Ocean Dead Zones

Dead zones are hypoxic (low-oxygen) areas in the world’s oceans and large lakes, caused by “excessive nutrient pollution from human activities coupled with other factors that deplete the oxygen required to support most marine life in bottom and near-bottom water. (NOAA)”. In the 1970s oceanographers began noting increased instances of dead zones. These occur near inhabited coastlines, where aquatic life is most concentrated. (The vast middle portions of the oceans, which naturally have little life, are not considered “dead zones”.)

Dead zones are human-caused. They occur when crop fertilizer and cow poop, containing high levels of nitrogen and phosphorous, get washed into streams and rivers and out to the ocean.

In March 2004, when the recently established UN Environment Programme published its first Global Environment Outlook Year Book (GEO Year Book 2003), it reported 146 dead zones in the world’s oceans where marine life could not be supported due to depleted oxygen levels. Some of these were as small as a square kilometre (0.4 mi²), but the largest dead zone covered 70,000 square kilometres (27,000 mi²).

Less oxygen dissolved in the water is often referred to as a “dead zone” because most marine life either dies, or, if they are mobile such as fish, leave the area. Habitats that would normally be teeming with life become, essentially, biological deserts.

Hypoxic zones can occur naturally, but scientists are concerned about the areas created or enhanced by human activity. There are many physical, chemical, and biological factors that combine to create dead zones, but nutrient pollution is the primary cause of those zones created by humans. Excess nutrients that run off land or are piped as wastewater into rivers and coasts can stimulate an overgrowth of algae, which then sinks and decomposes in the water. The decomposition process consumes oxygen and depletes the supply available to healthy marine life.

Dead zones occur in many areas of the country, particularly along the East Coast, the Gulf of Mexico, and the Great Lakes, but there is no part of the country or the world that is immune. The second largest dead zone in the world is located in the U.S., in the northern Gulf of Mexico.

One of the largest dead zones forms in the Gulf of Mexico every spring. Each spring as farmers fertilize their lands preparing for crop season, rain washes fertilizer off the land and into streams and rivers.

Causes of Dead Zones

The cause of such “hypoxic” (lacking oxygen) conditions is usually eutrophication, an increase in chemical nutrients in the water, leading to excessive blooms of algae that deplete underwater oxygen levels. Nitrogen and phosphorous from agricultural runoff are the primary culprits, but sewage, vehicular and industrial emissions and even natural factors also play a role in the development of dead zones.

Dead zones occur around the world, but primarily near areas where heavy agricultural and industrial activity spill nutrients into the water and compromise its quality accordingly. Some dead zones do occur naturally, but the prevalence of them since the 1970s—when dead zones were detected in Chesapeake Bay off Maryland as well as in Scandinavia’s Kattegat Strait, the mouth of the Baltic Sea, the Black Sea and the northern Adriatic—hints at mankind’s impact. A 2008 study found more than 400 dead zones worldwide, including in South America, China, Japan, southeast Australia and elsewhere.

Where are Dead Zones?

The has been a staggering increase in the number of dead zones worldwide over the past 60 years, from just 42 in 1950, to 405 in 2008. Dead zones now cover 95,000 square miles; this is the size of the United Kingdom.

The largest dead zone worldwide is the Baltic Sea. Overfishing of Baltic cod has greatly intensified the problem. Cod eat sprats, a small, herring-like species that eat microscopic marine creatures called zooplankton that in turn eat the algae. So, fewer cod and an explosion of zooplankton-eating sprats means more algae and less oxygen – a vicious cycle develops (Westman, 2010)

The second largest dead zone is the northern Gulf of Mexico, surrounding the outflow of the Mississippi River, in the summer of 2002 it covered 8,500 square miles (approximately the same size as the state of Massachusetts). The catchment area of the Mississippi River basin is vast, draining approximately 41% of the land area of the continental United States.

The Chesapeake Bay also has a significant dead zone, each summer it occupies about 40% of the area of the Bay and 5% of its volume, stretching from Baltimore Harbor all the way down below the Potomac into Virginia waters.

The first dead zone in the Chesapeake Bay was reported in the 1930’s and it has been growing larger over the years. The dead zone this year (2011) may be one of the Bay’s five worst ever, covering a massive 1,200 square miles. The average volume of hypoxic water (Dissolved Oxygen ¡Â2.0 mg-L 1) in the Chesapeake Bay is predicted to be 6.8 km©ø for June to September 2011. This would be the 4th highest hypoxic volume for the period of record (1985-2010) (Eco-Check). On the Severn River there have been reports of oxygen levels as low as 0.02 mg-L 1, dangerously low for fish, crab and shellfish.

Hypoxic conditions can also develop naturally, and may be permanent or semi-permanent. They can form in areas of upwelling, or when there are changes in wind and circulation patterns. The Black Sea has been anoxic in its deepest parts for millennia, due to its shallow sill which prevents mixing and oxygen replenishment. There are also areas of the eastern tropical Pacific Ocean and Northern Indian Ocean where minimum circulation occurs creating Oxygen Minimum Zones (OMZ).

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