Ocean Dead Zones: A Growing Threat to Marine Life

Ocean dead zones are areas of the marine ecosystem near the sea floor that are unable to support marine life due to hypoxic (low oxygen) conditions. Hypoxia is defined as having a dissolved oxygen concentration of less than 2 parts per million. When the oxygen concentration falls below this level, adult fish and mobile marine creatures leave the hypoxic zone if they are able, but many younger fish, shrimp and bottom-dwelling organisms such as oysters and clams cannot escape and will suffocate and die. Widespread loss of sea life is devastating to marine ecosystems including seabirds and animals that rely on fish and shellfish for food. Dead zones also negatively impact human activities such as the commercial fishing industry.

Since dead zones were first discovered in the 1960s, their numbers and size have been steadily increasing. At present, more than 400 ocean dead zones have been detected world-wide, with a combined area of 95,000 square miles, or about 2% of the world's seas.

Hypoxic zones occur most frequently near densely populated coastlines. The world's largest dead zone is located in the Baltic Sea, and others are found across the globe, off the coasts of Europe, Asia, Australia and South America. Extensive hypoxic areas are present along the east and south coasts of the United States, as well as certain parts of the west coast. A large percentage of the Chesapeake Bay is a hypoxic zone, causing the deaths of hundreds of thousands of fish during the summer months. The 8,500 square mile dead zone in the Gulf of Mexico, along the coasts of Louisiana and Texas, is the second-largest in the world.

The primary cause of ocean dead zones is ocean pollution due to runoff from chemical fertilizers containing nitrogen and phosphorus compounds. Rain washes the fertilizer from agricultural fields into streams and rivers, and from there it is transported to the sea. Excessive nitrogen and phosphorus in the water provide nutrients for phytoplankton, triggering massive algal blooms in a process called eutrophication. When the algae die, they sink to the bottom and are decomposed by bacteria. It is this decomposition process that removes oxygen from the water. The massive dead zone in the Gulf of Mexico is caused by 1.6 million tons of nitrogen washed down from the Mississippi River basin every year.

Another major cause of ocean dead zones is the burning of fossil fuels, particularly in coal-fired power plants. When fossil fuels are burned, they produce nitrogen oxides that remain airborne as smog. Rain carries these nitrogen compounds into the ocean.

Oxygen is less soluble in warmer water, so as the temperature of the sea water increases, the dissolved oxygen concentration decreases. Dead zones often fluctuate in size with the seasons, growing larger during the summer months and shrinking during the winter.

Some scientists are predicting an expansion of ocean dead zones in the future as a result of increasing carbon dioxide levels in the atmosphere combined with rising ocean temperatures. See this National Geographic News article for more information on the effect of climate change on ocean dead zones.

It has been possible for some dead zones to recover once the factors causing them are removed. A notable example is the Black Sea, which became an extensive dead zone in the early 1960s due to fertilizer runoff from the Danube River. In 1989, the collapse of the political system in the surrounding countries caused farming procedures to change. Fertilizer use was drastically reduced, and by the late 1990s, the dead zone had largely disappeared.

Reduction of fertilizer use and limiting the burning of fossil fuels by developing alternative energy sources would go far in helping dead zones recover, but other factors such as increasing temperatures and carbon dioxide in the atmosphere create a level of complexity that will not easily be resolved.

http://disc.sci.gsfc.nasa.gov/oceancolor/additional/science-focus/ocean-color/science_focus.shtml/dead_zones.shtml

http://news.mongabay.com/2010/0722-hance_deadzones_nasa.html