Évolution 2023-2025 des paramètres physico-chimiques des lacs Andraikiba et Andranobe (région d’Antsirabe, Madagascar)

Abstract

Small peri-urban lakes in the Malagasy Highlands are exposed to runoff, organic inputs, domestic uses and marked seasonality. This study analyses physicochemical measurements actually obtained from Lakes Andraikiba and Andranobe between 2023 and 2025. The dataset contains 36 observations following a balanced design of two lakes × three years × two seasons × three stations per lake. Ten variables were assessed: temperature, pH, electrical conductivity, turbidity, dissolved oxygen, total dissolved solids, nitrate, ammonium, biochemical oxygen demand and chemical oxygen demand. From 2023 to 2025, mean values at Andranobe showed increases of 34.8% in turbidity, 22.0% in nitrate, 35.2% in ammonium and 25.3% in chemical oxygen demand, while dissolved oxygen decreased by 9.5%. The corresponding changes at Andraikiba were +25.8%, +20.2%, +31.9%, +14.2% and -5.5%. In 2025, the contrast between lakes was particularly marked: relative to Andraikiba, Andranobe showed 76.4% higher turbidity, 94.9% higher ammonium and 67.1% higher chemical oxygen demand, while dissolved oxygen was 12.8% lower. Rainy-season observations were characterized by higher turbidity, nitrogen compounds and organic load together with lower dissolved oxygen. Because the monitoring period covers only three years, these changes are interpreted as observed variations over the study period rather than a firmly established long-term temporal trend. The findings support the strengthening of a standardized and regular monitoring programme for both lakes.