Methane Uptake by Inland Waters: Fact or Fiction?

Abstract

Inland waters are widely known as net emitters of methane (CH₄) to the atmosphere, contributing ∼15% of global CH₄ emissions. CH₄ is produced by anaerobic methanogens in aquatic sediments, but net CH₄ emissions at the water–air interface are modulated by CH₄ oxidation by methanotrophs in the water column. However, the prevailing paradigm is that CH₄ production outweighs oxidation, and that freshwaters are ubiquitously net emitters. Here, via data collation and solicitation of expert opinion, we show that diffusive CH₄ uptake (i.e., net CH₄ consumption) is reported across a diverse range of freshwater types, although fluxes are generally small (median = −2.2 mg CH₄ m⁻² d⁻¹). Although some reports of freshwater CH₄ uptake can be attributed to errors in data collection and analysis, others are harder to dismiss. Uptake is potentially driven by biological processes such as high rates of methanotrophy, or by physical processes such as mixing-induced introduction of low-CH₄ water from other layers or inflows. Our results question the paradigm that freshwaters are always sources of CH₄. We call for targeted field and lab work programs to more fully detect and understand this elusive but intriguing natural phenomenon.