A hybrid biomanipulation strategy to regulate seasonal dynamics of phytoplankton in a subtropical mesocosm experiment

Eutrophication in subtropical enclosed water ecosystems (EWEs) poses a critical environmental challenge, driven by excessive phytoplankton proliferation. These water bodies are characterized by high area-to-volume ratios, poor water circulation, limited hydrodynamics, and simplified ecosystems inhibiting natural recovery, heightening vulnerability to algal dominance and water quality degradation. This study proposes a hybrid biomanipulation strategy integrating top-down (filter-feeding fish (Hypophthalmichthys molitrix) and bivalves (Anodonta woodiana), FB) and bottom-up (submerged macrophytes (Vallisneria denseserrulata) and gastropod snails (Bellamya aeruginosa), SG) interventions to control phytoplankton dynamics. Through year-long mesocosm experiments and AQUATOX modeling, results showed that the combined FB/SG approach achieved over 64.5 % nitrogen and phosphorus removal, and 91.0 % chlorophyll-α reduction, outperforming individual FB or SG treatments (P < 0.001). Model simulations revealed that phytoplankton biomass declined exponentially with increasing FB biomass during warm seasons (spring, summer, and autumn, R² > 0.871), while SG enhanced zooplankton grazing pressure year-round by promoting resource competition and removing epiphytes. Notably, FB/SG overcame constraints of single approaches: FB alone increased water turbidity and nutrient recycling, whereas SG stabilized water clarity and facilitated nutrient uptake. To optimize long-term efficacy, this study proposes a dynamic management framework, involving the introduction of juvenile fish in autumn to suppress overwintering phytoplankton and reduce zooplankton predation under lower temperatures, followed by harvesting mature organisms the subsequent summer to remove accumulated nutrients and prevent biomass overload. This strategy aligns with seasonal metabolic shifts and leverages synergistic trophic cascades, offering a scalable restoration solution for subtropical EWEs. Our findings bridge temperate biomanipulation practices with subtropical ecological complexities, underscoring the necessity of adaptive seasonal interventions in combating eutrophication.