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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.

Israeli aquaculture began in the 1920s, with common carp monoculture. This was followed by polyculture of carp with tilapias, grey mullet, and planktivorous carp. Scientific research on polyculture started in the 1950s and has since contributed to the global science and practice of green water aquaculture, especially with novel polyculture approaches and concepts. Today, the industry is characterized by intensive freshwater polyculture, implemented in earthen fish ponds and reservoirs. In the Mediterranean coastal plain, fresh, brackish, and marine water polyculture is carried out in semi-intensive fishponds. Polyculture in Israel is an entrepreneurial activity that combines ecological principles of Chinese polyculture with local technologies and objectives. The Biofloc approach (active suspension ponds, ASP), periphyton, and aquaponics, were developed in the 1980s in response to rising public and policymakers’ concerns and regulations on land use, pollution, use of chemicals, and organic manures. R&D on marine integrated multi-trophic aquaculture (IMTA) systems began in the early 1970s at the National Center for Mariculture (NCM) in Eilat. It started with sea bream and mullet in earthen seawater ponds, whose plankton-rich water recirculated through bivalve and macroalgae biofiltration modules. An advanced form of the concept was deployed in the early 1980s and was studied in detail using nutrient budgets. Several system models with fish, bivalves, and algae, on small and pilot scales, were studied and quantified. Abalone, sea urchins, shrimp, brine shrimp, Salicornia, and periphyton, were added to the Eilat marine IMTA models, beginning in the 1990s. Upon entering the third millennium, Israeli research further examined the relationship between the sustainability and economics of IMTA in world aquaculture.