Drivers and ecological impacts of harmful cyanobacterial blooms in Ethiopian freshwaters and their management strategies

Abstract

Cyanobacteria are photosynthetic microorganisms that play essential ecological roles in oxygen production and nutrient cycling; however, under conditions of excessive nutrient enrichment they can proliferate into harmful algal blooms (HABs) that produce potent toxins and threaten freshwater systems. In Ethiopia, increasing nutrient inputs from agricultural runoff, urban wastewater discharge, land-use change, and climate variability have intensified bloom formation in key freshwater bodies, particularly Koka Reservoir, Lake Tana, and Legedadi Reservoir. This review synthesizes peer-reviewed literature published between 2010 and 2024 using a structured search strategy with defined inclusion and exclusion criteria to assess drivers, ecological impacts, toxin occurrence, and management responses associated with HABs in these systems. Available monitoring data indicate that elevated total nitrogen and total phosphorus concentrations are primary drivers of eutrophication, with documented microcystin concentrations in some cases exceeding the World Health Organization guideline value of 1 µg L−1 for drinking water during severe bloom events, posing significant public health risks. Dominant toxin-producing taxa reported include Microcystis aeruginosa, Dolichospermum spp., and Cylindrospermopsis raciborskii. Beyond human health implications, HAB events have been associated with hypoxia, reduced biodiversity, fish mortality, and increased water treatment costs, affecting fisheries, drinking water security, and local livelihoods. Despite growing evidence of toxin presence, monitoring efforts remain fragmented, with limited long-term datasets and insufficient integration of advanced tools such as remote sensing and real-time in situ monitoring. By integrating dispersed national data and comparing bloom dynamics across systems, this review identifies persistent research and regulatory gaps and highlights the urgent need for coordinated long-term surveillance, watershed-based nutrient management, comprehensive toxin profiling, and development of a national HAB early warning and response framework to safeguard Ethiopia’s freshwater resources.