Hypoxic (dead zones)
Hypoxia in aquatic systems refers to low dissolved oxygen levels in water, where levels fall below 2mg/L. In such evnvironments most aquatic life will leave the area, become stressed or die. This problem is quite severe in northern Gulf of Mexico where the Mississippi river drains into the ocean. The available scientific evidencepoints to eutrophication and nutrient overloading as the main culprits creating these dead zones.
(CC Image (right) of global aquatic dead zones by Robert Simmon & Jesse Allen (NASA Earth Observatory) [Public domain], via Wikimedia Commons) (Click to enlarge)
Eutrophication occurs when organic matter increases in the aquatic system, usually in the form of phytoplankton algae. When the algae dies and sinks the ocean floor, it starts to decay, when this happens in excess the decay process depletes the available dissolved oxygen levels beyond what most aquatic creatures can handle. This depletion creates stratified water layers where deeper waters fail to mix with water above due to water density.
Excess nitrogen and phosphorous fertilizing has been implicated in this increase. With the Mississippi river cutting through one of the most agricultural regions on the planet with heavy uses of chemical nitrogen runoff occurring, it is no surprise that much of the excess nitrogen is getting into the ocean. The question for those aquatic species and the people that depend on the productivity of the ocean is, what can be done?
(CC Image courtesy of eutrophication&hypoxia, credit: Hans W. Paerl via Flickr)