Growth, protein and carbohydrate contents in Ulva rigida and Gracilaria bursa-pastoris integrated with an offshore fish farm

Low-technology practices are generally the rule when cultivating marine macroalgae, and they do not necessarily comply with sustainability requirements. When integrated with other marine organisms in land-based setups, seaweed culture can be sustainable also providing environmental benefits. Major challenges of such integrated aquaculture systems, however, are in sea-based setups. The current study examined the growth rates of Ulva rigida and Gracilaria bursa-pastoris, as well as their protein and carbohydrate contents, when exposed to different distances from an offshore, fed-fish cage system. Nutrient levels in seawater were consistently high downstream from the fish cages, significantly enhancing the specific growth rates and cellular contents of starch and soluble protein in these two seaweeds. Specifically, daily maximal growth rates were 17 % day⁻¹ for U. rigida and 10 % day⁻¹ for G. bursa-pastoris, maximal starch contents were 22 and 21 %, respectively, and maximal protein contents were on a dry weight basis 7 and 13 %, respectively. When repositioned at low ambient nutrient levels for 48 h, the starch and the carbohydrate levels increased by 129 and 131 % for U. rigida and by 198 and 150 % for G. bursa-pastoris, respectively. Altogether, this study supports implementing future viable mean of sustainable macroalgae cultivation by taking advantage of excessive nutrients released from an offshore fish farm.