Floating Aquatic Macrophytes Decrease the Methane Concentration in the Water Column of a Tropical Coastal Lagoon: Implications for Methane Oxidation and Emission

In wetlands, the knowledge accumulated on the role of aquatic plants in the methane cycle focused on emergent macrophytes, to the detriment of other typologies. Herein, we evaluated whether the free-floating macrophyte Salvinia auriculata Aubl. and the floating-leaved macrophyte Eichhornia azurea (Sw.) Kunth. decrease the water column methane concentrations compared to a plant-free surface. We prepared microcosms by inserting an individual of S. auriculata or of E. azurea into chambers filled with lagoon water previously bubbled with CH4. Another set of chambers was incubated only with the prepared water, representing the plant-free surface. Half of the chambers were kept in the dark and half in sunlight to simulate a diel cycle. We observed greater loss of CH4, higher O-2 uptake and lower CO2 outflow in the plants treatments. The decrease in methane concentrations in the E. azurea treatments was 93.5% in the light and 77.2% in the dark. In the S. auriculata treatments, the decreases were 74.2% and 67.4% in the light and in the dark, respectively. In plant-free surface the decrease was 58.7% in the light and 36.3% in the dark. These results indicate a role of floating aquatic macrophytes in the methane cycle in the water column. Moreover, our results suggest a diel variation of methane oxidation and methane emission, according to the differences observed in O-2 uptake and CO2 outflow between dark and light conditions. Thus, future predictions of global methane budget should include the role played by floating aquatic macrophytes.