Even though the old adage that oil and water do not mix is partially untrue, oil does tend to concentrate at the water's surface or, especially if absorbed in sediment, on the bottom. This means that the impact of oil on a marine ecosystem is not uniform but is greater on organisms living at or near the ocean's surface—intertidal life, neuston (small ocean-surface dwellers), and sea birds—and those organisms living on the seabed, the benthos. The following discussion is a summary of the knowledge of the effects of oil on some major marine communities and environments.

BIRDS AND MAMMALS

The layman's picture of the biological effects of oil spills is mainly of oil-fouled birds on beaches. The obviousness of this effect together with our feelings toward warm-blooded animals have meant that the oiling of marine mammals and especially birds has received much publicity and has been the subject of much literature ().

Many dead sea birds have been observed after most spills of crude and heavy fuel oils. The death toll has usually been estimated by counting the number of oiled birds washed onto beaches, but these estimates often do not include dead birds not reaching shore. It has been estimated that only 5 to 15 percent of those birds killed by oil actually wash onto shores (). Nonetheless, 40 thousand to 100 thousand birds were reported killed by the Torrey Canyon spill (), 3,686 by the Santa Barbara blowout (), and 7 thousand by the San Francisco Bay spill (). Such spills have often killed significant proportions of some bird species within the affected areas.

The primary effect of oil on birds has been the fouling of feathers, to which oil clings. The small feather barbules are disheveled, thus allowing a disruption of the insulation and buoyancy of the feathers. Birds may sink and