Petroleum is a naturally occurring substance thought to be formed from decaying plants and animals under pressure and heat over a long period of time. Petrogenic and biogenic hydrocarbons coexist in the environment and it is often impossible to distinguish their origins. The complexity of an oil spill in the ocean is much greater than in other places - such as on land. Petroleum, or its various products, enter the ocean by accidental release from subsurface blowouts during offshore exploration or production, the grounding or sinking of a tanker, the rupture of an underwater oil pipeline, human activity on land or offshore, or discharge of oil over an extended period of time.

Once an oil spill occurs, responding agencies need to first decide whether to treat it. If treated, the best type of remediation measure should be considered. The natural dispersion of spilled oil determines what species are potentially influenced and their level of exposure, while the geological environment and weather conditions greatly influence the fate. If water and air temperatures are high, volatile fractions evaporate quickly from the surface, decreasing the toxic components that can enter the water column and impact aquatic organisms. Conversely, marine animals that inhale the volatile fractions may develop brain lesions and disorientation. If wind and currents are actively transporting spilled oil further offshore, reduction in the bioavailability of toxic components along the coast can result in reduced impacts to near-shore organisms. However, if such components are persistent, then transfer offshore can influence populations of other species. Therefore, when considering an oil spill event, a broad picture of the tradeoffs between species and effects should be evaluated.

Since petroleum and its products are complex mixtures, it is very difficult to estimate their impacts on different organisms - and even more so on an entire ecosystem. There is currently little knowledge on the synergistic and antagonistic interactions of petroleum hydrocarbons. Moreover, as the various components are dependent on their origin, the exact composition of petroleum at each spill is different. The most notable toxic effect is short-term mortality. However, chronic exposure to sublethal concentrations is also important to consider - and whether an ecosystem can recover over a long period of time. In this article, the composition of crude oils, their transfer and fate in the ocean, and their toxicity to marine organisms and ecosystems are discussed.

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