Oxic methane production in shallow productive lakes: linking field and in vitro experimental evidence

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 to what extent this pathway contributes to ecosystem OMP rates. Shallow lakes are often productive and feature high phytoplankton biomass, implying that OMP rates could be high and contribute substantially to CH₄ emissions. Here we present results of an extensive field mesocosm study carried out in three shallow and productive lakes in the Pampean Plain (Argentina), designed to assess ambient OMP dynamics. We combined this with in vitro experiments to quantify the potential CH₄ production by dominant phytoplankton strains from these systems. We demonstrate that OMP occurred in all lakes, albeit at rates that were lower than expected given their productivity; all phytoplankton strains produced CH₄, yet our results suggest that phytoplankton CH₄ production contributed up to 14% 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, suggesting that sediment CH₄ production is the main source for CH₄ emissions in these ecosystems.