Eddy Covariance Evaluation of Ecosystem Fluxes at a Temperate Saltmarsh in Victoria, Australia Shows Large CO₂ Uptake

Recent studies highlight the important role of vegetated coastal ecosystems in atmospheric carbon sequestration. Saltmarshes constitute 30 % of these ecosystems globally and are the primary intertidal vegetation outside the tropics. Eddy covariance (EC) is the main method for measuring biosphere-atmosphere fluxes, but its use in coastal environments is rare. At an Australian temperate saltmarsh site on French Island, Victoria, we measured CO2 and water gas concentration gradients, temperature, wind speed and radiation. The marsh was dominated by a dense cover of Sarcocornia quinqueflora. Fluxes were seasonal, with minima in winter when vegetation is dormant. Net ecosystem productivity (NEP) during the growing season averaged 10.54 g CO2 m-2 day-1 decreasing to 1.64 g CO2 m-2 day-1 in the dormant period, yet the marsh remained a CO2 sink due to some sempervirent species. Ecosystem respiration rates were lower during the dormant period compared with the growing season (1.00 vs 1.77 μmol CO2 m-2 s-1) with a slight positive relationship with temperature. During the growing season, fluxes were significantly influenced by light levels, ambient temperatures and humidity. Evapotranspiration peaked at 0.27 mm h-1. We cautiously estimate the annual NEP budget at this marsh to be 753 (±112.7) g C m-2 y-1 which is similar to carbon uptake by temperate saltmarshes in Europe and within the range measured at some US saltmarshes. This value is higher than the value hypothesised for global saltmarshes of 382 g C m-2 y-1 but is only half the mean value estimated for global mangroves.