Effects Of Crude Oil On Marine Life

The population of marine microorganisms that can metabolize soluble hydrocarbons increases dramatically in the vicinity of an oil spill. The growth of these microorganisms in the ocean is limited by the hydrocarbon content of the water, so they multiply at a much greater rate when there is a big increase in the hydrocarbon content of the seawater. As noted earlier, nitrogen is the growth-limiting nutrient in the presence of excess hydrocarbons.

Almost all forms of life absorb hydrocarbons and are affected by crude oil. The acute toxicity (an imminent health hazard) of crude oil is proportional to the percentage and structures of the aromatic compounds present. Soluble aromatic hydrocarbons containing two or more condensed rings have the highest acute toxicity. Ingestion of crude oil by birds and mammals inevitably leads to their death. The bulk of the oil ingested is a result of an animal's attempts to remove the oil from feathers or fur. Ingestion is also a consequence of eating oiled plants or animals. For example, seals can easily catch and eat oiled birds that are unable to fly. Plants and coral reefs are also susceptible to the toxic effects of crude oil. Sea grasses and mangrove forests have been killed by crude oil.

Birds and mammals are weakened by oil that binds fur or feathers together. This greatly decreases the insulating capacity of the fur or feathers, with the result that the animals lose heat at a much more rapid rate than before. Consequently they must eat much more food to maintain their body temperature. This is a serious problem for mammals like polar bears and sea otters, which spend a lot of time in the sea and are unable to escape from the vicinity of the spill. These animals also experience eye irritation as a consequence of their surfacing in the oil and getting it in their eyes. Laboratory studies and other studies have demonstrated that crude oil is more toxic to eggs and developing young marine life than it is to adults. Such studies also indicate that marine life does not instinctively avoid oil spills.

The toxic effects of the crude oil appear to be short-lived. Most marine life repopulates a previously affected area within one to two years, and there is little visible effect on the ecosystem. The rate of recovery is strongly dependent on the activity of the ocean on the shoreline. In areas where there is active surf, the ocean constantly scrubs the beach and the rocks, and the oil is dispersed. Oil spilled in a protected area may persist for many years, so it takes correspondingly longer for the environment to return to its prespill condition.

The indirect effects of oil spills are not well known, but these are potentially as important as the direct effects just noted. It is known that fish and other forms of marine life are stimulated in their search for food, in their escape from predators, and in their mating by the organic compounds in the sea. The messages received by the fish may be masked by the presence of petroleum, or the compounds present in the crude oil may convey incorrect messages.

We now turn to some of the numerous laboratory studies to determine the effects of crude oil on various types of marine life as well as observations made at the sites of oil spills.

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