Biofiltration allows for environmentally sustainable mariculture. An intensive, biofiltered recirculating integrated system producing fish and seaweed on a semi-commercial scale was evaluated with respect to production and to nutrient and heat budgets. The system consisted of abalone (Haliotis discus hannai) and sea urchin (Paracentrotus lividus) tanks, an intensive fishpond (Sparus aurata), and a three-stage Ulva lactuca biofilter, which cleaned and recirculated 50% of the effluent back to the fishpond. To preserve water heat, the shellfish and fishpond units were both covered with greenhouses; the biofilter unit was covered with a greenhouse only during winter. Seaweed yield was variable and averaged 94 and 117 g m 2 day 1 (fresh weight) in periods with and without greenhouse cover, respectively. Protein content of U. lactuca averaged above 34% of dry weight. The biofiltration of only 50% of the water through the seaweed biofilter reduced the export of dissolved nutrients to the environment by nearly 30%. Peak ammonia excretion by the morning-fed fish coincided with maximum seaweed light-dependent ammonia uptake and concentrations of ammonia in the fishpond remained within nontoxic limits. Also, daytime photosynthesis of U. lactuca (uptake of CO2) met fish respiration (production of CO2), thus balancing fishpond pH levels within safe limits regarding ammonia toxicity. Daytime oxygen demand by the fish was partially met by the photosynthetically generated oxygen. Before covering the biofilter with a greenhouse, it lost much heat, reducing the temperature in the fishpond. Following the greenhouse covering of the biofilter, heat loss ceased and consequently the fishpond temperature was raised. Recirculation through the biofilter improved system sustainability; it reduced water use, lowered negative environmental impact, and maintained stable and safe water quality conditions in the fishpond.