Nutrient mass balance of koi–basil aquaponics production systems

The viability of aquaponics as a sustainable practice in water-scarce regions hinges on how efficiently water and nutrients cycle between fish rearing and crop production. We operated three coupled aquaponics (CAP) systems (250 L), each pairing koi fish with basil grown in deep water culture, across three sequential 32–41-day growth cycles. Treatments differed only in source water relevant to the Texas Panhandle: reverse osmosis (RO), Ogallala (“fresh”) groundwater, and Dockum (“brackish”) groundwater. Objectives were to quantify (1) the effect of source water on fish growth, crop yield, and operational ease, and (2) the efficiency of nutrient transfer from fish feed to plant biomass—assessed via a multi-cycle total mass balance design, an approach applied in few previous aquaponics studies. Water source produced no detectable effect on fish. Evapotranspiration ranged 0.86–1.6 cm/day, water use efficiency was 361–615 g wet biomass per 1,000 L, and daily system volume loss was approximately 1%. Na accumulated to a greater degree in RO-grown basil than in Ogallala-grown basil, possibly reflecting competitive suppression of Na uptake by Ca and Mg in hard groundwater. Basil growth declined significantly in the brackish water source (mixed-effects ANOVA), coincident with electrical conductivity (EC) rising above 3 mS/cm by cycle 3. Fe and Mn accumulated preferentially in roots rather than shoots across all treatments, suggesting impaired Fe mobility given that basil presented chlorosis. Salinity buildup was driven by Cl - and Na + accumulation in all three water sources, although this was somewhat confounded by Cl - added as part of nutrient supplementation for the brackish water source. Thus, production-scale CAP operations will require periodic drain events to manage salinity stress. Plant nutrient use efficiency was low for N (2.19–9.48%), typical for P (2.06–9.14%), and high for K (10.2–47.5%). The mass balance frequently closed within ± 25% error for N, Cu, and Mg but was difficult to close for most other nutrients. Koi–basil CAP production is feasible in the semi-arid Texas Panhandle using local groundwater sources, offering the potential for high-value aquatic and crop agricultural outputs with lower water demand than open-field production.