Brackish-water aquaculture producers often face challenges with nitrate accumulation to the point it becomes toxic to animals. Disposing of high-nitrate water wastes both water and salt, can be toxic to terrestrial crops, and often cannot be discharged into municipal sewer systems. Using aquaponics to reduce waste discharge while producing marketable plants is gaining popularity globally; however, brackish water producers are typically thought to be limited to halophytes, or salt-tolerant plants only. Although halophytes grow well and remove nitrogen in brackish water systems, the plants are often unknown to consumers, or are not palatable. Finding non-halophyte species that are tolerant of salt and better known to consumers may allow producers to utilize aquaponics and reduce water and salt use.
Over 10 different plants have been tested at KSU to determine their suitability for brackish water aquaponics. Each plant is evaluated for salinity tolerance by growing plants in 0, 5, 10, 15, and 20 salinity. Plants that show high survival and growth at elevated salinities are used in further trials that examine plant performance in situations representative of shrimp farming conditions. Variables such as varying nutrient concentrations, plant age, acclimation strategies, decoupled versus coupled aquaponics with shrimp, and the effects of supplemental iron on chloride tolerance have been tested.
Kale has been a standout performer with 100% survival even at 20 salinity. Mustard has also shown high survival while only showing moderate growth reduction at higher salinities. A primary finding is the importance of an acclimation period. This acclimation process involves germinating plants in freshwater, then increasing salinity over a two-week period. Many plants show survival at 5, 10, and even 15 salinity when using this acclimation period. Changes in nutrient uptake and tissue concentrations have been noted, particularly replacement of Ca and K by Na, while Mg seems to be unaffected. Some plants appear to store excess Na in leaf tissue, while other plants (notably kale), appear to have lower amounts of Na in the leaves and can reduce Na uptake at the roots or have a Na transport/evacuation ability. Nitrate uptake rates tended to decrease as salinity increases across all plants, however significant amounts of nitrogen were removed from the water, even at 15 and 20 salinty. Other findings are the increased necessity of dosing certain nutrients at high levels compared to freshwater, particularly iron. This line of research has opened potential opportunities for brackish-water aquaculture producers. Further research will include human sensory profiles of the plants and consumer acceptance studies.