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 ± 1.7 cm, n=107) sediment porewater in 4 oligotrophic lakes was in the order of 0.04 ± 0.03 mM (0–0.14, n=25), 0.24 mM in one oligotrophic reservoir to high values of 1.4 ± 0.96 mM (0–2.8, n=18) in 5 eutrophic-hypereutrophic lakes and 1.4 ± 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 ± 0.36 mM but slightly lower for the same eutrophic lakes with values around 1.35 ± 0.9 mM and reservoirs [1.0 ± 0.8 (0.1–3.2, n=23)]. Surface sediment CH₄ concentrations were low in 3 acidotrophic lakes [0.2 ± 0.3 (0–1, n=22)] and very high in geothermal lakes [2.4 ± 0.6 (1.8-3, n=3)]. The CO₂ concentrations in acidotrophic lakes were, however, high [0.9 ± 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 as related to aquatic surface emissions. One study in an 11-m deep tropical high elevation reservoir observed that surface losses represented 10% of the sediment diffuse flux of CH₄ (thus, 90% was oxidized at the SWI or in the water column). For CO₂ it is suspected that 20% of the surface emissions come from sediment sources. The sediments represent an important repository of carbon which contributes gases to overlying waters. These fuel the activities of microorganisms and substantially contribute to oxygen depletion in overlying waters as well as contributing to climate change.