Floating algal beds and aquatic methane emissions:a potential positive ecosystem-climate feedback loop

Eutrophic shallow freshwater ecosystems often develop floating filamentous microalgae on their surface during spring and summer. In recent years, this phenomenon has become more pronounced due to rising temperatures and drier conditions, with floating algae sometimes even covering the entire surface of water bodies. These floating mats, known as Floating Algal Beds (FLAB), are primarily composed of phytoplankton from the group Chlorophyte. New evidence suggests that phytoplankton can produce methane (CH₄), raising the possibility that these floating beds may represent overlooked sources of CH₄ emissions to the atmosphere. However, FLAB may also reduce CH₄ emissions by decreasing the CH₄ diffusion at the air-water interface and/or trapping CH₄ bubbles. Consequently, the net impact of FLAB on CH₄ emissions in freshwater ecosystems remains unclear. To address this knowledge gap, this study aims to investigate how FLAB influence CH₄ dynamics by examining both biological processes (such as CH₄ production pathways and CH₄ oxidation) and physical factors (as CH₄ bubble retention). To achieve this, we conducted field mesocosm experiments in a eutrophic ditch in the Netherlands during the summer of 2024. Eight mesocosms were deployed, with four containing FLAB on their surface and four controls without FLAB. The mesocosms were closed at the sides to prevent lateral transport and open at the surface and bottom allowing for the inclusion of CH₄ sediment production, CH₄ oxidation, CH₄ bubble dissolution, CH₄ diffusive flux at the air-water interface, and potential CH₄ production in the water column (including contributions from FLAB). Over a five-day period, we monitored all these CH₄ pathways alongside several other limnological parameters in both the treatment and control mesocosms. Additionally, we also incubated sediment, water and FLAB separately, to test for CH₄ production and oxidation in each one of these compartments. Preliminary results indicate that mesocosms with FLAB exhibited CH₄ diffusive emissions on average ten times higher compared to the control mesocosms. Further analysis is needed to determine whether these elevated emissions originate from CH₄ production by FLAB, increased sediment and/or water column CH₄ production, or reduced CH₄ oxidation in the presence of FLAB; but these preliminary findings already suggest that FLAB significantly influences CH₄ dynamics in eutrophic systems. This points to a potential increase in the climate-ecosystem feedback loop, were climate change drives higher temperatures and periods of drought, leading to more stagnant waters. This, in turn, promotes the growth of FLAB, which enhances CH₄ emissions.