Background

Eutrophication

Excessive growth of marine plant life – eutrophication – is potentially one of the most damaging of the many harmful effects that humans have on the oceans, both in its scale and its consequences. It can turn parts of the sea into wastelands. Plants in the oceans, as on land, need adequate nourishment from minerals and organic substances if they are to grow well. Life is far more profuse in coastal waters, which are rich in these nutrients, than in the open oceans. Areas with poor supplies of nutrients support little life; their transparent and apparently 'clean' blue waters may be aesthetically attractive, but biologically they resemble deserts on land.

Waters, however, can have too much nutrient. When this happens, usually because of pollution from the land, plant life – phytoplankton or algae – proliferates. Long-term increases in phytoplankton, and their decay near the seabed, can deplete oxygen over large areas, either periodically or permanently – and dramatically alter ecosystems. Coastal areas with relatively little circulation of their waters are particularly vulnerable. A "dead zone" with far too little oxygen, for example, appears off Louisiana in the Gulf of Mexico each summer; excessive nitrogen from agricultural fertilizer used upstream, and flushed down the Mississippi River, has been blamed.

Increases in the abundance of phytoplankton also make water less transparent, and thus reduce the penetration of sunlight into the sea. Coral reefs, seagrass beds – and other ecosystems that depend on light – can suffer. And the reefs can be threatened in another way too. Eutrophication can cause seaweeds on the ocean floor to grow so fast that they outstrip the corals and smother them; the reefs stop growing and start to erode, and much of the diversity of the ecosystem is lost.

Eutrophication can also cause explosive blooms of algae – such as 'red tides' – which cover the surface of the sea. And changes in the relative amounts of different nutrients can stimulate the growth of toxic or otherwise harmful algae. The toxins can accumulate in shellfish and poison people who eat them. One explosion of algae in Chesapeake Bay, for example, killed thousands of fish, made dozens of people ill, and sent sales of crabs, oysters and fish plummeting. The poisons can also be blown to land, at times causing eye irritation, respiratory problems, and other complaints.

Toxic algae can also harm other marine life – including whales, dolphins and other marine mammals – and cause hundreds of millions of dollars worth of damage to commercial fisheries. They devastate tourism in areas like the Adriatic, and damage aquaculture, with massive economic and social costs. There are indications that the blooms, toxic or otherwise, are increasing.

GESAMP (IMO/FAO/UNESCO-IOC/WMO/WHO/IAEA/UN/UNEP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). 2001. A sea of troubles. Rep. Stud. GESAMP No. 70, 35 pp. ISBN 82-7701-010-9.