Eutrophication

Eutrophication cascades begin when excess nutrients stimulate massive phytoplankton and algal growth. When blooms die and sink, bacterial decomposition consumes oxygen, creating hypoxic (dissolved oxygen <2 mg/L) or anoxic 'dead zones.' The Gulf of Mexico dead zone — fueled by Mississippi Basin agriculture — covers up to 22,000 km² in summer. Harmful algal blooms (HABs) involving cyanobacteria or dinoflagellates produce toxins that kill fish directly, contaminate shellfish, and poison wildlife. Climate warming accelerates eutrophication by increasing temperature stratification and reducing mixing that would otherwise distribute oxygen deeper.