From environment to biomass: trait-mediated effects on submerged macrophyte biomass in a subtropical shallow restored lake

Recovery of submerged macrophyte communities is crucial for the restoration of shallow eutrophic lakes, as macrophyte biomass here plays a key role in maintaining a clear-water state. However, the influence of environmental conditions on macrophyte functional traits mediating biomass production is unclear. We examined trait-mediated responses of Vallisneria denseserrulata to abiotic (water and sediment) and biotic (fish, neighbor plants) variables in 20 isolated restored experimental units in the shallow urban Lake Yiai, China, to elucidate how these variables affected trait expressions and, consequently, the biomass of V. denseserrulata. Vallisneria denseserrulata biomass varied widely across the restored units and was positively related to the contents of organic matter and nitrogen in the sediment and negatively so with water total nitrogen, turbidity, and fish biomass. However, the biomass and diversity of neighboring plants showed no significant relationship with the biomass of V. denseserrulata. Path analysis further indicated that water, sediment, and fish variables indirectly influenced biomass by modulating the traits of V. denseserrulata. High sediment nutrients content was associated with conservative plant traits, such as a high carbon content and a high carbon to phosphorus ratio, and enhanced aboveground allocation, i.e., increased shoot biomass, leaf length, and leaf number, leading to higher total biomass. In contrast, elevated water nutrients and turbidity favored belowground allocation, reducing total V. denseserrulata biomass. Fish biomass indirectly suppressed V. denseserrulata biomass by degrading water quality and subsequently promoting belowground allocation, while no significant direct contribution by grazing was found. Sediment, water, and fish control submerged macrophyte biomass through trait-mediated pathways. This study enhances our understanding of how environmental conditions affect the functional traits and biomass of submerged macrophytes, illustrating the role of trait-mediated processes in improving the success of submerged macrophyte recovery in subtropical shallow lakes.