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 CO 2 atmospheric emissions (10-1000 mmol•m -1 ), except in vegetated tidal flats and marshes, particularly those at freshwater sites, where sediments may be CH 4 -saturated. CH 4 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 CH 4 emissions at low salinity (<10) only.

Seasonal variability of methane in the rivers and lagoons of Ivory Coast (West Africa)

“…Surface waters were always over-saturated in CH 4 with respect to equilibrium with the atmosphere, with CH 4 concentrations ranging from 34 nmol l -1 (i.e., 1496% of saturation) to 1004 nmol l -1 (i.e., 51843% of saturation). These CH 4 over-saturations are within the range reported for temperate and tropical estuarine environments ranging between *70 and 160000% (Bange et al 1994;Upstill-Goddard et al 2000;Middelburg et al 2002;Abril and Borges 2004;Bange 2006;Shalini et al 2006).…”

“…The CH 4 oversaturation in estuarine waters is the result of a complex combination of sources, sinks and transport. In estuarine channels, net CH 4 inputs from the sediments to the water column and CH 4 production in the water column are generally low because oxic and suboxic respiration dominate (Abril and Borges 2004). Consequently, CH 4 in estuarine waters originates from two major sources: (1) rivers, which receive CH 4 from soils, groundwater, wetlands and floodplains on the watershed (De Angelis and Lilley 1987;Richey et al 1988) and (2) tidal wetlands and mud flats, which are generally vegetated and enriched in organic matter to support methanogenesis (Bartlett et al 1987;Chanton et al 1989;Kelley et al 1995;Middelburg et al 2002;Abril and Borges 2004).…”

“…In estuarine channels, net CH 4 inputs from the sediments to the water column and CH 4 production in the water column are generally low because oxic and suboxic respiration dominate (Abril and Borges 2004). Consequently, CH 4 in estuarine waters originates from two major sources: (1) rivers, which receive CH 4 from soils, groundwater, wetlands and floodplains on the watershed (De Angelis and Lilley 1987;Richey et al 1988) and (2) tidal wetlands and mud flats, which are generally vegetated and enriched in organic matter to support methanogenesis (Bartlett et al 1987;Chanton et al 1989;Kelley et al 1995;Middelburg et al 2002;Abril and Borges 2004). Majors sinks of CH 4 in estuarine channels are the export to the adjacent coastal zone that dominates in the case of estuaries with a high freshwater discharge and a short residence time (Scranton and McShane 1991;Middelburg et al 2002), the emission to the atmosphere and the bacterial oxidation in the water column and sediment.…”

“…Majors sinks of CH 4 in estuarine channels are the export to the adjacent coastal zone that dominates in the case of estuaries with a high freshwater discharge and a short residence time (Scranton and McShane 1991;Middelburg et al 2002), the emission to the atmosphere and the bacterial oxidation in the water column and sediment. The emission of CH 4 to the atmosphere usually dominates bacterial oxidation by a factor of 1-20 (De Angelis and Scranton 1993;Lilley et al 1996;Abril and Iversen 2002;Abril and Borges 2004). In addition, methanotrophic activity in estuaries is strongly inhibited by salinity (Scranton and McShane 1991;De Angelis and Scranton 1993), but is enhanced by high turbidity (Abril et al 2007).…”

Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes

“…However, it was not until the late 1990s that a regional synthesis of fluxes of CO 2 out of the estuaries was made available by Frankignoulle et al (1998), who reported that CO 2 emissions from inner estuaries in Europe (average 50 mol m −2 yr −1 ) was a significant term in the regional CO 2 budget, equivalent to 5-10% of the total anthropogenic CO 2 emissions from western Europe. The first compilation of global inner estuarine fluxes of CO 2 was that of Abril and Borges (2004) and was based on only a handful of data. The datasets (Borges 2005;Borges et al 2005) grew gradually, and in 2009 an assessment including 32 estuaries was published (Chen and Borges 2009).…”