Fate of methane in aquatic systems dominated by free-floating plants

Worldwide the area of free-floating plants is increasing, which can be expected to alter methane (CH4) emissions from aquatic systems in several ways. A large proportion of the CH4 produced may become oxidized below the plants due to the accumulation of CH4 as a result of a decrease in the diffusive water-atmosphere flux and the entrapment of part of the ebullitive CH4, in combination with suitable conditions for methane oxidizing (MOX) bacteria in the aerobic rhizosphere. We used a set of essays to test this hypothesis and to explore the effect of different densities for three widespread free-floating species: Azolla filiculoides, Salvinia natans, and Eichhornia crassipes. The gas exchange velocity, proportion of CH4 bubbles trapped by the plants, occurrence of radial oxygen loss from roots, and MOX rates on the roots were assessed. We subsequently used the outcome of these experiments to parameterize a simple model. With this model we estimated the proportion of the produced CH4 that is oxidized, for different plant species and different densities. We found that in a shallow (1 m) system up to 70% of the CH4 produced may become oxidized as a result of a strong decrease in gas exchange combined with high MOX activity of the rhizosphere microbiome. As floating plants also are likely to increase CH4 production by organic matter production, especially when their presence induces anaerobic conditions, the overall effect on CH4 emission will strongly depend on local conditions. This explains the contrasting effects of floating plants on CH4 emissions in literature as reviewed here. As the effect of floating plants on CH4 emissions, including the high MOX rates we show here, can be substantial, there is an urgent need to consider this impact when assessing greenhouse gas budgets. [Display omitted] •In shallow waters up to 70% of CH4 produced is oxidized below free-floating plants.•This is partly caused by a strong decrease (up to 90%) in gas exchange velocity.•Trapped CH4 is oxidized by CH4 oxidizing bacteria (MOX) in the aerobic rhizosphere.•Simultaneously increased CH4 production explains varying effects on net emission.