Biomass production and nutrient dynamics in a low-trophic IMTA system integrating sea urchins, polychaetes and macroalgae

Integrated multi-trophic aquaculture (IMTA) can improve resource efficiency by converting dissolved and particulate wastes from fed species into biomass of extractive organisms. This study investigated the performance of a three-component IMTA system combining the sea urchin Paracentrotus lividus as the fed species and the polychaete Hediste diversicolor and the macroalga Ulva rigida as the extractive species. A 5-week trial was conducted with three treatments differing in system configuration: (i) U. rigida monoculture; (ii) IMTA including all three species; (iii) IMTA with P. lividus and H. diversicolor . Specific growth rate (SGR) and survival were assessed together with nitrate and phosphate concentrations and the biochemical composition of the extractive species. Polychaetes exhibited high SGR (7.1–7.8% day −1 ) and survival (85–86%), with protein and lipid contents of approximately 44% and 10% DW, respectively, and no significant differences between treatments. Nitrate concentrations ranged from 50 to 73 mg L −1 in U. rigida monoculture, declined from 8.8 to 4.5–5.9 mg L −1 in IMTA with U. rigida , and remained within 6.2–8.4 mg L −1 in IMTA without macroalgae. Phosphate concentrations ranged from 3.5 to 5.0 mg L −1 in monoculture and decreased from 2.6 to 2.7 to approximately 2.0 mg L −1 at mid-trial in both IMTA treatments, before increasing in the final week. U. rigida showed a non-significantly higher mean SGR in monoculture than in IMTA (4.1 vs 2.0% day −1 ; p = 0.111), whereas IMTA-grown biomass showed numerically higher total fatty acid concentrations (25 μg mg −1 DW) and an n-3/n-6 ratio of approximately 11. These results support the short-term technical feasibility of this novel low-trophic IMTA configuration for producing nutritionally valuable polychaete and macroalgal biomass from sea urchin waste-derived nutrients.