Using Digital Earth Africa to monitor chlorophyll-a concentration and detect harmful algal blooms in Suguti Bay, Lake Victoria, Tanzania

Abstract

Lake Victoria is a vital source of drinking water for millions, necessitating continuous monitoring of chlorophyll-a (Chl-a) concentrations to assess its trophic status and support sustainable water resource management. This study aimed to provide a scalable, cost-effective alternative to traditional in situ monitoring, which is limited in spatial and temporal coverage. Sentinel-2A imagery, integrated within the Digital Earth Africa (DE Africa) Platform, was used alongside the optimised Ocean Color 3 (OC3) algorithm to estimate Chl-a concentrations in Suguti Bay. Validation using 13 in situ samples collected between March 2018 and June 2019 showed strong agreement with satellite- derived data (R ² = 0.97, RMSE = 1.23µg l⁻¹, NSE = 0.97). Results revealed elevated Chl-a levels reaching up to 100 µg l⁻¹, and frequent harmful algal blooms (HABs) between 2016 and 2024, indicating significant eutrophication. Spectral analysis consistently showed higher green than blue reflectance, aligning with Chl-a characteristics. Spatial analysis identified the southeastern bay as a hotspot, likely influenced by runoff from the Nyasangu River and anthropogenic pressures. These findings emphasise poor water quality and the urgent need for targeted nutrient management. Overall, this study demonstrates that combining Sentinel-2A and DE Africa offers a reliable, replicable framework for inland water quality monitoring in Africa.