Methane oxidation at the sediment-water interface of shallow eutrophic Lake Loosdrecht and deep meso-eutrophic Lake Vechten

Abstract

Methane (CH₄) oxidation and CH₄ release into the water column were determined at the sediment-water interface of a shallow eutrophic lake (Lake Loosdrecht) and a deep meso-eutrophic lake (Lake Vechten). Oxygen (O₂) and CH₄ profiles were measured in the littoral and profondal sediments. The CH₄ flux from the anoxic part of the sediments into the oxic surface layer and the flux from the oxic surface layer into the overlying water were calculated. The flux into the overlying water was <5% of the anoxic flux in all of the measured sediments. Consequently, almost the complete diffusive flux of CH₄ (>95%) was oxidized in the oxic surface layer.

CH₄ release into the overlying water of incubated sediment cores was also <5% of the flux out of the anoxic layer into the oxic surface layer for all of the tested sediments. In Lake Vechten, CH₄ release into the overlying water from profondal sediments was 7.9 mmol m⁻²0 d⁻¹ under anoxic conditions in the water column and <0.05 mmol m⁻² d⁻¹ under oxic conditions. The diffusive flux from the sediments into overlying water under anoxic conditions was close to the calculated flux from the anoxic part of the sediments into the oxic surface layer under oxic conditions (9.4 mmol m⁻² d⁻¹).

CH₄ oxidation rates were highest in the profondal gyttja sediments of Lake Vechten (7.9 to 9.4 mmol m⁻² d⁻¹) at 7°C. Rates in littoral gyttja sediments of Lake Loosdrecht were 1.5 mmol m⁻² d⁻¹ at 5°C and 6.5 mmol m⁻² d⁻¹ at 22°C. The lowest values (0.1 mmol m⁻² d⁻¹ at 7°C and 0.4 mmol m⁻² d⁻¹ at 22°C) were measured in the littoral sandy sediments of Lake Vechten. The contribution of CH₄ oxidation to the total sediment O₂ consumption was 58–65% in the profondal sediments, 34% in the littoral gyttja sediments and 2–4% in the littoral sandy sediments. Significant amounts of CH₄ were released into the water column only during hypolimnetic anoxia, while bubble ebullition provided an export of CH₄ directly to the atmosphere.