An algal bloom is a rapid explosion of algae in water. It happens when nutrients, mainly nitrogen and phosphorus, flow into a lake or coastal area. Fertilizer runoff, sewage, and livestock waste are common sources. Warm temperatures and calm water make blooms worse.

Some blooms are harmless. Others are toxic. Cyanobacteria, often called blue-green algae, produce toxins that harm fish, birds, pets, and people. Swimming in contaminated water can cause rashes and illness. Drinking it can be fatal. Even non-toxic blooms kill life by blocking sunlight and consuming oxygen when they decompose.

The result is often a dead zone. As algae die, bacteria break them down and strip oxygen from the water. Fish and shellfish suffocate. The Gulf of Mexico hosts a dead zone the size of New Jersey most summers, fed by nutrients from the Mississippi River. Reducing fertilizer runoff is the main solution, and it is politically difficult.

Common causes

• Fertilizer runoff
• Sewage discharge
• Livestock waste
• Warm water temperatures
• Slow-moving or stagnant water