Nutrient Pollution

Choking the Seas

Not all ocean pollution arrives there directly; rivers and streams act as arteries to carry manmade toxins into the seas. It’s called nutrient pollution, which might seem to be something of an oxymoron. After all, nutrients such as nitrates and phosphates are essential to all living organisms and play a vital role in the functioning of life on Earth. But, says Boyce Thorne-Miller, senior scientist with SeaWeb, “We’re introducing massive amounts of nutrients-way above normal levels-into coastal waters, through things like fertilizer and sewage from agriculture, and the burning of fossil fuels by factories and cars. And these are having devastating effects.” Indeed, the Environmental Protection Agency (EPA) has listed “nutrients” as the major cause of impaired estuaries in this country, while a panel of scientists in 1997 described nitrogen pollution as arguably “the most serious human threat to the integrity of coastal marine ecosystems.”

Water pollution is implicated in a worrying outbreak of coral disease–like this case o fbleaching in Cayman Islands pillar coral.Photo: © Mike Bacon/Tom Stack & Associates

Among many effects, nutrient pollution is believed to be a major contributor to what Dr. Ted Smayda, of the Graduate School of Oceanography at the University of Rhode Island, has termed a “global epidemic of harmful algal blooms.” Algal blooms are natural phenomena, and an integral component of marine productivity. In recent years, however, there has been an increase in the duration, distribution and extent of “nuisance” blooms, including some which are highly toxic.

Pfiesteria piscicida, the so-called “cell from hell” which has killed billions of fish and caused illness in fishermen, scientists and others along the east coast, is believed to be stimulated by nutrients from agricultural operations. Blooms of another toxic species, Gymnodinium breve, have been implicated in the deaths of humpback whales in 1987 and Florida manatees in 1982 and 1996.

Some algal species bloom in such large quantities that they outstrip the ability of predators to feed on them. Huge numbers of dead algae then sink to the bottom, where they are decomposed by bacteria which consume oxygen in the water. Eventually, oxygen supplies in the water column and in sediments become depleted, choking resident marine life and causing mobile species to flee the area. The result is a large area, known as a “dead zone” where fish, shellfish and marine plants cannot live. In the Gulf of Mexico, there is a “dead zone” which lasts about eight months a year and covers an area of up to 5,400 square miles. In the Baltic Sea, the same phenomenon has led to the virtual extinction of bottom-dwelling animal life over an area of about 42,000 square miles.