Attribution of Seasonal and Interannual Turbidity Variations in Lake Sofia (Sofia Region, Madagascar) to Land-Cover and Climate Change

Lake Sofia, located in the mountains of northern Madagascar (1100 m a.s.l.), has experienced increased sediment loads over the past century, which have adversely affected its biodiversity and reduced its lifespan. Lake Sofia provides critical ecosystem services and is an important habitat for threatened species, e.g., the critically endangered Madagascar Pochard (Aythya innotata). Suggested causes of the high sediment load in Lake Sofia are water erosion, wind erosion, and the stirring of mud from the bottom of the lake by recently introduced tilapia (Oreochromis spp.). Using publicly available weather reanalysis, satellite data, land cover data, soil data, and elevation data, we attribute high turbidity levels in Lake Sofia to water erosion, which explains over 90% of the variance in lake turbidity. Wind erosion was not significant, and tilapia did not contribute to sediment influx. Increased precipitation and decreased vegetation cover in agricultural areas contributed most to increased erosion risk, whereas lavaka (erosion gullies) contributed less. Erosion risk over crop lands has increased by 2% annually and is expected to increase further because of climate change and expected increased precipitation intensity. The erosion model identifies areas most at risk to be targeted for interventions, and is applicable to other catchments in Madagascar and elsewhere.