Freshwater plant communities influence water column greenhouse gases

Abstract

Freshwater ecosystems can be large sources of greenhouse gases (GHGs), such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Aquatic primary producers (e.g., submerged and floating macrophytes, phytoplankton) directly and indirectly influence GHG production, transport, and emissions; however, it is unclear how the dominant primary producer community influences GHGs. Here, we conducted a 4-month field experiment to investigate the effects of different plant communities on CO2, CH4, and N2O. We used in-pond mesocosms to establish three different aquatic plant communities: submerged macrophytes, submerged and floating macrophytes, and no macrophytes (phytoplankton only). We measured surface and bottom water concentrations and diffusive fluxes of CO2, CH4 and N2O as well as ebullitive CH4 flux and physicochemical parameters in the water column. We found that the treatment with submerged and floating macrophytes had the lowest dissolved oxygen and highest water column dissolved organic carbon. These communities also typically had the highest dissolved CO2 and CH4 but the lowest N2O concentrations. Despite differences in concentrations, we observed no differences in overall emissions across communities, possibly due to floating plants blocking fluxes to the atmosphere and oxidizing CH4. Thus, individual traits may mask effects that plant communities have on total emissions. Our study shows that aquatic plant communities and plant trait composition can have distinct effects on GHG production and transport through the water column. Ultimately, plant communities can have a strong impact on aquatic biogeochemical cycles and may improve models and the upscaling of aquatic GHG dynamics.