Lake Water Quality Under Biomass Removal Scenarios: Integrating Observations and Modeling Approaches

Eutrophication, driven by excessive nutrient inputs from agriculture, wastewater, and aquaculture, remains a pressing challenge for freshwater ecosystems. In response, the EU Nature Restoration Law (2024) sets ambitious targets for restoring degraded ecosystems, emphasizing the need for effective and scalable lake management strategies. In this study, we assessed current water quality in Lake Simnas (Lithuania) and applied dynamic modeling to evaluate two in-lake restoration scenarios: removing scum-forming cyanobacteria and harvesting emerged macrophytes. While both interventions reduced local biomass, neither led to substantial improvements in chlorophyll-a concentrations or total phosphorus levels. Macrophyte harvesting was particularly ineffective because of the low phosphorus content, limited spatial coverage, and slow growth. In contrast, simulations showed that a 50% reduction in external phosphorus inputs led to a significant improvement in water quality, including a 58% drop in mean TP and a 47% decrease in peak chlorophyll-a. These findings support prioritizing catchment-scale nutrient reduction over isolated biomass removal and highlight the importance of sustained monitoring and integrated management for restoring shallow eutrophic lakes.