Evaluating the Sustainability of Using Phoslock and Daphnia to Mitigate the Effects of Eutrophication Under Climate Change Conditions

Eutrophication, driven by excessive amounts of nitrogen and phosphorus, remains one of the most serious threats to freshwater ecosystems. Algal blooms cause the biodiversity loss and a decline in water quality. Its impacts are expected to intensify with climate change, as global warming accelerates algal proliferation and alters species interactions. Thus, sustainable management requires methods that integrate biological and chemical processes to stabilise aquatic systems. This study aimed to evaluate the effectiveness of various eutrophication control strategies in reducing algal biomass. Specifically, in the experiment it was compared the impact of biotic regulation via Daphnia magna (Straus) grazing as a top-down mechanism, abiotic phosphorus binding by Phoslock® as a bottom-up approach and the combined application of these methods. A 22-day laboratory experiment was conducted using the green algae Tetradesmus obliquus. All treatments received the same algal supply of 1 mg Corg L-1, and two temperature regimes (18°C and 24°C) were applied to reflect the influence of climate warming. Responses were assessed using chlorophyll concentration as an indicator of algal biomass. Alongside the life-history traits of Daphnia was evaluated, such as growth rate, reproduction, body size and population dynamics. The results showed that Phoslock® alone was unable to suppress algal proliferation at either temperature, as chlorophyll concentrations continued to increase despite phosphorus binding. Daphnia grazing reduced algal biomass in both thermal regimes, although the reduction was statistically significant only at 24°C. The combined treatment was the most effective, significantly lowering chlorophyll concentrations at both temperatures. This synergy of top-down grazing and bottom-up phosphorus binding offers a sustainable strategy to mitigate eutrophication and maintain water quality under current and future climate conditions.