Diffuse Flux of Greenhouse Gases — Methane and Carbon Dioxide — at the Sediment-Water Interface of Some Lakes and Reservoirs of the World

Abstract

Sediments were cored from 19 lakes and 4 reservoirs and were analyzed for CO2 and CH4. Additionally sediment concentration profiles were measured in 107 cores. Concentrations of methane in the surface (2 {\textpm} 1.7 cm, n=107) sediment porewater in 4 oligotrophic lakes was in the order of 0.04 {\textpm} 0.03 mM (0--0.14, n=25), 0.24 mM in one oligotrophic reservoir to high values of 1.4 {\textpm} 0.96 mM (0--2.8, n=18) in 5 eutrophic-hypereutrophic lakes and 1.4 {\textpm} 1.0 mM (0--3.5, n=24) in 2 eutrophic reservoirs. Concentrations of porewater CO2 in oligotrophic lakes were ten times higher than methane concentration with values in the order of 0.4 {\textpm} 0.36 mM but slightly lower for the same eutrophic lakes with values around 1.35 {\textpm} 0.9 mM and reservoirs [1.0 {\textpm} 0.8 (0.1--3.2, n=23)]. Surface sediment CH4 concentrations were low in 3 acidotrophic lakes [0.2 {\textpm} 0.3 (0--1, n=22)] and very high in geothermal lakes [2.4 {\textpm} 0.6 (1.8-3, n=3)]. The CO2 concentrations in acidotrophic lakes were, however, high [0.9 {\textpm} 0.7 (0.1--2, n=14)]. Diffuse flux of the two greenhouse gases - CH4 and CO2 --- from the surficial sediments across the sediment-water interface (SWI) were calculated from Fick's first law of diffusion. These resulted in low methane fluxes from oligotrophic systems (lakes and reservoirs) of 0.2--0.4 mM CH4·m−2·d−1 (3--6 mg CH4·m−2·d−1) to much higher fluxes from eutrophic systems [3.9 mM CH4·m−2·d−1 (62 mg CH4·m−2·d−1) for lakes and 5.2 mM (83 mg) for reservoirs] to very high fluxes at the two geothermal lakes [14 mM CH4·m−2·d−1 (220 mg CH4·m−2·d−1)]. Dissolved CO2 fluxes were double the methane fluxes in oligotrophic lakes, about equal for eutrophic lakes [3.8 mM CO2·m−2·d−1 (167 mg CO2·m−2·d−1)] and slightly lower than methane fluxes in eutrophic reservoirs [4.3 mM CO2·m−2·d−1 (190 mg CO2·m−2·d−1)]. Diffuse fluxes of CO2 in acidotrophic systems [3.3 mM CO2·m−2·d−1 (143 mg CO2·m−2·d−1)] were almost the same as observed in eutrophic lakes. Even though it is unclear why there are such great differences between temperate and tropical ecosystems, CO2 gas fluxes at the SWI in one tropical reservoir [Lobo Broa at 16 mM CO2·m−2·d−1 (700 mg CO2·m−2·d−1)] were much higher than temperate ecosystems while CH4 diffuse fluxes [9 mM CH4·m−2·d−1 (140 mg CH4·m−2·d−1)] are only slightly higher than temperate ones. There are few data to evaluate the importance of sediment diffuse fluxes of these two greenhouse gases a…