Temporal dynamics outweigh spatial gradients in shaping water quality across a regulated Afrotropical reservoir cascade.

Water quality in regulated tropical reservoirs is shaped by the interplay of seasonal hydrology, spatial habitat heterogeneity, land use, and climate variability, yet the relative importance of these drivers remains poorly quantified in African river basins. We aimed to partition the variance in water quality among temporal, spatial, land-use, and climate drivers across the Kainji-Jebba reservoir cascade in Nigeria and to identify sentinel parameters capable of consistently and timely detecting water-quality change. Monthly water-quality surveys were conducted at six sites spanning two dams and three habitat types (riverine, ecotonal, and lacustrine) from April 2024 to March 2025 (n = 72 samples). A modified National Sanitation Foundation Water Quality Index (WQI) was calculated for each sample. Variance partitioning using partial redundancy analysis (RDA), exploratory Granger causality testing, and six complementary changepoint detection methods were applied to disentangle the relative contributions of space, time, land use, and climate. Temporal variation (season and month) explained 28.3% of the variance in water quality, whereas spatial factors accounted for 1.8% and land-use effects for 0.4%. These results suggest that seasonal hydrological dynamics were the dominant structuring force within the study period, although the limited spatial extent of sampling (six sites across two reservoirs) should be considered when interpreting the comparatively weak spatial signal. After Benjamini-Hochberg false-discovery-rate correction, no climate-water-quality relationships were statistically significant across 480 stratified Granger causality tests; given the exploratory scope of the analysis and the 1-year dataset, this null result is treated as hypothesis-generating rather than conclusive. WQI, electrical conductivity (EC), and relative humidity (RH) showed the highest changepoint consensus detection rates (71.4% each) and are proposed as sentinel parameters for water-quality monitoring, although the role of RH warrants further investigation before operational adoption. These findings suggest that monitoring programmes in regulated tropical reservoir cascades may benefit from prioritising targeted seasonal windows, the wet-season runoff peak (June-July) and the black flood period (January-February), rather than expanding spatial coverage, pending validation across longer time series. The integrated analytical framework, combining WQI, variance partitioning, exploratory Granger causality, and multi-method changepoint detection, offers a transferable template for other data-poor tropical river systems where monitoring resources must be allocated efficiently.