Are elusive anaerobic pathways key methane sinks in eutrophic lakes and reservoirs?

Abstract

Collectively, freshwaters constitute a significant source of methane to the atmosphere, and both methane production and methane oxidation can strongly influence net emissions. Anaerobic methane oxidation (AOM) is recognized as a strong regulator of marine methane emissions and appreciation of AOM’s importance in freshwater is growing. In spite of this renewed interest, recent work and reactive-transport modeling results we present in this paper point to unresolved pathways for AOM. Comparison of recent observations from a eutrophic reservoir, Lacamas Lake, with predictions of a 1D steady-state model of water column methane dynamics indicates that high rates of methane oxidation measured via bottle assays cannot be explained with conventional electron acceptors (O2, NO2 −, NO3 −, SO4 2−, Mn4+, and Fe3+). Reactive-transport modeling suggests that solute oxidant concentrations at the thermocline would have to be around 10 times higher than observed to explain the measured methane consumption. Organic acids—a major constituent of organic matter—may account for part of this unexplained AOM given their abundance in eutrophic systems, although the details of these pathways remain elusive (e.g., which species are involved, seasonal renewal of reduced species, contribution of particulate versus dissolved phases). We point to several observations consistent with organic acid-mediated AOM, both in Lacamas Lake and in other systems. Nevertheless, direct evidence of this pathway is still lacking and testing for this remains an important direction for future work. To this end, we identify several new avenues of research that would help quantify the role of organic acid-mediated AOM relative to other electron acceptors.