The process of nutrient cycling is critical to all ecosystem services. However, when the flux of nutrients into the environment exceeds the ability of natural systems to absorb them, aquatic ecosystems feel the greatest impact. An oversupply of nutrients in aquatic systems can cause excessive growth of algae, reduce species diversity, damage coral reefs, and, in extreme cases, create oxygen-depleted "dead zones."
Eutrophication--the presence of excessive nutrients, usually nitrogen or phosphorus--is the most ubiquitous freshwater quality problem in the world (see UNEP's 2007 Global Environmental Outlook for more information). Since industrialization, fertilizer application, nitrogen-fixing crops, and air pollution have more than doubled the flow of nitrogen through air, water, and land. The flux of phosphorus has tripled during the same time period. (EarthTrends' data sets on fertilizer consumption and air pollution show recent growth in some of the inputs and sources of eutrophication.)
The effects of nutrient overloads were first observed in the 1960s, when harmful algal blooms, increased turbidity, and fish kills were noticed in the lakes, rivers, and coastal areas of industrialized nations. By 1993, the United Nations Environment Program surveyed eutrophic conditions in more than one-half of all lakes in Europe and Asia.
In coastal areas, however, research into eutrophication and hypoxic zones has been more limited in scope. To close these gaps in knowledge, WRI, together with the Virginia Institute of Marine Science, has identified and mapped 415 eutrophic and hypoxic coastal systems worldwide. Of these, 169 are documented hypoxic areas, 233 are areas of concern and 13 are systems in recovery (see the map below).
Source: World Resources Institute 2008.
A larger version of this map can be found here.
The map shows three types of eutrophic zones:
(1) Documented hypoxic areas - Areas with scientific evidence that hypoxia was caused, at least in part, by an overabundance of nitrogen and phosphorus. Hypoxic areas have oxygen levels low enough to inhibit the existence of marine life.
(2) Areas of concern - Systems that exhibit effects of eutrophication, including elevated nitrogen and phosphorus levels, elevated chlorophyll 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.
(3) 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.
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.
A more detailed analysis of this data set will be available in February 2008 in a policy note entitled Eutrophication and Hypoxia in Coastal Areas: A Global Assessment of the State of Knowledge (a list of related publications can be found here).
Adapted from Choking Coastal Waters by Mindy Selman at wri.org.
RELATED LINKS:
The Gulf of Mexico Hypoxia Watch
Eutrophication in Europe's Coastal Waters
EarthTrends
Nutrient Overload: Unbalancing the Global Nitrogen Cycle
November 2006 Monthly Update: The Value of Coastal Ecosystems
Dirty Water: Pollution Problems Persist
