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

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

doi:10.5194/bg-18-3087-2021

Abstract

Abstract. Small boreal lakes are known to contribute significantly to global CH4 emissions. Lake Lovojärvi is a eutrophic lake in southern Finland with bottom water CH4 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 CH4 removal by chemical profiling and through incubation experiments. δ13C-CH4 profiling of the water column revealed a methane-oxidation hotspot just below the oxycline and zones of CH4 oxidation within the anoxic water column. In incubation experiments involving the addition of light and/or oxygen, CH4 oxidation rates in the anoxic hypolimnion were enhanced 3-fold, suggesting a major role for photosynthetically fueled aerobic CH4 oxidation. We observed a distinct peak in CH4 concentration at the chlorophyll-a maximum, caused by either in situ CH4 production or other CH4 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 CH4 oxidation, yet additions of Fe(III), anthraquinone-2,6-disulfonate and humic substances also stimulated anoxic CH4 oxidation. Surprisingly, nitrite seemed to inhibit CH4 oxidation at all other depths. Overall, this study shows that photosynthetically fueled CH4 oxidation can be a key process in CH4 removal in the water column of humic, turbid lakes, thereby limiting diffusive CH4 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.

Highlights

Lacustrine water bodies represent a substantial natural source of atmospheric methane (CH4), a major contributor to global warming
Lake Lovojärvi incubation experiments and the natural abundance δ13C-CH4 signal in the water column suggest that natural CH4 oxidation rates are highest at 3 and 7 m depth (Figs. 1 and 4)
CH4 oxidation rates were highest directly at the oxycline (∼ 1 μM d−1 at 3 m; Fig. 4), confirming that aerobic methanotrophs are most active at the oxic–anoxic transition, where both substrates (CH4 and O2) overlap and conditions are most favorable for aerobic CH4 oxidation (Rudd et al, 1976, Blumenberg et al, 2007; Fenchel and Blackburn, 1979)

Summary

Introduction

Lacustrine water bodies represent a substantial natural source of atmospheric methane (CH4), a major contributor to global warming. They may release up to ∼ 72 Tg CH4 a−1 (12 % of total global emissions) (Bastviken et al, 2011), despite covering a relatively small proportion of the land surface area (> 3 %; Downing et al, 2006). CH4 can diffuse into the water column and may be emitted to the atmosphere at the water–air interface. Large fractions of this CH4 may, be consumed by microbial CH4 oxidation, decreasing the CH4 concentration and emissions. Research has shown that microbial CH4 oxidation may be the single most important control on CH4 emissions from lakes and other ecosystems (Chistoserdova, 2015)

Methods

Results

Discussion

Conclusion