Experimental integrated aquaculture of fish and red seaweeds in Northern Portugal

Three potentially valuable red seaweeds, Chondrus crispus Stackhouse, Gracilaria bursa pastoris (S.G. Gmelin) P.C. Silva and Palmaria palmata (L.) O. Kuntze, collected in northern Portugal, were cultivated using the nutrient-rich effluents from a local turbot (Scophthalmus maximus Linnaeus) and sea bass (Dicentrarchus labrax Linnaeus) farm. The algae were cultivated in a two level cascade system. Several arrangements of the cascade system, stocking densities (3, 5, 7 and 8 kg m^− 2) and water fluxes (140 and 325 l h^− 1) were tested to optimize biomass yield and nitrogen uptake rate and efficiency. The yield and the total ammonium nitrogen (TAN) uptake of the three species were highly seasonal. Palmaria could not survive culture conditions during the summer when water temperature was above 21 °C. In the spring, Palmaria had an average yield of 40.2 (± 12.80) g DW m^− 2 day^− 1 and a nitrogen uptake efficiency (NUE) of 41.0% (± 17.26%). NUE expresses, in percentage, the average reduction in TAN concentration between the inflows and the outflows of the tanks. Chondrus performed better in summer with an average yield of 37.0 (± 11.10) g DW m^− 2 day^− 1 and removing 41.3% (± 17.32%) of nitrogen. Gracilaria grew year round, but also performed better during spring/summer, producing an average of 29.1 (± 2.90) g DW m^− 2 day^− 1, and only 7.3 (± 5.08) g DW m^− 2 day^− 1 during autumn. Yield of C. crispus did not differ significantly when grown at two different stocking densities (5 kg m^− 2 and 8 kg m^− 2). On the other hand, Gracilaria had significantly higher yields at 5 than at 7 kg m^− 2. Better NUE, on average 76.7% (± 22.13%), was also obtained with 5 kg m^− 2 stocking density and only 63.8% (± 24.62%) with 7 kg m^− 2. The yield of Gracilaria increased significantly with the increase of water flux from 140 to 325 l h^− 1 and more nitrogen was removed from the water. However, NUE decreased from 48.4% to 33.4% at 140 and 325 l h^− 1, respectively. Biofiltration was highly improved by a cascade system with a NUE as high as 83.5%.