While it's relatively common to spot unidentified dark or reddish patches on the surface of the ocean in coastal areas around the U.S., it's not always easy to discern by sight what the substance is that's creating the disturbance.Often, offshore patches of discolored water are the result of algal blooms or oil slicks.

Algal blooms occur when colonies of algae—simple ocean plants that live in the sea—grow out of control. While algal blooms come in many colors (and some have no color at all), they are popularly known as 'red tides' because some are deep red in color.

Oil slicks, on the other hand, are simply films of oil floating on top of the water. While some slicks may be a few inches thick, most are thinner than a human hair. They may form naturally, but they are often introduced by man in incidents ranging from refined fuels or crude oil spilled from a ship to larger events such as last year's Deepwater Horizon oil spill. Oil sometimes emulsifies under certain conditions. Emulsified oil is a mixture of oil and water that often resembles chocolate mousse or pudding.

How do you tell the difference? It can be difficult. Even the experts can be fooled, especially when looking at the ocean from an aircraft.

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