Résumé
Study region: Lake Guiers, Senegal, West Africa Study focus: Water quality degradation from agriculture threatens freshwater lakes worldwide. This study investigated the spatiotemporal hydrochemical dynamics of Lake Guiers, which supplies 50 % of Dakar's drinking water, and examined natural and anthropogenic drivers of water mineralization. The approach combined multivariate statistics, hydrogeochemical analysis (Piper, Gibbs), and Water Quality Index (WQI) assessments of samples collected over five seasonal campaigns (2022-2023) as well as hydroclimatic analysis. Seasonal variations and spatial gradients of 22 parameters (physicochemical, major ions, biological, heavy metals) were explored to inform effective freshwater management strategies. New hydrological insights for the region: Multidisciplinary approach highlights impact of intensified agriculture and inflows on spatiotemporal variations in water quality. WQI values categorized 67-87 % of samples as poor water quality, with conditions worsening during the rainy season. Several parameters including heavy metals, cyanobacteria, chlorophyll-a and turbidity exceeded WHO standards. Temporal variations were influenced by evaporation during the dry season, agricultural discharge, and the combination of runoff and inflow waters during the rainy season. Sediment loaded inflow from the Taouey canal notably leads to greater turbidity and suspended particulate matter in the north of the lake, while runoff and associated mobility of nutrients favour cyanobacteria proliferation during the rainy season. Targeted interventions to reduce pollution sources and continuous monitoring of transitory drainage and runoff in selected hotspots are required to preserve this vital semi-arid freshwater lake.