Adverse Impacts on Marine Ecosystems

Many years ago, the vastness of the oceans made it easy to assume that nothing that humans could do to it could have any long-term or widespread effect. Today, however, our species has become so successful that nothing in the biosphere can be held to be insensitive to our activities.

Overfishing. Improvement of fishing technology led to overfishing in many marine fisheries. There are many instances of this. Peru led the world in its catch with 4 million metric tons in 1960. In 10 years it increased the harvest to 13 million metric tons. But this high level, combined with a series of El Nino events, devastated the industry until it was reduced to 100,000 tons in 1985. Cod and herring are seriously depleted in the North Atlantic. The Pacific sardine has been reduced to commercial extinction, in which it is no longer profitable to harvest it. Of 11 species of large whales that have been hunted commercially, eight are now commercially extinct.

When a fishery becomes overfished, instead of prudently reducing its take, fishermen instead increase their efforts by adding more boats and more efficient methods. One of these methods is called drift net fishing, in which a 7-m-wide net up to 80 km long is deployed vertically overnight, then hauled in the next morning. This method ensnares turtles, birds, and marine mammals that are not intended for harvesting. Furthermore, about 1600 km of these nets are lost each season, becoming "ghost nets'' that drift and kill for decades.

Oil Pollution. Oil pollution in the ocean is sometimes very dramatic. However, spills due to accidents are a small part of the total discharge: only 13% in 1985. A significant fraction, about 8%, is of natural origin. A major source (about one-third) is due to routine flushing of oil tankers when loading and unloading. Another third comes from use and disposal on land via rivers and streams. Once in the water, fractions of crude oil may disperse by evaporating, dissolving, forming emulsions, or settling to the bottom. Floating oil slicks may directly harm wildlife at the surface, especially birds, and can devastate benthic organisms of the littoral zone. These effects are most severe and may persist for many years. The dispersed fractions are biodegraded or assimilated by zooplankton and higher organisms, thus entering the food chain. Compounds absorbed by organisms have toxic effects at all trophic levels. Also, they may affect the commercial use of the resource by imparting an unpleasant taste to fish, shellfish, and so on. Spills of refined hydrocarbons can be more damaging than crude oil, since it contains lighter fractions and additives that are more easily taken up by organisms and are often more toxic.

Water Pollution. Coastal areas are affected by discharge of water pollutants from land sources via rivers, direct pipeline disposal, or dumping by barges. Barge dumping was practiced by communities in the New York Metropolitan Area up to the early 1990s. Originally, sewage sludge dumping was confined to a 10-km-square site some 20 km from the coast of New Jersey and Long Island. When it became known that sludge was accumulating at that site, an interim site was chosen in deeper waters at a distance of 196 km from New York Harbor. The new site provided enhanced dilution. Ultimately, ocean dumping was banned altogether in favor of land-based alternatives.

When these discharges are used as an alternative to treatment, they discharge toxic materials and nutrients to the marine environment. The nutrients can cause algal blooms in excess of what might occur naturally. Sometimes, the algae themselves are toxic to marine life. Other times, the algae have been responsible for fish or benthos kills due to deoxygenation, when the algae decompose. Discharge of sewage and sludge have also been blamed for diseases of finfish, including "black gill'' and "fin rot.''

Severe cases of hypoxia (low-dissolved-oxygen conditions) have been observed in the New York Bight and in the Gulf of Mexico. The New York Bight is the continental shelf area into which the Hudson River discharges. In one incident in the late 1980s, dissolved oxygen levels dropped to low levels over a 300 x 100 km area in the Bight, falling all the way to zero in a portion of the region off Atlantic City, New Jersey. A similar incident occurred in a 420-km band along Louisiana in the Gulf of Mexico. Even in the absence of such catastrophes, continuing sanitary pollution causes public health risks to swimmers in the ocean and eliminates many shellfish beds from use as a resource due to contamination by pathogens.

Coral Diseases. In 1983, scientists discovered a disturbing disease of coral called coral bleaching. For unknown reasons, the coral will expel its zooxanthellae, leaving the coral a pale color. Without their symbionts, coral cannot deposit calcium carbonate and are subject to erosion. If it does not regain its zooxanthellae, the coral dies.

This was first noticed in the Pacific but has since been found in Caribbean coral reefs as well. The cause of this disease is still unknown and is being sought with some urgency by marine biologists. Some blame a combination of pollution and high temperatures. The original observation occurred during a particularly severe El Nino event in the eastern Pacific, when sea surface temperatures averaged 30 to 31 °C for 5 to 6 months.

Coral are vulnerable to other problems as well. The crown-of-thorns sea star Acantha-ster planci is destroying coral reefs in the western Pacific. A number of other diseases are found on coral, including white-band disease. Anecdotal evidence points to many of these problems as having proliferated recently. Thus, human activities are being blamed, although there is no strong evidence for it.

Thermal Pollution. Industries, especially steam-powered electrical generating plants, often discharge large amounts of heated water to adjacent surface waters. At low levels these may provide an energy subsidy to the ecosystem, encouraging growth. The benefits may vary by organism, resulting in population shifts. Detrimental effects include stimulation of fungal organisms, which may cause disease for aquatic organisms. Cyanobacter have been found to dominate at temperatures above 35°C, green algae between 28 and 35°C, and diatoms at lower temperatures. Oxygen consumption rates increase with temperature, but oxygen solubility decreases, both effects increasing oxygen depletion. Stress may occur if the heated discharge is suddenly halted due to operating considerations of the source.

The sensitivity of animals to temperature may be measured by the upper lethal temperature (ULT), the temperature at which 50% mortality occurs over long-term exposure. The ULT for the fishes Salmo trutta and Salmo gairdneri was measured at 25 and 24°C, respectively. Others, including the fish Carassius auratus, Cyprinus carpio, and Lepomis macrochirus and the invertebrates Gammarus fasciatus, Procladius, and Cryptochirono-mus had ULT between 30 and 35°C. At lower temperatures, growth and reproduction will be inhibited.

Use of large volumes of surface water for cooling can have a direct effect as the water is passed through pumps and heat exchangers. Shear and pressure forces can kill or damage organisms. Discharge of water from below the thermocline in reservoirs can produce a low-termperature anomaly. These usually have nonlethal effects, such as population shifts or inhibition of spawning of fish.

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