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The growth, tissue content and nutrient removal rates of Ulva spp., when exposed to moderate to high nitrogen (0.5–5 mmol L−1) and phosphorus (0.01–0.9 mmol L−1) concentrations, were examined to get a better understanding of recirculating IMTA (Integrated Multi-Trophic Aquaculture) systems with fish and seaweed. It was hypothesized that fish waste effluents might lead to unfavorable nutrient stoichiometry and/or toxic conditions, which might harm seaweeds and, specifically for the present study, reduce Ulva spp. performance. Results demonstrate that: (I) the unfavorable N:P stoichiometry (N:P ≠ Atkinson atomic ratio of 30:1) did not restrict Ulva spp. growth nor tissue content; this indicates that supply of both nutrients exceeded the minimum requirements; (II) a high orthophosphate concentration (0.9 mmol L−1) was toxic to Ulva spp., whereas (III) a high nitrate concentration (5 mmol L−1) did not inhibit phosphorus uptake; (IV) Ulva’s growth was not enhanced when nitrate was exchanged for similarly high ammonium concentrations. However, tissue nitrogen content was 1.4 times higher when exposed to ammonium than nitrate, suggesting that the former N-form was stored faster in the seaweed’s tissue. Therefore, other factors must have limited growth with the high ammonium concentrations. This study also highlights the importance of relatively long acclimatization periods (one week) when maintenance uptake (Vm) is evaluated, as surge uptake (Vs) may result in considerably different and more variable rates. Results of this study contribute to a better understanding of the application of Ulva spp. as extractive component in closed IMTA systems, thus advancing sustainable and circular production techniques.

One of the bottlenecks for commercial implementation of integrated multi-trophic aquaculture (IMTA) is the difficulty in quantifying its environmental performance. We reviewed a large body of literature to determine the variability in nutrient dynamics within different IMTA systems (open sea-cages, land-based flow-through and recirculating aquaculture systems), with the aim to provide a generic framework to quantify nutrient retention efficiencies in integrated aquaculture systems. Based on the ecophysiological requirements of the cultured species, as well as the response of “extractive” species to waste from “fed” species, the maximum retention efficiency was defined for a conceptual four-species marine IMTA system (fish–seaweed–bivalve–deposit feeder). This demonstrated that 79%–94% of nitrogen, phosphorus and carbon supplied with fish feed could theoretically be retained. In practice, however, various biological and environmental factors may limit retention efficiencies and thereby influence the bioremediation of IMTA systems. These biological (waste production, stoichiometry in nutrient requirements) and environmental (temporal and spatial connectivity) factors were therefore evaluated against the theoretical reference frame and showed that efficiencies of 45%–75% for closed systems and 40%–50% for open systems are more realistic. This study is thereby the first to provide quantitative estimates for nutrient retention across IMTA systems, demonstrating that a substantial fraction of nutrients released from fish culture units can be retained by extractive species and subsequently harvested. Furthermore, by adapting this framework to the design and the condition prevailing for a specific IMTA system, it becomes a generic tool to analyse the system's bioremediation potential and explore options for further improvement.