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

van Grinsven, Sigrid; Oswald, Kirsten; Wehrli, Bernhard; Jegge, Corinne; Zopfi, Jakob; Lehmann, Moritz F.; Schubert, Carsten J.

Small boreal lakes are known to contribute significantly to global inline-formulaCH4 emissions. Lake Lovojärvi is a eutrophic lake in southern Finland with bottom water inline-formulaCH4 concentrations up to 2 inline-formulamM. However, the surface water concentration, and thus the diffusive emission potential, was low (inline-formula< 0.5 inline-formulaµM). We studied the biogeochemical processes involved in inline-formulaCH4 removal by chemical profiling and through incubation experiments. inline-formulaδ13C-inline-formulaCH4 profiling of the water column revealed a methane-oxidation hotspot just below the oxycline and zones of inline-formulaCH4 oxidation within the anoxic water column. In incubation experiments involving the addition of light and/or oxygen, inline-formulaCH4 oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic inline-formulaCH4 oxidation. We observed a distinct peak in inline-formulaCH4 concentration at the chlorophyll-inline-formulaa maximum, caused by either in situ inline-formulaCH4 production or other inline-formulaCH4 inputs such as lateral transport from the littoral zone. In the dark anoxic water column at 7 inline-formulam depth, nitrite seemed to be the key electron acceptor involved in inline-formulaCH4 oxidation, yet additions of inline-formulaFe(III), anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic inline-formulaCH4 oxidation. Surprisingly, nitrite seemed to inhibit inline-formulaCH4 oxidation at all other depths. Overall, this study shows that photosynthetically fueled inline-formulaCH4 oxidation can be a key process in inline-formulaCH4 removal in the water column of humic, turbid lakes, thereby limiting diffusive inline-formulaCH4 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.

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van Grinsven, Sigrid / Oswald, Kirsten / Wehrli, Bernhard / et al: Methane oxidation in the waters of a humic-rich boreal lake stimulated by photosynthesis, nitrite, Fe(III) and humics. 2021. Copernicus Publications.

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Rechteinhaber: Sigrid van Grinsven et al.

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