Abstract: Intensive aquaculture generates nutrient-rich effluents that contribute to eutrophication if discharged untreated. Seaweed cultivation offers a sustainable approach to mitigate this impact while producing valuable biomass. This study evaluated the seasonal biofiltration efficiency and biomass production of Ulva spp. under simulated flatfish aquaculture effluent conditions using two experimental designs: parallel tanks and serial tanks with varying hydraulic retention times (HRTs). Ulva spp. showed higher nutrient removal and growth in summer compared to winter. In the parallel system, ammonium and phosphate removal exceeded 70% in summer but dropped below 50% in winter. In the serial system, nutrient removal peaked at a 24 h HRT, with ammonium and phosphate removal above 90%, while longer retention (36 h) did not improve performance. Biomass production followed a similar seasonal pattern, with higher yields in spring and summer. These results demonstrate that Ulva spp. are effective biofilters for aquaculture effluents, with performance strongly influenced by season and HRT. A two-tank serial configuration with 24 h HRT optimizes nutrient removal without compromising biomass production, providing useful insights for the design of sustainable seaweed-based biofiltration systems.
