Growth rates of Gracilariopsis longissima, Gracilaria bursa-pastoris and Chondracanthus teedei (Rhodophyta) cultured in ropes: implication for N biomitigation in Cadiz Bay (Southern Spain)

Cultures of three native macroalgal species (Gracilariopsis longissima, Gracilaria bursa-pastoris and Chondracanthus teedei) were developed in open coastal waters of a shallow, potentially eutrophic bay in Southern Spain. Experimental trials to assess the technical feasibility of seaweed cultivation and nitrogen biofiltration potential were conducted between October 2014 and December 2015. Seaweeds were cultivated using submersible rafts in five periods of 9 weeks to identify the most suitable seasons and culture duration (3, 6 or 9 weeks). Using generalized linear models (GLMs), the most relevant environmental factors controlling the growth of the three seaweeds were identified. Maximum net growth rates varied between 1.39% day in C. teedei (winter, 6 weeks) and 4.71% day in G. longissima (autumn, 3 weeks). Overall, the best period to cultivate seaweeds in Cadiz Bay was from mid-winter to early summer. No clear effects of duration of the culture were observed for C. teedei and G. bursa-pastoris. Short culture periods of 3 weeks were more suitable for G. longissima. Tissue N contents generally were lower than the critical quota, and GLMs suggested a critical role of N limiting seaweed growth in this bay. Dissolved inorganic nitrogen was biofiltered from winter to early summer and revealed G. bursa-pastoris as the main biomitigator of nitrogen (up to 80 mg N m month in spring). This species also showed positive growth rates virtually during the entire study period and was the most suitable species for cultivation in this area.