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Choking Coastal Waters

My team at WRI, together with Dr. Bob Diaz at the Virginia Marine Institute, has identified and mapped 415 eutrophic and hypoxic coastal systems worldwide through an extensive literature review. Of these, 169 are documented hypoxic areas, 233 are areas of concern and 13 are systems in recovery.

Our findings highlight the dramatic growth of areas receiving the endflows of nitrogen and phosphorus created by agriculture, increasing industry, fossil fuel combustion, and population growth. More than 1,000 scientists estimated, in the Millennium Ecosystem Assessment, that, as a result of human activities over the past 50 years, the flux of nitrogen has doubled over natural values while the flux of phosphorus has tripled.

The effects of nutrient pollution in coastal waters may include excessive growth of algae, including harmful algae species that can cause fish kills and shellfish poisoning in humans; reduced species diversity and dominance of gelatinous organisms such as jellyfish; damage to coral reefs; and formation of oxygen-depleted "dead zones."

The map shows three categories:

  • Documented hypoxic areas - Areas with scientific evidence that hypoxia was caused, at least in part, by an overabundance of nitrogen and phosphorus.
  • Areas of concern - Systems that exhibit effects of eutrophication, including elevated nitrogen and phosphorus levels, elevated chlorophyll a levels, harmful algal blooms, changes in the benthic community, damage to coral reefs, and fish kills. These systems are impaired by nutrients and are possibly at risk of developing hypoxia. Some of the systems may already be experiencing hypoxia, but lack conclusive scientific evidence of the condition.
  • Systems in recovery - Areas that once exhibited low dissolved oxygen levels and hypoxia, but are now improving. For example, the Black Sea recovery is largely due to the economic collapse of Eastern Europe in the 1990s, which greatly reduced fertilizer use. Others, like Boston Harbor in the United States and the Mersey Estuary in the United Kingdom also have improved water quality resulting from better industrial and wastewater controls.


The actual extent and prevalence of eutrophication is only beginning to be studied. Some countries, such as the United States and European Union, have undertaken comprehensive coastal surveys in the past five years, and have the most comprehensive coastal data on eutrophication. However, data do not exist or are not publicly available for areas that may be suffering from the effects of eutrophication.

Given the state of global data, the actual number of eutrophic and hypoxic areas around the world is likely to be greater than the 415 listed here. The most under-represented region is Asia. Asia has relatively few documented eutrophic and hypoxic areas despite large increases in intensive farming methods, industrial development, and population growth over the past 20 years. Africa, South America, and the Caribbean also have few reliable sources of coastal water quality data.

It is critical that we begin to close the gaps in our knowledge about where eutrophication is occurring globally.

As part of WRI's work on this topic, we will release a policy note in March 2008 entitled Eutrophication and Hypoxia in Coastal Areas: A Global Assessment of the State of Knowledge.

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