FRESHWATER AQUAPONICS: LOW-TECH ALTERNATIVE FOR NUTRIENT REMOVAL IN SWEDISH AQUACULTURE?

Abstract

The growing world population is putting conventional aquaculture to the test. Available land for food production is becoming scarcer, and excess nutrients can cause eutrophication of soils and waters. Aquaponics, the coupling of recirculating aquaculture systems (RAS) with the production of plants in growbeds without soil, so called hydroponics are a promising way to improve modern aquaculture. Due to a rising demand for seafood, it is therefore necessary to identify plant species suitable for nutrient removal from RAS and potential challenges. To achieve this, we cultivated green kale (Brassica oleracea), a leafy vegetable requiring relatively high amounts of nitrogen for good yields, in ebb- and flow hydroponic systems with process water from a RAS farming African catfish (Clarias gariepinus), an important upcoming species within Swedish aquaculture. We compared plantgrowth and nutrient levels in the water against a control system with a standard fertilizer solution for hydroponics. The daily growth rate of the plants in the aquaponic treatment was significantly lower at 0.57 g/d than in the control at 0.97 g/d within the first 29 days of the experiment. The growth correlated with the nitrogen uptake of 49.81 mg/d for aquaponics and 87.37 mg/d for fertilised. Low leaf-pigment concentration in the aquaponic treatment suggested a lack of trace elements and/or suboptimal pH. The addition of trace element solution and pH adjustment with sulfuric acid led to a clear positive effect on the plant’s growth and pigmentation. Focusing on researching sustainable and alternative ways to maintain optimal pH and a balanced nutritional profile for all organisms within aquaponic systems could improve growth and nutrient absorption, producing a valuable secondary product, while employing more circularity and sustainability.