Growth and Nutrient Uptake of Palmaria palmata in Small-Batch Cultures with Effluent Water from a Commercial Salmo salar Recirculating Aquaculture System

Dulce (Palmaria palmata) is a high-value macroalga that is increasingly being cultivated, with strong potential for waste valorisation in nutrient-rich aquaculture systems (RASs). This study evaluated P. palmata growth in, and nutrient uptake from, commercial Atlantic salmon RAS effluent. A 12-week bench-scale experiment cultivated wild-collected P. palmata (average 10 g fresh weight, FW). These were grown in 1 L glass beakers at three effluent dilutions (25%, RAS25; 50%, RAS50; 100%, RAS100) and in seawater (SW) using 10 replicates. The water samples were analysed for ammonium nitrogen (NH4-N), nitrate nitrogen (NO3-N), and orthophosphate (PO4-P) using a spectrophotometer. RAS25 and RAS50 exhibited 100% survival and maintained a dark red colour, with RAS50 achieving the highest specific growth rate (0.49 ± 0.13% day−1), significantly higher than that of SW and RAS25. In contrast, RAS100 led to complete disintegration by 4–8 weeks with significant colour degradation. SW also exhibited reduced colour and 50% mortality by week 12. Sori’s presence was highest in RAS25/RAS50 (up to 80% at week 8 in RAS50), low in SW (10%), and absent in RAS100. The NH4-N uptake was notably 3× higher than that of NO3-N (0.16 vs. 0.05 mg g FW−1 day−1), without differences among the groups. The PO4-P uptake was significantly higher for RAS50 (0.07 mg g FW−1 day−1) than for RAS100. P. palmata performed best in the diluted RAS effluents, as undiluted conditions led to acute tissue disintegration; the nitrogen and phosphorus uptake from the RAS effluents demonstrates significant potential for nutrient valorisation.