Seasonal depth-dependent water quality profile in the Koga Reservoir, Ethiopian highlands

ABSTRACT

Conceptual diagram of hydrothermal stratification and depth-dependent water quality in Koga Reservoir. Schematic illustration of vertical water quality gradients in Koga Reservoir, showing surface, middle, and bottom layers with seasonal differences in temperature, oxygen, and nutrients.

Vertical temperature shapes vertical water quality gradients, yet depth-dependent variability in tropical highland reservoirs remains poorly documented. This study evaluated seasonal vertical water quality observations in the Koga Reservoir during the dry (February 2025) and wet (August 2025) seasons. Samples from surface, middle, and bottom layers were collected with a Van Dorn sampler, and physicochemical parameters were measured using standard field and laboratory procedures. Observations indicated that temperature decreased by about 9 °C from surface to bottom in both seasons, while dissolved oxygen declined from 5.95 to 2.75 mg/L (dry) and from 4.73 to 1.40 mg/L (wet), with bottom waters below the recommended guideline (>5 mg/L). Nitrate remained within FAO limits but decreased with depth, suggesting potential eutrophication risk. Phosphate and ammoniacal nitrogen increased toward bottom waters, exceeding FAO thresholds (up to 5.98 and 7.15 mg/L, respectively). Turbidity and suspended solids were markedly higher in the wet season (596 NTU and 166.5 mg/L) compared to the dry season (296 NTU and 85.6 mg/L). The findings provide preliminary observations of seasonal vertical thermal and chemical gradients within the Koga Reservoir and suggest that both in-reservoir processes and seasonal catchment conditions may contribute to observed water quality variability.