Eutrophication is the enrichment of water with nutrients, mainly nitrogen and phosphorus. It sounds harmless. It is not. Extra nutrients trigger explosive growth of algae and plants. When they die, bacteria decompose them and consume oxygen. The water becomes hypoxic. Fish and shellfish die. The ecosystem collapses.

Agriculture is the main source. Fertilizer applied to fields runs off into streams and rivers. Livestock waste adds more. Sewage treatment plants discharge nutrients even after processing. In lakes, phosphorus is often the limiting nutrient. In coastal waters, nitrogen usually controls. Knowing which one limits growth tells managers where to focus.

The problem is global. Lake Erie has suffered from toxic algal blooms that shut down drinking water for cities. The Baltic Sea has a dead zone the size of Denmark. China's Taihu Lake turned green with cyanobacteria in 2007. Fixing eutrophication requires cutting nutrient runoff at the source. That means changing farming practices, upgrading sewage plants, and restoring wetlands that filter water naturally.

Main nutrients

• Nitrogen
• Phosphorus
• Potassium (less often limiting)
• Agricultural runoff
• Sewage and detergents