Assimilation of inorganic nutrients from salmon (Salmo salar) farming by the macroalgae (Saccharina latissima) in an exposed coastal environment: implications for integrated multi-trophic aquaculture

This paper investigated the assimilation of dissolved inorganic nitrogen (DIN) in Saccharina latissima in proximity to salmon cages in coastal waters. The bioassays were performed on plants from three stations located in the vicinity of a salmon farm (Salmo salar) in exposed waters at Tristein (63A degrees 52' N, 9A degrees 37' E) in Central Norway. The growth, the C and N content, and the nitrogen isotope ratios (delta N-15) of S. latissima were monitored over 1 year. The DIN concentrations in seawater were higher at the salmon farm stations than at the reference station during the winter, and the N/P ratio at the salmon farm stations was higher from September to January and in June. S. latissima at the salmon farm stations grew faster than at the reference station. The length of S. latissima increased by 50 % when integrated with the salmon farm compared to the reference station. The N content of S. latissima was positively correlated to the DIN concentration in seawater (p < 0.05), but the increased N supply from salmon did not result in N accumulation in S. latissima at the salmon farm station because of the dilution by a higher growth rate. The delta N-15 in S. latissima was higher at the salmon farm station from April to June and changed in the direction of the delta N-15 signature in urine. This indicated that N in S. latissima at the salmon farm station partly originated from the salmon. One hectare of S. latissima may absorb 0.8 similar to 1.2 t N during one growth season. Large-scale cultivation of S. latissima should be considered to mitigate the environmental effects of DIN wastes from salmon farms.