Dead zone: Encyclopedia II - Dead zone - Causes of dead zones

Dead zone - Causes of dead zones

Aquatic and marine dead zones can be caused by the process of eutrophication, triggered by an excess of plant nutrients (nitrogen and phosphorus) from fertilizers, sewage, combustion emissions from vehicles, power generators, and factories. In a cascade of effects, the nutrients trigger a bloom of phytoplankton at the bottom of the marine food chain, allowing zooplankton to proliferate. As phytoplankton and zooplankton die and sink below the photic zone where photosynthesis can occur, a bloom of natural bacterial degradation exhausts the water's dissolved oxygen.

Dead zones can also be produced by the natural event of river flooding. Large amounts of fresh water empty into the ocean forming a thick layer of fresh water atop the denser salt water, effectively forming a barrier between the ocean water and oxygen in the atmosphere. (Osterman, 2004)

Remains of organisms found within sediment layers near the mouth of the Mississippi River indicate four hypoxic events before the advent of artificial fertilizer. In these sediment layers, anoxia-tolerant species are the most prevalant remains found. The periods indicated by the sediment record correspond to historic records of high river flow recorded by instruments at Vicksburg, Mississippi.

Adapted from the Wikipedia article "Causes of dead zones", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki