyperspectral data to assess MPB PP rates (Méléder et al, 2018). Recently, and in the light of the work of Brito et al (2013) Ibrom, Andreas and Rannik, Üllar and Moncrieff, John and Foken, Thomas and Kowalski, Andy S and Martin, Philippe H and Berbigier, Paul and Bernhofer, Ch and others, 1999;Zemmelink et al, 2009;Polsenaere et al, 2012). Similarly, the aquatic Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-325 Manuscript under review for journal Biogeosciences Discussion started: 10 July 2018 c Author(s) 2018.…”

“…The study of air-water and sediment-water exchanges through simultaneous atmospheric and aquatic EC measurements could 5 allow quantifying the importance of metabolic fluxes during immersion and emersion periods but also the coupled processes between the benthic and pelagic compartments such as MPB resuspension. Microphythobenthic community resuspension can significantly contribute to planktonic gross PP and, in turn, explain lower CO 2 fluxes from the water column to the atmosphere at high tide during the day than at night (Guarini et al, 2008;Polsenaere et al, 2012). To date, the modelling effort put on the physically-driven (tides and waves) resuspension processes of MPB is still limited (see Mariotti and Fagherazzi, 2012).…”

Eddy covariance for quantifying trace gas fluxes from soils

“…Only very recently have researchers started to also extend eddy covariance measurements to ever more challenging and less widespread ecosystem types, such as mangroves Jha et al, 2014), deserts (Honrath et al, 2002;Liu et al, 2012;Li et al, 2014), intertidal flats (Polsenaere et al, 2012), and screenhouses (Tanny et al, 2006).…”

Net uptake of atmospheric CO 2 by coastal submerged aquatic vegetation

“…Therefore, the method has been widely used to estimate terrestrial atmospheric CO 2 fluxes (Lee et al, 2004). The method has also been used to estimate air-sea CO 2 fluxes in the ocean and coastal seas (McGillis et al, 2001;Vesala et al, 2006;Kondo & Tsukamoto, 2007;Zemmelink et al, 2009;Huotari et al, 2011;Polsenaere et al, 2012) and to estimate sediment-water O 2 fluxes (Berg et al, 2003;Kuwae et al, 2006). However, several studies have reported that results from the eddy covariance method are inconsistent with results from other methods, such as the bulk formula method (Kondo & Tsukamoto, 2007;Zemmelink et al, 2009).…”