Lake Victoria Turns Toxic Green Amid Ecological Collapse

The waters of Lake Victoria, the world’s largest tropical lake, have shifted to a persistent toxic green. What were once seasonal algal blooms have become a permanent feature. The lake supports nearly 47 million people across Kenya, Tanzania, and Uganda for drinking water, transport, and fisheries. Scientists now warn that decades of nutrient pollution have pushed the ecosystem beyond a critical threshold.

Nutrient Overload and Algal Dominance

Harmful algal blooms arise from eutrophication, a process driven by excessive nitrogen and phosphorus. In Lake Victoria’s densely populated basin, fertiliser runoff, untreated sewage, industrial discharge, soil erosion, and atmospheric deposition feed the lake with nutrients. Wet seasons intensify the crisis as rainfall washes accumulated waste into tributaries.

Recent basin studies show alarming nitrate levels in inflowing rivers. During peak rains, the Nzoia River alone delivers over 22,000 kilograms of nitrate per day into the lake. Manure and sewage are identified as dominant contributors. Rapid population growth in riparian towns compounds the pressure on sanitation and waste systems.

A Century of Ecological Change

Sediment core analyses from Mwanza Gulf reveal that eutrophication began around the 1920s, coinciding with land use transformation. Between 1920 and 1990, primary productivity rose steadily. Since the 1990s, cyanobacteria have become dominant.

The ecological imbalance predates visible fishery collapses. Zooplankton biomass declined sharply around 1960, disrupting the food web. This preceded the loss of many native cichlid species in the 1980s. Rising lake levels and altered hydrology further destabilised habitats.

Toxic Blooms and Hidden Health Risks

Cyanobacteria such as Microcystis now produce microcystin, a liver toxin frequently exceeding World Health Organization safety limits in Winam Gulf. Blooms are not always visible. Toxins can persist even in murky waters without surface scums.

Genomic research has identified multiple cyanobacterial species contributing to blooms, including Dolichospermum. Scientists have also detected hundreds of uncharacterised genes, suggesting the presence of unknown bioactive compounds. The shifting composition of blooms signals a dynamic and unpredictable threat.

Important Facts for Exams

• Lake Victoria is the world’s largest tropical lake by surface area.
• Eutrophication is driven by excess nitrogen and phosphorus inputs.
• Microcystin is a toxin produced by certain cyanobacteria affecting liver function.
• Lake Victoria is shared by Kenya, Tanzania, and Uganda.

Dead Zones and Fishery at Risk

Oxygen depletion now affects deeper waters, creating dead zones incapable of sustaining aquatic life. The lake’s fishery, exceeding 300,000 tonnes annually and valued at around $600 million, faces growing instability. Nile perch, Nile tilapia, and dagaa dominate catches, yet fluctuating oxygen levels and food web disruption threaten their survival. Expanding hypoxic zones narrow the boundary between isolated fish kills and systemic ecological collapse.