Abstract
<jats:p>Eutrophic shallow freshwater ecosystems often develop floating filamentous microalgae on their surface during spring and summer, known as floating algal beds (FLAB). In recent years, FLAB has become more frequent and pronounced due to rising temperatures and drier conditions, sometimes even covering the entire surface of water bodies. New evidence suggests that phytoplankton can produce methane (CH4) and it has also been reported that algal-derived carbon can enhance CH4 production in the sediments, raising the possibility that FLAB may increase CH4 emissions. However, FLAB may also reduce CH4 emissions by decreasing CH4 diffusion at the air-water interface and/or trapping CH₄ bubbles below the mat, thereby increasing the opportunity for microbial CH4 oxidation before release to the atmosphere. Consequently, the net impact of FLAB on carbon emissions from freshwater ecosystems remains unclear. To address this knowledge gap, a field experiment consisting of a treatment covered with FLAB and a control (no FLAB) was carried out in a eutrophic shallow ditch. Diffusive and ebullitive CH4 emissions increased by 12-fold and 5-fold, respectively, in the presence of FLAB, resulting in a 5-fold increase in CO2-equivalent emissions. This increase appears to be linked to enhanced CH4 production in the sediments fuelled by FLAB derived organic carbon, which was mostly relevant when anoxia developed beneath the mat. Overall, these findings indicate that FLAB blooms can substantially amplify carbon emissions from shallow aquatic ecosystems, with effects driven not only by FLAB biomass but also by mat-induced changes in water column dynamics and by intrinsic sediment properties.</jats:p>