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. Small boreal lakes are known to contribute significantly to global CH4 emissions. Lake Lovojärvi is a eutrophic lake in southernFinland with bottom water CH4 concentrations up to 2 mM. However, the surface water concentration, and thus the diffusive emissionpotential, was low (&lt; 0.5 µM). We studied the biogeochemical processes involved in CH4 removal by chemical profiling andthrough incubation experiments. δ13C-CH4 profiling of the water column revealed a methane-oxidation hotspot just belowthe oxycline and zones of CH4 oxidation within the anoxic water column. In incubation experiments involving the addition of light and/oroxygen, CH4 oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobicCH4 oxidation. We observed a distinct peak in CH4 concentration at the chlorophyll-a maximum, caused by either in situCH4 production or other CH4 inputs such as lateral transport from the littoral zone. In the dark anoxic water column at7 m depth, nitrite seemed to be the key electron acceptor involved in CH4 oxidation, yet additions of Fe(III),anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic CH4 oxidation. Surprisingly, nitrite seemed to inhibitCH4 oxidation at all other depths. Overall, this study shows that photosynthetically fueled CH4 oxidation can be a key processin CH4 removal in the water column of humic, turbid lakes, thereby limiting diffusive CH4 emissions from boreal lakes. Yet, italso highlights the potential importance of a whole suite of alternative electron acceptors, including humics, in these freshwater environments inthe absence of light and oxygen.

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