BIOMES OF THE EARTH - OCEANS Phần 9 pptx

Bahamas-registered Prestige, sank off the coast of Spain in

November 2002, releasing fuel oil onto Spanish fish and

shellfish grounds and polluting more than 185 miles (300

km) of coastline. The wreck has the potential to release

twice as much oil as the Exxon Valdez spill of 1989.

Heavy metals

Domestic and industrial wastewater, and runoff from pol￾luted land, may contain trace amounts of heavy metals such

as mercury, lead, and cadmium that are poisonous to many

forms of life.

The most dramatic case of heavy-metal pollution hap￾pened in Minamata Bay, Japan, in the mid-1950s. A local

plastics factory began discharging mercury-contaminated

Crude oil from the

Exxon Valdez floating

on the sea surface in

Prince William Sound,

Alaska (Courtesy of Flip

Nicklin/Minden Pictures)

204 OCEANS

THE HEALTH OF THE OCEANS 205

wastewater into the sea. The wastewater contained a particu￾larly harmful mercury-based substance, methyl mercury. This

substance enters the cells of organisms more readily than

mercury itself. An unusually large number of local people

began experiencing a range of symptoms: headaches, shak￾ing, paralysis, and even blindness. It took about 10 years for

the authorities to be convinced that the contaminated waste￾water was the cause of the outbreak. By that time, several

hundred people had “Minamata disease” and were disabled

by it. Eventually, more than 100 people were to die as a direct

result of the methyl-mercury poisoning.

Studies at Minamata showed that plankton were taking in

methyl mercury and plankton-eating fish and shellfish were

consuming the poison and concentrating it within their

bodies. If organisms cannot break down or get rid of a poi￾son, then it is passed to animals higher up the food chain

that can accumulate large amounts of it. This process is

called biomagnification. In Minamata Bay locals who ate

plenty of seafood were consuming dangerously high levels

of methyl mercury.

Today, most governments recognize the threat to human

and environmental health posed by heavy-metal pollution.

They impose laws to stop companies from discharging metal￾contaminated wastewater.

Exxon Valdez oil spill

On March 24, 1989, the supertanker Exxon Valdez struck a reef in Prince William Sound,

Alaska. About 38,500 U.S. tons (35,000 tonnes) of oil spilled into the sea. The oil spill

eventually covered more than 1,200 miles (1,950 km) of shore in an unsightly sludge that

smothered or poisoned wildlife. Experts estimated that up to 300,000 seabirds, 5,000 sea

otters, and 300 harbor seals died in the incident. The local herring- and salmon-fishing

industry was devastated. However, the affected shores and coastal waters are slowly

recovering. By 2000 the salmon were back and so were many seabirds, although harbor

seals and herring had yet to return. Today safety vessels escort all tankers that enter Prince

William Sound.