Ocean Dead Zones

Ocean dead zones are areas depleted of oxygen, which cannot support marine life. The number of dead zones is increasing, primarily due to rising sea temperatures and fertilizer run-off. Dead zones typically occur along the coast, especially near the mouth of rivers where fertilizer run-off collects.

Healthy Oceans

Phytoplankton living on the surface of the water produce organic matter, which sinks to the bottom of the ocean. Bacteria break down this matter through a process called bacterial respiration, which uses oxygen and gives off carbon dioxide. Other marine life on the bottom of the ocean, such as crabs; shrimp and fish; compete with this bacteria for oxygen. Phytoplankton replenish this oxygen by using the carbon dioxide and releasing oxygen through photosynthesis. In a healthy ecosystem, a natural balance exists between the levels of oxygen and carbon dioxide that are being used and produced by the bacteria and phytoplankton. NASA Creeping Dead Zones (May 1, 2008)

Causes

Dead zones can occur for several reasons, such as more phytoplankton, restriction of natural water flow, fertilizer run-off, other pollutants and rising sea temperatures.

An increase of phytoplankton means more organic matter, which leads to an increase of bacterial respiration. This takes more oxygen out of the water forcing other marine life to find oxygen elsewhere or die, hence, creating a dead zone. Fertilizer run-off and other pollutants contain high levels of nitrogen, which is a nutrient for phytoplankton and contributes to an increase in their numbers.

When natural water flow is restricted, the levels of oxygen are not replenished as quickly; therefore, the oxygen is used by bacteria and marine life faster. Water flow restriction is usually caused by natural disasters, like drought or hurricanes, or man-made obstructions, such as dams.

Rising ocean temperatures can also cause dead zones because oxygen dissolves slower in warmer water Red Orbit: Fish at Risk From Ocean Dead Zones (September 30, 2008)