Internal phosphorus cycling in macrophyte-dominated eutrophic lakes and its implications

Macrophyte-dominated eutrophication (MDE) generally exhibits different characteristics from phytoplanktondominated eutrophication (PDE). However, the significance of P cycling on eco-environmental management of MDE lakes is still not fully recognized. In this study, P-cycling mechanism was studied in a typical MDE lake (Wuliangsu Lake, China) based on a Before-After-Control-Impact design, taking advantage of the contrasting states between its 13 sub-lakes (with/without macrophytes and with/without external water flow). Our study demonstrated that P cycling predominantly occurs through macrophyte 4-* sediment in the MDE lakes, rather than water 4-* sediment as in PDE lakes; the biodynamics of the macrophytes acts as a primary driving force for this self-enforced P cycling. Our findings challenge the present lake eutrophication management strategies, which primarily limited to the water nutrient content, and demonstrate that successful MDE lake restoration should focus on stressors caused by the sustainable macrophyte-sediment P cycling. Macrophyte harvesting immediately before withering is recommended as the most sustainable technique in environment management for periodically frozen shallow MDE lakes. By this technique, sediment P can be gradually pumped up by the overgrown macrophytes each year until the advent of an alternative stable state (low sediment P, small biomass, and higher diversity), thereby forming a positive feedback loop macrophyte harvesting-* less sediment P-* less macrophyte-* higher diversity. Also, the catastrophic shift from MDE to PDE is no longer a concern. Furthermore, the macrophyte debris will not pose a problem as long as the macrophytes are removed during the harvest.

Automated summary

In macrophyte-dominated eutrophication (MDE) lakes, the cycling of phosphorus (P) predominantly occurs through macrophyte sediment, challenging the current strategies for lake eutrophication management. This study suggests that successful restoration of MDE lakes should focus on stressors caused by sustainable macrophyte-sediment P cycling. The technique of macrophyte harvesting before withering is recommended as the most sustainable method for managing periodically frozen shallow MDE lakes.