Ulva spp. performance and biomitigation potential under high nutrient concentrations: implications for recirculating IMTA systems

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 (V_m) is evaluated, as surge uptake (V_s) 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.