Evaluating nutrient dynamics, energy utilization, and performance of aquaponics systems for smallholder farmers' sustainability

Abstract

Aquaponics is promoted as a climate-smart food production system, yet little empirical evidence exists on its resource use efficiency, nutrient dynamics, and productivity under East African conditions, limiting adoption by smallholder farmers. This study, conducted at the Aquaculture Research and Development Centre, Kajjansi, evaluated the effectiveness of aquaponics systems in utilizing resources (nutrient, water, energy). The experiment evaluated the aquaponics systems using one factor – fish type (African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus)) as nutrient source (treatments) for lettuce (Lactuca sativa) in deep-water culture (for four seasons/growth cycles) and tomatoes (Solanum lycopersicum) in media-filled beds (for one season). Each treatment replicated three times contained fish tanks, filters (mechanical, biological), and plant zones (media-filled bed, deep-water culture) which formed the aquaponics zones. The quality of water from fish tanks was monitored onsite using a seneye device to measure DO, pH, NH₃ and NH₄⁺ while nitrogen (N), phosphorous (P), and potassium (K) nutrients were analyzed in the laboratory at Makerere University. Fish were fed on the same feed, and the quantity was determined using the body weight of fish. The fish’s response was monitored for over 250 days. To determine a system adoptable to a smallholder farmer, photovoltaic (PV) system sizing using PVsyst software was carried out accompanied by a financial evaluation that assessed feasibility of an off-grid solar-powered aquaponics system. Results show that the quality of water from tanks of both African catfish and Nile tilapia remained optimal, though varied significantly in DO, pH, and NH₃ by season and aquaponics zones while NH₄⁺ varied by only season (p ≤ 0.05). Nutrient analysis showed N was significantly different by treatment and zone, P by treatment only, and K by treatment and season (p ≤ 0.05). African catfish systems produced higher N (48.5 mg/L) than Nile tilapia (30.5 mg/L), while Nile tilapia systems yielded greater P (99.3 mg/L) and K (291.2 mg/L) concentrations, indicating species-specific influences on nutrient dynamics in aquaponics. Lettuce yield was significantly different among the four growth cycles, while tomatoes significantly varied across treatments i.e., African catfish and Nile tilapia (p ≤ 0.05). Nile tilapia systems produced better yields than African catfish systems. PV sizing indicated that a 2.4 kWp solar system meets 95% of aquaponics power needs at 70% performance ratio. Furthermore, the financial evaluation showed that the sized system is able to pay back its investment in less than four years with an annual return on investment of ~22%. Therefore, focus should be on Nile tilapia aquaponics for a higher productivity and African catfish aquaponics for diversity.