Water reuse is a focus of inland brackish water recirculating aquaculture systems (RAS) due to the cost of salt and challenges with saline effluent discharge. Aquaponics systems are designed to minimize water discharge and utilize plant growth as a means of nutrient removal from system water, notably nitrate that builds up over time in RAS. Brackish water aquaponics research has been limited and mainly focused on halophytic plants. Marketing halophytic species can be a challenge, especially for inland locations. Kale is marketable and we have established its salt tolerance up to 20 ppt. salinity. This trial analyzed coupled (C) and decoupled (DC) aquaponics systems at 15 ppt. salinity with an 8-week growout of Winterbor kale and Pacific white shrimp.
Replicated treatments in UVI-based systems (1718 L) were salted using a least-cost salt mix and homogenized with reused shrimp water to attain an initial nitrate concentration of 100 mg/L. Shrimp were stocked at 300 m-3 and fed a commercial growout diet (35% protein). Kale seeds were germinated in fresh water and acclimated to salt over a 15-day period; each system was stocked with 66 kale plants. Weekly plant growth metrics (height, number of leaves) were measured on a fixed random selection to include 10% of the plants per system. Plant metrics measured on selected plants at harvest included height, number of leaves, chlorophyll content index (CCI), leaf area index (LAI), wet and dry weights. Total plant wet weights (biomass, roots) from each system were also measured. Water quality parameters measured twice daily included temperature, dissolved oxygen, pH, salinity and conductivity. Total ammonia-nitrogen, nitrite and nitrate were measured thrice weekly; phosphate and potassium were measured weekly and iron was measured twice weekly with additions of DTPA 10% chelated iron throughout the trial to maintain levels at 3 mg/L.
DC systems had significantly higher pH than C systems; C systems had significantly higher Fe levels than DC systems (58.6 g Fe added to C systems, 65.7 g added to DC). There were no significant differences in any other water quality parameter measured. Higher nitrite spikes in DC systems resulted in significantly less total feed input than in C systems, which showed more stable water quality overall. Coupled systems reduced nitrate levels by 19.1% and DC systems by 6.3%. Overall, daily nitrogen uptake per plant was 7.5 mg in C systems and 2.7 mg in DC systems. Plants in C systems outperformed those in DC systems in all metrics except CCI and root to shoot ratio, with significantly higher wet and dry weights, height and system wet weights (roots and shoots). Plant survival in C systems was 95.5%, DC systems was 92.4%. Future research will involve engineering the UVI design to better suit shrimp production and biofloc technology, and modification of DC system protocols to improve plant production.