Aeration is a critical component of the success of seaweed tumble culture, as it improves the growth and bioremediation potential of seaweeds. Specifically, aeration is used to tumble the seaweeds, thinning the diffusive boundary layer (DBL) and enabling seaweeds to absorb nutrients from the water while also fragmenting them. Optimizing aeration is important to the success of tumble culture, including the costs of operation. Ulva species have been considered ideal biofilters to remove dissolved nutrients in integrated multi-trophic aquaculture (IMTA) systems, and Ulva biomass has wide applications for human and animal food, biofuel, medicine, etc. Therefore, it is important to understand the effects of aeration on the performance of Ulva and associated cost : benefits in IMTA systems.
This study was conducted in an IMTA system at Hubbs-SeaWorld Research Institute in Carlsbad, California. White seabass (Atractoscion nobilis) were stocked in a 26,000 L raceway at a density of 20.0 kg/m³, with effluent gravity-fed into replicate 175L tanks containing Ulva australis stocked at 0.5 kg/m². Three aeration levels were set at 0.09 (low), 0.14 (medium), and 0.29 (high) L air/L seawater/min, with three replicates for each aeration level. U. australis was also cultivated in three tanks as that received raw seawater without nutrient supplementation and aeration set at medium. U. australis biomass was recorded and re-set to the original stocking biomass weekly. Seawater exchange rate was set at 63 vol./day. TAN, NO2–N, NO3–N, PO4–P, and inorganic carbon (IC) were measured weekly. Photosynthetic parameters were assessed using a PAM fluorometer. Temperature, light intensity, dissolved oxygen, pH, and total dissolved gas were recorded daily. The study was run for four weeks.
Preliminary findings indicated that aeration levels did not significantly impact the specific growth rate (SGR) and productivity of U. australis (p=0.58 and 0.61) during the experimental period. SGR ranged from 20.89 to 34.89%/day, 25.70 to 32.83%/day and 17.71 to 34.20%/day for low, medium, and high aeration levels, respectively, and accordingly, productivity was in the range of 11.29-36.06, 12.96-37.85 and 8.55-39.94 g DW/m2/day. SGR and productivity also were not different between treatments in each sampling week (p=0.51-0.67 for SGR, 0.29-0.86 for productivity). U. australis had significantly higher SGR cultivated in effluent from A. nobilis tanks than raw seawater when aerated at low and medium aeration levels (p=0.020 and 0.016, respectively), but no difference in productivity (p=0.51 and 0.76). Under high aeration, U. australis did not show a difference in SGR and productivity (p=0.57 and 0.65). These initial findings suggest a potential cost reduction in the tumble culture of U. australis by lowering aeration levels to as low as 0.09 L air/L seawater/min. Results for water quality, nutritional composition, and photosynthetic capacities are still pending analysis.