Evaporation occurs when the lighter substances within the oil mixture become vapors and leave the surface of the water. This process leaves behind the heavier components of the oil, which may undergo further weathering or may sink to the bottom of the ocean floor. Spills of lighter refined products such as kerosene and gasoline contain a high proportion of flammable components known as light ends. These may evaporate completely within a few hours, causing minimal harm to the environment. Heavier oils leave a thicker, more viscous residue. Wave action from rough seas increases both evaporation and natural dispersion.

Oxidation occurs when oil contacts the water, and oxygen combines with the oil hydrocarbons to produce watersoluble compounds. This process affects oil slicks mostly around their edges. Thick slicks may only partially oxidize, forming tar balls. These dense, sticky black spheres may linger in the environment, washing up on shorelines long after a spill.

Biodegradation occurs when microorganisms such as bacteria feed on oil hydrocarbons. A wide range of microorganisms is required for a significant reduction of the oil. To sustain biodegradation, nutrients such as nitrogen and phosphorus are sometimes added to the water to encourage the micro-organisms to grow and reproduce. Biodegradation tends to work best in warm water environments.

Emulsification is the process that forms emulsions, which are mixtures of small droplets of oil and water. Emulsions. are formed by wave action, and they greatly hamper weathering and cleanup processes. Two types of emulsions exist: water-in-oil and oil-inwater. Water-in-oil emulsions are frequently called "chocolate mousse,"

After oil is spilled, the most toxic substances in it evaporate quickly. Therefore, plant, animal, and human exposure to the most toxic substances are reduced rapidly with time, and are usually limited to the initial spill area. Although some organisms may be seriously injured or killed very soon after contact with the oil in a spill (lethal effects), non-lethal toxic effects are more subtle and often longer lasting. For example, marine life on reefs and shorelines is at risk of being smothered by oil that washes ashore or of being slowly poisoned by long-term exposure to oil trapped in shallow water or on beaches.

Sensitivity of marine habitats

The marine environment is made up of complex interrelations between plant and animal species and their physical environment.

Harm to the physical environment will often lead to harm for one or more species in a food chain, which may lead to damage for other species further up the chain. Where an organism spends most of its time -- in open water, near coastal areas, or on the shoreline will determine the effects an oil spill is likely to have on that organism.

In open water, marine organisms such as fish and whales have the ability to swim away from a spill by going deeper in the water or further out to sea, reducing the likelihood that they will be harmed by even a major spill. Marine animals that generally live closer to shore, such as turtles, seals, and dolphins, risk contamination by oil that washes onto beaches or by consuming oil-contaminated prey. In shallow waters, oil may harm sea grasses and kelp beds, which are used for food, shelter, and nesting sites by many different species.

Spilled oil and cleanup operations can threaten different types of marine habitats, with different results.

Coral reefs are important nurseries for shrimp, fish, and other animals as well as recreational attractions for divers. Coral reefs and the marine organisms that live within and around them are at risk from exposure to the toxic substances within oil as well as smothering.

Exposed sandy, gravel, or cobble beaches are usually cleaned by manual techniques. Although oil can soak into sand and gravel, few organisms live full-time in this habitat, so the risk to animal life or the food chain is less than in other habitats, such as tidal flats.

Sheltered beaches have very little wave action to encourage natural dispersion. If timely cleanup efforts are not begun, oil may remain stranded on these beaches for years.