Low contribution of oxic methane production in shallow productive lakes

Whereas the occurrence of oxic methane (CH₄) production (OMP) in the oxygenated water column of lakes is widely accepted, its mechanisms, isotopic signature, and contribution to total CH₄ emissions remain uncertain. Evidence suggests that phytoplankton produces CH4, but it is unclear how this pathway contributes to ecosystem OMP rates. Shallow lakes are often productive and feature high phytoplankton biomass, which could potentially lead to high OMP rates and a substantial contribution to CH4 emissions. Here we present results of a field mesocosm study carried out in three shallow lakes in the Pampean Plain (Argentina), designed to assess their ambient OMP dynamics. We combined this with laboratory experiments designed to estimate the potential CH4 production by phytoplankton strains from these systems. We demonstrate that OMP occurred in all lakes, albeit at low rates; all tested phytoplankton strains produced CH4, yet this production contributed up to 15% to OMP rates, implying that other pathways dominate the observed OMP. The contribution of OMP to lake CH4 diffusive emissions was low for all lakes and likely influenced by lake morphometry, suggesting that, despite their high phytoplankton abundances, other sources-such as sediment CH4 production and/or lateral inputs-dominate CH4 emissions in these ecosystems.