Role of submerged macrophytes with different growth forms in CO2 and CH4 emissions: Insights from a year-long mesocosm study

Abstract Submerged macrophytes play a vital role in the carbon cycling of lake ecosystems. However, the extent to which contrasting macrophyte growth forms—bottom-dwelling versus canopy-forming—control annual CO2 and CH4 emissions is unresolved, limiting evidence-based guidance for lake restoration aimed at carbon mitigation. We conducted a fully replicated, year-long outdoor mesocosm experiment under natural temperature and light regimes to quantify greenhouse-gas fluxes from monospecific stands of four widespread macrophytes: bottom-dwelling Vallisneria denseserrulata and canopy-forming Ceratophyllum demersum, Myriophyllum spicatum and Hydrilla verticillata. Monthly diffusive flux measurements were integrated with high-resolution data on water chemistry, macrophyte biomass, zooplankton, phytoplankton and the functional genes mcrA and pmoA for methanogenic and methanotrophic communities. Canopy-forming macrophytes reduced annual CO2 fluxes by 5-13 mol m-2 yr-1 relative to bottom-dwelling treatments, with Hydrilla and Myriophyllum systems functioning as net carbon sinks (negative CO2-equivalent balance), whereas Vallisneria and Ceratophyllum remained sources. Canopy-forming macrophytes exhibited higher biomass than bottom-dwelling forms, enabling greater nutrient uptake and correspondingly higher CO2 fixation via photosynthesis. CH4 release was strongly modulated by plant biomass and associated redox conditions. These results demonstrate that canopy-forming macrophytes offer superior potential for CO2 mitigation and CO2-equivalent balance, providing essential trade-off information for managers selecting plant assemblages for climate-smart lake restoration