Lake Victoria turns green and toxic from nitrogen and phosphorus runoff, threatening East Africa's fisheries

Key facts

• Lake Victoria, the world’s largest tropical lake, now shows near-permanent green algal blooms loaded with liver-damaging microcystin.
• Nyando River dumps over 6,000 kg of nitrate daily during peak rains, while Nzoia River adds about 22,000 kg daily, fuelling explosive cyanobacterial growth.
• Microcystis and Dolichospermum dominate the blooms, producing toxins that create dead zones and crash fish populations.
• The lake supports a $600 million fish export industry and hundreds of thousands of tons of annual catch now at risk from oxygen depletion and food-web collapse.
• Kenya, Tanzania, and Uganda rely on the lake for drinking water and livelihoods, but seasonal die-offs have become systemic and recovery may be impossible on human timescales.