Spatio-temporal dynamics of nutrient enrichment and eutrophication risk in Dadin Kowa Reservoir, Nigeria

This study investigates the spatio-temporal variation of nitrogen and phosphorus compounds in Dadin Kowa Reservoir, particularly seasonal hydrology and nutrient drivers that remains poorly quantified in Nigerian reservoirs. A stratified random sampling design was employed to capture spatial variability across the reservoir, and the water samples collected from two stations (SWD1 and SWD2) over six months were analyzed for nitrogen and phosphorus compounds alongside key physico-chemical parameters. The analysis revealed markedly elevated concentrations of nitrogen and phosphorus compounds (5.291- 55.4 mg/l and 0.0031-0.646 mg/l) that exceeded USEPA thresholds, indicating strong anthropogenic loading from agricultural runoff, domestic effluents, and internal nutrient regeneration. Principal component analysis showed the first component (F1), explained 48-66% of variance across sampling stations, represents a dominant nutrient-enrichment axis integrating bioavailable nitrogen, phosphorus, and chlorophyll-a, while the second component (F2; 14-27%) reflects hydrological and physicochemical processes linked to wet-season mixing, turbidity, and dissolved oxygen variation. PCA loadings showed co-alignment of N and P variables along F1, indicating dual-nutrient enrichment and potential N:P co-limitation, particularly during the dry season when reduced inflow concentrates nutrients. Spatially, SWD1 exhibited strong clustering of N and P forms, whereas SWD2 displayed greater nutrient heterogeneity associated with upstream land-use pressures. Temporally, nutrient surges in May-June reflected dry-to-wet season transitions, while July-September indicated active internal cycling driven by runoff and phytoplankton dynamics, and December showed decomposition that driven oxygen depletion. These findings emphasize the urgent need for integrated reservoir management strategies focusing on nutrient source reduction, sediment control, and seasonal oxygenation to mitigate eutrophication risk and sustain aquatic ecosystem health.