Carbon Dioxide and Methane Emissions from Estuaries

Abstract

Carbon dioxide and methane emissions from estuaries are reviewed in relation with biogeochemical processes and carbon cycling. In estuaries, carbon dioxide and methane emissions show a large spatial and temporal variability, which results from a complex interaction of river carbon inputs, sedimentation and resuspension processes, microbial processes in waters and sediments, tidal exchanges with marshes and flats and gas exchange with the atmosphere. The net mineralization of land- and marsh-derived organic carbon leads to high CO₂ atmospheric emissions (10–1000 mmol·m⁻²·d⁻¹ i.e. 44–44 000 mg·m⁻²·d⁻¹) from inner estuarine waters and tidal flats and marsh sediments. Estuarine plumes at sea are sites of intense primary production and show large seasonal variations of pCO₂ from undersaturation to oversaturation; on an annual basis, some plumes behave as net sinks of atmospheric CO₂ and some others as net sources; CO₂ atmospheric fluxes in plumes are usually one order of magnitude lower than in inner estuaries. Methane emissions to the atmosphere are moderate in estuaries (0.02–0.5 mmol·m⁻²·d⁻¹ i.e. 0.32–8 mg·m⁻²·d⁻¹), except in vegetated tidal flats and marshes, particularly those at freshwater sites, where sediments may be CH₄-saturated. CH₄ emissions from subtidal estuarine waters are the result of lateral inputs from river and marshes followed by physical ventilation, rather than intense in-situ production in the sediments, where oxic and suboxic conditions dominate. Microbial oxidation significantly reduces the CH₄ emissions at low salinity (<10) only.