Brackish water recirculating aquaculture systems (RAS), especially in inland locations, have unique challenges in water reuse and disposal of effluent due to high salt content. Although there are marine and halophyte plant species that can be used in brackish water aquaponics systems, marketability of these species may be challenging. Brackish water aquaculture is established in Kentucky and the surrounding region, presenting a need for sustainable effluent management. Kale belongs to the Amaranthacaea family, members of which show a general tolerance to sodic soils. Kale has become a very popular leafy green in recent years and performs well in freshwater aquaponics systems. This study examined the salt tolerance of kale and its viability for production in a brackish water aquaponics system.
Three trials were conducted using 18 L tanks, each aerated with a 5 cm ceramic air stone and located under LED grow lights providing 12 hours of light per day. In each study, four plants were stocked in each bucket and floated using 2.5 cm polystyrene foam; each treatment contained three, randomly assigned replicate tanks. The first 21- day study was conducted at five salinities (0, 5, 10, 15 and 20 ppt.) to determine salt tolerance and effects on plant performance. A second 28-day trial analyzed two acclimation times (A1 and A2) based on plant age (28 and 35 day old seedlings), two salinities (10 and 20 ppt.) and a treatment assessing the plant's response to excess nitrate (80 mg/L NaNO₃). Treatments included A1-10-N, A1-10, A1-20-N, A1-20, A2-10-N, A2-10, A2-20-N and A2-20. The third 21-day trial compared unused, clean water with reused shrimp water, and the effect of shrimp (Litopenaeus vannamei) on plant performance. Treatments included clean water with plants and fertilizer (CW-P-F), clean water with shrimp and plants (CW-S-P), clean water with fertilizer and no plants (CW-NP-F), shrimp water with plants (SW-P), shrimp water with shrimp and plants (CW-S-P) and shrimp water with no plants (SW-NP). At the conclusion of each trial, plants were harvested and a series of growth parameters measured. Ammonia, nitrite and phosphorus were measured weekly, and initial and final water samples were analyzed for a suite of dissolved nutrients and elements.
Although growth was reduced with increased salinity, kale had 100% survival in all trials and salinities and produced marketable foliage. Acclimation to brackish water using older plants resulted in better plant performance overall. Average chlorophyll content index (CCI) was highest at 10 and 15 ppt. with significant differences between both salinity and acclimation time. Average dry weights of kale biomass were consistent across salinities except for the youngest plants in the acclimation trial that showed significantly lower weight with increased salinity. Results from the third trial are pending. Future studies will focus on a kale grow out with multiple harvests in a brackish water aquaponics system with shrimp, in addition to testing other kale varieties and evaluating consumer preferences for brackish water-produced kale.