Comparative analysis of nutrient dynamics, growth performance, and economic returns of lettuce (Lactuca sativa) in coupled aquaponic, decoupled aquaponic, and conventional hydroponic systems

Quantitative information on the fate of macronutrients derived from fish effluents, their concentrations in culture water, and their assimilation by plants across hydroponic and aquaponic systems remains limited. To bridge this gap, this study examined the growth response of lettuce ( Lactuca sativa ) and the dynamics of key nutrients, nitrogen (N), phosphorus (P 2 O 5 ), potassium (K 2 O), calcium (Ca), and magnesium (Mg), in water and plant tissues under three production strategies: coupled aquaponics (T1), conventional hydroponics (T2), and nutrient supplemented decoupled aquaponics (T3) integrated with African catfish ( Clarias gariepinus ). Over a forty-day cultivation period, lettuce grown in conventional hydroponics achieved higher biomass values than plants in either of the aquaponic systems. T1 produced intermediate growth, while T3 showed the lowest biomass. Morphological observations revealed greater shoot development in T2, moderate shoot formation in T1, and comparatively higher root biomass in T3, indicating differences in nutrient uptake and allocation among treatments. Although measured macronutrient concentrations suggested sufficient availability, plant productivity was not directly proportional to total nutrient levels, highlighting the role of nutrient assimilation efficiency. Economic evaluation indicated that although T2 achieved the highest vegetable yield, T1 generated the highest gross income due to the combined fish and crop sales, while T3 achieved the highest return on investment and the shortest payback period. Overall, this study provides quantitative insight into nutrient dynamics, plant uptake, and economic performance across hydroponic and aquaponic systems. These findings contribute to a better understanding of resource use efficiency and support the optimization of nutrient management and system design for sustainable integrated aquaculture production.