Less mobile or immobile animals, such as mussels or crabs, cannot move to waters with more oxygen and are often killed during hypoxic events. Mobile animals (e.g., adult fish) can typically survive a hypoxic event by moving to waters with more oxygen. Why is it important to reduce hypoxic zones?ĭirect effects of hypoxia include fish kills, which deplete valuable fisheries and disrupt ecosystems. Visit the Gulf of Mexico Dead Zone site, developed by the Science Museum of Minnesota. Without mixing, oxygen in the bottom water is limited and the hypoxic condition remains. This stratification prevents the mixing of oxygen-rich surface water with oxygen-poor water on the bottom of the Gulf. In addition to the saline gradient caused where the freshwater and saline water meet, the freshwater is warmer than the deeper ocean water, further contributing to the stratification. As nutrient-laden water from the Mississippi flows into the Gulf, this freshwater is less dense and remains above the denser saline seawater.
The hypoxic zone in the Gulf of Mexico forms every summer and is a result of excess nutrients from the Mississippi/Atchafalaya River and seasonal stratification (layering) of waters in the Gulf.
Discharges from sewage treatment plants.
