Nitrogen Budget Illustrates the Efficiency of Co-Farming Sea Lettuce (Ulva fenestrata) with Cold-Water Fish in a Marine Recirculating Aquaculture System

Integrated multitrophic level aquaculture (IMTA) in land-based recirculating aquaculture systems (RAS) has been proposed as an effective nitrogen bioremediation strategy. It can improve the utilization of added nitrogen through further biomass production, which increases the sustainability and efficiency of the farming system. Few studies have evaluated how co-farming in cold-water land-based marine recirculating aquaculture systems affects the nitrogen utilization; here, we quantify the effects of co-farming sea lettuce (Ulva fenestrata) with Atlantic wolffish (Anarhichas lupus) and rainbow trout (Oncorhynchus mykiss) in such a system. Nitrogen budgets were used to evaluate the efficiency of the system by summarizing all processes, both inputs and outputs. In alternating four-week periods for four months, the recirculating system held either only fish or fish together with sea lettuce. Fish biomass held 90–91% of the system nitrogen, while 9–10% was available as dissolved inorganic nitrogen. The nitrogen uptake by the sea lettuce biomass was quantified to 1.0–1.5% of the total nitrogen input. Extrapolations based on these results show that a wet weight ratio of sea lettuce:fish of 0.7 would be optimal for remediation of all added nitrogen.