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.