Remediation of nutrient-rich, brackish fjord water through production of protein-rich kelp S. latissima and L. digitata

Nutrient-rich, brackish seawater arises in the southwestern fjords of Norway where much freshwater runoff from the mountains is combined with nutrients from agriculture and smolt farms located along the fjords. This work evaluates the potential of short-term remediation of the nutrients from the fjord water by means of macroalgae. In the first experiment, seawater with a salinity of 25–34‰ without nutrients was compared with brackish water with a salinity of 18‰ with added nitrate and phosphate. After 13-day exposure to the elevated nutrient level, the protein and N content of young sporophytes of Saccharina latissima and Laminaria digitata increased by 25–47 and 31–33%, respectively. In a second 5-day experiment, elevated nutrients increased the N content by 40% in young sporophytes of S. latissima while it was unaffected in older sporophytes. The N content in L. digitata increased 34% in young sporophytes and 20% in older sporophytes. Nitrate contributed only to a small fraction of the total N content in both species even with nutrient supply (maximum 6% of the total). The N:P ratio (6–8 w/w) in the algae was unaffected by the nutrient exposure in both experiments. In experiment 1, elevated nutrients decreased the content of ß-glucans from 11.2% (w/w) to 4.1% (w/w) in S. latissima while it was unaffected in L. digitata (about 5% w/w). Iodide content decreased to about half in the nutrient-rich, brackish water in S. latissima while it was unaffected in L. digitata. Due to the rapid change in the protein content caused by nutrient supply, it was suggested that moving floating kelp-rigs to nutrient-rich locations could be used to enhance the protein content of the algae prior to harvest. Since nitrate uptake was found to take place also in darkness, a cultivation system that can be submerged to deeper nutrient-rich fjord water during night and brought up to the surface at daytime was suggested.