Introduction
Aquaponics is a farming method where the nutrient-rich wastewater from a recirculating aquaculture system (RAS) is utilized to grow plants. In an aquaponic system, the ammonia produced by fish metabolism is oxidized by biofilter nitrification microbes to nitrate which is an essential nutrient for the plants but is also well tolerated by fish. Fish feed contains also other nutrients that are important for plants. It is important to manage the water quality in an aquaponics system and to maintain the optimal growing conditions for fish and plants. Fishes thrive in clean water while the plants require nutrient-rich medium for their growth. In RAS cyanobacteria, Actinomycetes and myxobacteria are known to produce off-flavor compounds geosmin (GSM) and 2-methylisoborneol (MIB) which can easily accumulate in fish flesh resulting in earthy and musty flavor and making them unmarketable. Thus, the removal of these compounds by depurating them in flow-through water is necessary before marketing. The aims of this study were to investigate the growth of baby spinach (Spinacia oleracea) and rainbow trout (Oncorhynchus mykiss) and to compare water quality in RAS, hydroponics (i.e. soilless culture of plants) and in a coupled aquaponic system. Special emphases were given to off-flavor compounds in fish flesh and plants as well as the onset of nitrification process in RAS and aquaponics.
Materials and methods
A six-week experiment was performed with three replicated RAS, aquaponic and hydroponic systems. Twenty rainbow trout (initial weight 108 g) were stocked in each of the six fish tanks: three for RAS and three for aquaponic systems. Twenty-five baby spinach (Spinacia oleracea) plants were transplanted in each of the six deep water culture rafts (3 for aquaponics and 3 for hydroponics). Biomass for the fish and plants were recorded in the beginning and at the end of the experiment. Chloride, phosphate, sulfate, nitrate, nitrite, GSM and MIB were compared between RAS, aquaponics and hydroponics at the start and end of the experiment. The water quality data (total ammonia nitrogen, nitrite, nitrate, and pH) were collected during the experiment and the nitrification process was compared between RAS and aquaponics treatments.
Results
The fish grown in aquaponics exhibited statistically higher weight (137.1 ± 11.29) compared to fish grown in RAS (109.3 ± 3.05). Specific growth rates were also higher for fish grown in aquaponics (1.95 ± 0.12) than in RAS (1.67 ± 0.08). The feed conversion ratios (feed fed/weight gain) were lower for the fish grown in aquaponics (0.86 ±0.08) than in RAS (1.06 ±0.03). The concentrations of GSM were lower in fish flesh grown in aquaponics (376.0 ± 24.99) than in RAS (466.3 ± 39.40). The concentrations of MIB were at similar levels in fish flesh grown in aquaponics and RAS. The concentrations of GSM and MIB did not differ statistically significantly between the water of the two systems. The onset of nitrification was faster in the aquaponic system than in RAS (Figure 1).
In the aquaponics system, the plant dry weight of spinach was 43 % higher (40 % for shoot and 70 % for root) than in hydroponics. The concentration of GSM was significantly higher in the roots of spinach grown in aquaponics (3579 ± 1682.8 ng/L) compared to spinach grown in hydroponics (191.80 ± 48.78 ng/L), but in the shoots the level of GSM was very low in both treatments (~8 ng/L). The concentration of MIB was higher in the shoots of spinach grown in aquaponics (704.4 ± 73.08 ng/L) than in those grown in hydroponics (278 ± 158.2 ng/L).
Conclusions
The aquaponic system was better than RAS in terms of water quality, onset of nitrification, fish growth and feed conversion ratio and also lower concentrations of GSM accumulated in fish flesh. Spinach grew better in aquaponics than in hydroponics but the difference was not statistically significant.
Acknowledgments: We would like to thank University of Jyväskylä, JAMK University of applied Sciences, Sisä-Suomen kalatalousryhmä, Maa- ja Vesitekniikan Tuki Ry, Niemi-säätiö and Luonnonvarakeskus (Luke) for their contribution towards this research work.