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

[摘要] Small boreal lakes are known to contribute significantly to global CH 4 emissions. Lake Lovojärvi is a eutrophic lake in southern Finland with bottom water CH 4 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 CH 4 removal by chemical profiling and through incubation experiments. δ 13 C - CH 4 profiling of the water column revealed a methane-oxidation hotspot just below the oxycline and zones of CH 4 oxidation within the anoxic water column. In incubation experiments involving the addition of light and/or oxygen, CH 4 oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic CH 4 oxidation. We observed a distinct peak in CH 4 concentration at the chlorophyll- a maximum, caused by either in situ CH 4 production or other CH 4 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 CH 4 oxidation, yet additions of Fe(III) , anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic CH 4 oxidation. Surprisingly, nitrite seemed to inhibit CH 4 oxidation at all other depths. Overall, this study shows that photosynthetically fueled CH 4 oxidation can be a key process in CH 4 removal in the water column of humic, turbid lakes, thereby limiting diffusive CH 4 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.

[学科分类] 大气科学