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 wa- ters 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 es- tuarine waters are the result of lateral inputs from river and marshes fol- lowed by physical ventilation, rather than intense in-situ production in the sediments, where oxic and suboxic conditions dominate. Microbial oxida- tion significantly reduces the CH4 emissions at low salinity (<10) only.