Coastal Dead Zones

Coastal dead zones, areas of oxygen depleted waters incapable of supporting marine life, continue to spread along coastal areas around the world.

Additional Resources Great Lakes Algal Blooms Toxic Algal Blooms Beach Pictures and Info Beach Water Quality

Coastal dead zone episodes in United States coastal waters are seasonal occurrences, whose cause can be traced to one of two different explanations.

The most common dead zones are found along areas where river mouths intersect with coastal waters. Nutrients from agriculture runoff collect and flow down river to the ocean, creating a super supply of food for the local phytoplankton, single-celled plants, that grow into large groups called algae blooms. When the blooms die and fall to the ocean floor, the decomposition process depletes the area of oxygen.

The Gulf of Mexico dead zone is the largest in the United States. Agriculture run off from the farm belt around the Mississippi River flows to the Gulf of Mexico causing a seasonal dead zone. Researchers from the National Oceanic and Atmospheric Administration (NOAA) have been studying the Gulf's dead zone for over twenty years and recently reported that the 2007 dead zone will reach record levels.

Natural causes serve as the primary triggers for the second type of dead zone. Over the past five years, irregular northerly summer winds along the Oregon Coast have cause coastal waters to upwell, or turn over. The colder water that is pushed to the surface contains sufficient nutrients to support the algae bloom and decomposition process.

The 2006 dead zone episode lasted close to four months and covered the northern half of the Oregon coast with probable intrusion into the southern Washington coast. Dead crabs, sea cucumbers and other marine organisms were documented along the length of the dead zone.

NOAA provides a good reference on the topic. It comes complete with descriptions, satellite photographs and maps of dead zones around the world.

The Baltic Sea, for example, represent another dead zone type that fits both the natural and man-made categories. The map in the link shows a mostly enclosed sea. The lack of water exchange between the Baltic Sea and the Atlantic and Arctic Oceans makes the sea water naturally more stagnent and prone to natural dead zone episodes. Nutrient loading from agriculture activities of states surrounding the Baltic Sea exacerbates the problem, leading to persistent dead zones. (see Baltic Sea Dead Zone)

A subset of algae blooms consist of toxic species of phytoplankton. The link in the additional resources box provides additional information.

© 2007 Patricia A. Michaels