Characterization of ranomafana lake water quality - antsirabe madagascar

Abstract

Analyses of water quality parameters in Ranomafana Lake showed that the lake is hypereutrophic. It receives untreated wastewater from Antsirabe municipality by three main inlets: the North West inlet, the North inlet and the North East inlet. And the measured values in different stations indicate that, horizontally, the water is not well mixed; peak values are recorded at station 3 located downstream the North West inlet. For all measured parameters, daily variation was noticed. The surface water registered a temperature difference of 2°C from morning to afternoon. The water pH was slightly alkaline and ranged from 7.17 to 8.12 in the surface and from 7.1 to 7.95 in the bottom. The dissolved oxygen in the morning was between 6mg/l to 12 mg/l in the surface layer and between 4 mg/l to 8 mg/l at the bottom water. This amount increased from morning to the afternoon due to the photosynthesis. Regarding the nutrient level, the total nitrogen concentration in the lake water varied from 7.6 mg/l to 10.6 mg/l in February; from 6.6 mg/ to 10.8 mg/l in March and from 6.3 mg/l to 12.1 mg/l in April. The total phosphorus concentration ranged from 0.94 mg/l to 2.23 mg/l in February; from 0.99mg/l to 2.23 mg/l in March and from 0.94 mg/l to 3.85 mg/l in April. The quantity was always higher in the afternoon. Ranomafana Lake water also had high chlorophyll a concentration: 106 mg/m3 to 232 mg/m3 in February, 88 mg/m3 to 142 mg/m3 in March and 131 mg/m3 to 238 mg/m3 in April. Despite its hypereutrophic state, Ranomafana Lake water does not experience oxygen depletion. The whole water column is aerobic due to high photosynthesis. The main problems are high phosphorus concentration and algae concentration. They contribute the most to the increase of water turbidity and to the decrease of Secchi disk depth in this Lake. It is then necessary to reduce the nutrient level and the chlorophyll a concentration in order to remediate the water quality. And for that purpose, our recommendations consist of reducing the nutrient loads by treating the wastewater prior to their discharge into the lake, increasing the nutrient uptake from the lake water by promoting algae growth and then removing the excess of algae to clarify the water.