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EUTROPHICATION AND WATER QUALITYIntroduction Coral reefs have evolved in the lowest nutrient environment in the world, the tropical ocean, where plants often consume all available nitrogen and phosphorus, at which point new growth is limited to rates at which these elements are provided by decomposition of dead organisms. Although there is abundant nitrogen and phosphorus in the deep sea it cannot reach surface layers where bright light promotes the growth of plants and animals because a thick layer of warm waters floating on top of deep cold nutrient-rich water prevents its being mixed upward to the surface. Tiny increases in nutrients above the near zero level are probably beneficial to corals, but it takes only very small increases for the net effect to turn negative. This is not because high nutrients harm corals directly, but because corals are quickly overgrown by much faster growing algae which need higher nutrient levels than corals. Only very little excess nutrients are needed to turn healthy coral reefs into waving fields of algae which smother and kill corals. This phenomenon is called eutrophication. Eutrophication takes place in all ecosystems and is responsible for green scummy layers of algae covering ponds into which sewage and manure flows. Coral reefs go eutrophic at the lowest level of nutrients of any aquatic ecosystem: nutrient levels which would be regarded as very low in any other marine or freshwater habitat will kill coral reefs. This web site has papers discussing these problems, and their solutions, in more detail. Major sources of excessive nutrients include sewage, livestock manures, agricultural fertilizers, soils eroding away after deforestation, and upwelling of deep ocean waters. Where nutrient inputs are episodic, for example where there are strongly seasonal rivers, tourist sewage, or upwelling inputs reefs may be eutrophic part of the year only. Where nutrients continue to increase, coral will be killed. Due to the large increase of nutrients released into coastal waters from sewage discharged directly into the ocean or delivered via rivers and ground waters, the reefs off all coastal areas which are densely populated or developed for tourism are already eutrophic or quickly turning so. This can happen very rapidly, and in only a few years healthy reefs can be turned into coral graveyards. Probably no coral reef country is free of this problem, not even the smallest. Critical Levels of Nutrients These values are all regarded as extremely low levels, almost undetectable, in coastal waters of temperate and cold zones. For years researchers measured concentrations in this range but thought that values were too low to possibly cause problems to reefs. This was wrong because they used irrelevant standards for acceptable nutrient levels. It is essential that appropriate water quality standards be applied in coral reef ecosystems if they are to be protected against eutrophication. These standards must be below the levels given above. In general, where water quality standards have been applied for tropical waters, they are often based on uncritical adoption of nutrient standards from North America and Europe that are irrelevant to the tropics because cold ecosystems are normally exposed to much higher nutrient levels. Many nutrient water quality standards available are related to human health and are even more worthless for coral reefs because humans can drink water with nutrient levels hundreds of times higher than coral reefs can stand. This site includes papers on the relationship between nutrients, coral health, and algae abundance. For more information on water quality standards and data on nutrients and coral reefs we recommend reading the papers by Brian Lapointe on the Reef Relief web site, which can be accessed directly from our LINKS page.
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