Fish aquaculture produces a nutrient-rich (nitrogen and phosphorus) water that may affect the surrounding environment . Aquacria Piscícolas S.A. is a company dedicated to an intensive aquaculture of Solea senegalensis , which operates in recirculating aquaculture system (RAS). Although these systems minimize negative impacts, part of the RAS water is still discharged in the environment. I ntegrating a seaweed cultivation unit with these fish farms may be an effective way to reduce nutrients levels from these waters but also to growth biomass for valorization – stimulating sustainability and circular economy.
To improve nutrient bioremediation by seaweed , the production factors culture density and flow rate were optimized during a 3-month trial (January to March) . Ulva sp. was selected , based on its growth rate and nutrient removal. Th ree density levels (1, 2 and 3 kg m-2) and two water flows levels (low–LWF and high–HWF) were tested. Physicochemical parameters were measured and seaweed growth rate (RGR) and productivity were assessed every week. N utrient removal was measured monthly . Cultivation conditions are currently being optimized throughout the year, thus these are preliminary results .
LWF was 57.1 ± 18.6 L h-1 and HWF was 165.0 ± 21.9 L h-1. LWF led to lower temperatures and pH in the seaweed cultivation tanks than HWF. Salinity was similar for both water flows (table 1). In general, RGR and productivity increased from January to March at all densities. T he highest RGR (13.6 ± 1.6% day-1 ) was registered for the 1 kg m-2 of density. P roductivity was similar for all densities, averaging 234.0 ± 61.4 g DW m-2 wk-1 . Considering N removal, the highest values were obtained in January, at the lowest density.
Results indicate that the best conditions to achieve higher growth rates and N removal are low cultivation density and higher flow. Increase overtime may be related to higher light availability. Since RAS water temperature is around 20 ºC, higher flows during this period (January to March ) resulted in a more stable temperature, leading to higher growth.