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.