Integrating field monitoring and remote sensing to identify CyanoHAB hotspots and human health risks in Kisumu Bay, Lake Victoria

Freshwater cyanobacterial harmful algal blooms (CyanoHABs) are an increasing threat to public health, fisheries and ecosystem services in eutrophic aquatic systems worldwide. Kisumu Bay, located within the Nyanza Gulf of Lake Victoria, supports over half a million people, including fishing communities and residents of Kisumu City, yet is increasingly impacted by wastewater discharge, agricultural runoff, and industrial activities that contribute to declining water quality and elevated health risks. This study assessed the spatial distribution of CyanoHABs and associated human health risks from point and nonpoint sources within Kisumu Bay between 2015 and 2016. Water quality parameters, including chlorophyll-a, nutrients, dissolved oxygen, fecal indicator bacteria, cyanobacterial cell density and cyanotoxins were monitored over an eight-month period. Field observations were integrated with Landsat-derived chlorophyll-a data to evaluate spatial and historical trends in eutrophication within Kisumu Bay and the wider Nyanza Gulf. Based on World Health Organization recreational water quality guidelines, 86% of samples were classified as high risk due to elevated cyanobacterial cell densities. Microcystins were detected in 51.7% of samples, with 48.9% exceeding United States Environmental Protection Agency advisory levels for vulnerable populations. Approximately half of the water samples exceeded recommended limits for fecal contamination, while dissolved oxygen concentrations fell below 3.5 mg L-¹ during nearly 20% of sampling events, indicating conditions that may adversely affect fisheries. Elevated concentrations of total phosphorus, ammonium, chlorophyll-a, cyanobacterial cells and Microcystis spp. were concentrated along the northwestern sector of Kisumu Bay, identifying this area as a priority for intervention. Remote sensing analyses corroborated field observations and further identified Homa Bay as a potential CyanoHAB hotspot within Nyanza Gulf. These findings demonstrate the value of integrating field monitoring, remote sensing and community knowledge to support targeted monitoring, early warning systems and management interventions aimed at reducing CyanoHAB risks in Lake Victoria.