Eutrophication

Eutrophication is the over-enrichment of water with nutrients, mainly nitrogen and phosphorus from agricultural runoff, sewage, and atmospheric deposition, that fuels excessive algal growth. When the resulting bloom dies and decomposes, bacterial respiration strips oxygen from bottom water, producing hypoxic dead zones such as the seasonal one in the Gulf of Mexico. It is the central problem of semi-enclosed seas like the Baltic, the target of HELCOM nutrient-reduction targets.