Unique roles of functional group of submerged macrophytes on ecosystem functions

The presence of submerged plant communities and the restoration of their diversity have critical impacts on aquatic ecosystem function, especially in eutrophic lakes. However, we lack a holistic understanding of how specific species and functional groups and their richness of submerged macrophytes affect the community productivity and water quality in aquatic ecosystems, and how these effects vary across water nutrient gradients. Here, we performed a mesocosm experiment by manipulating species richness and functional richness from a species pool of 22 submerged macrophytes to test what are the ecological implications of shifts in macrophyte diversity under two nutrient levels. The results showed that species richness significantly enhanced community productivity only at low nutrient levels, while functional richness had no marked effects on community productivity across nutrient levels. The improvement of water quality by increasing submerged plant diversity (species and functional group richness) is often efficient, despite its context-dependent effect (e.g., nutrient level, specific parameters). Specifically, our results demonstrated a remarkable role in distinct functional groups (FG). That is, the presence of a particular functional group characterized by high leaf dry mass content (LDMC) and small specific leaf area (SLA) significantly enlarged the reduction of total nitrogen (TN) across nutrient levels, as well as the reduction of total phosphorus (TP) and chemical oxygen demand (COD" "Mn")at low nutrient llevels Additionally, the presence of FG characterized by long florescence and great SLA significantly affected nitrate reduction at high nutrient level and the reduction of TN, "COD" "Mn", and conductivity at low nutrient level. The presence of FG, characterized by low LDMC and short shoot height, significantly enhanced the reduction of dissolved oxygen and pH. Our findings highlighted that the unique contributions of functional groups of macrophyte communities to water quality improvement should be taken into account when designing restoration strategies.