During the Lower Mekong River’s annual flood-pulse, the surface area of Cambodia’s Tonle Sap Lake (TSL) expands nearly tenfold. This creates steep horizontal and vertical redox gradients conducive to methane production and oxidation in formerly dry, terrestrial floodplain environments. Methanogens and methanotrophs adapted to these redox gradients may be important components of basal productivity within local food webs. Carbon transferred into the food web via these microbial pathways is highly depleted in carbon-13 and traceable using bulk isotope analyses. TSL is home to Southeast Asia’s most productive freshwater fishery. Understanding the ecosystem functioning underlying its productivity, such as the annual flood-pulse, is vital to maintaining regional food security as hydropower development intensifies in the Lower Mekong. We measured net methane production and oxidation rates in TSL surface waters, bottom waters, and sediments in pelagic, littoral, and terrestrial floodplain environments. We compared these forms of net carbon production and oxidation (i.e., respiration) to 1) gross photoautotrophic primary production and 2) total ecosystem respiration estimated using diel oxygen curves in a Bayesian framework. We also measured carbon isotope ratios in the dorsal muscle of fishes sampled in each environment.
Overall, we find significantly higher methane concentrations and net methane oxidation rates during the rising-water and high-water stages of the flood-pulse than the falling-water and low-water stages of the flood-pulse. Methane production and oxidation comprise a greater proportion of gross primary production and ecosystem respiration, respectively, in floodplain environments than in littoral or pelagic lake environments. Several subsistence fish species are also highly depleted in carbon-13, indicating a methanogenic origin. This suggests that the flood-pulse mediates the entry of carbon into the TSL food web and local fishing communities.