Methane oxidation in the waters of a humics-rich boreal lake stimulated by photosynthesis, nitrite, Fe(III) and humics

Abstract. Small boreal lakes are known to contribute significantly to global methane emissions. Lake Lovojärvi is a eutrophic lake in Southern Finland with bottom water methane concentrations up to 2 mM. However, the surface water concentration, and thus the diffusive emission potential, was low (< 0.5 μM). We studied the biogeochemical processes involved in methane removal by chemical profiling and through incubation experiments. δ¹³C-CH₄ profiling of the water column revealed methane-oxidation hotspots just below the oxycline and within the anoxic water column. In incubation experiments involving the addition of light and/or oxygen, methane oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic methane oxidation. A distinct peak in methane concentration was observed at the chlorophyll a maximum, caused by either in-situ methane production or other methane inputs such as lateral transport from the littoral zone. In the dark anoxic water column at 7 m depth, nitrite seemed to be the key electron acceptor involved in methane oxidation, yet additions of Fe(III), anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic methane oxidation. Surprisingly, nitrite seemed to inhibit methane oxidation at all other depths. Overall, this study shows that photosynthetically fueled methane oxidation can be a key process in methane removal in the water column of humic, turbid lakes, thereby limiting diffusive methane emissions from boreal lakes. Yet, it also highlights the potential importance of a whole suite of alternative electron acceptors, including humics, in these freshwater environments in the absence of light and oxygen.