It is often difficult to tell the difference between oil slicks and algal blooms. When in doubt, report a suspected spill to the National Response Center.

four images of oil slicks and algal blooms

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. Often, offshore patches of discolored water are the result of algal blooms and not oil slicks.

Algal blooms occur when colonies of phytoplankton—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 slicks can vary in thickness, most are thinner than a human hair. They may form from natural oil seeps, but they can also be introduced by man in incidents ranging from refined fuels or crude oil spilled from a ship to larger events such as 2010's Deepwater Horizon oil spill.

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.

Color. Oil can vary greatly in color, from the commonly-expected black or dark brown, to red, orange, yellow, and even some more exotic colors. When oil becomes emulsified*, its color will change. For example, South Louisiana crude oil is generally a dark red, but appears bright red to orange when emulsified—as it did during the Deepwater Horizon spill. If the oil has a reddish hue, it may be mistaken for so-called "red tide." If the emulsion breaks, the color reverts back to its normal dark color. Once an oil slick spreads out and becomes very thin, the color varies from gray to rainbow. Both oil and algal blooms can have a large range in colors that are similar, and algal blooms may create sheens.

*Oil emulsifies under certain conditions. Emulsified oil is a mixture of oil and water that often resembles chocolate mousse or pudding.

Odor. The most reliable difference is odor. Oil slicks nearly always have a characteristic petroleum smell. Algal blooms may have a strong smell as well, but the smell is distinctly different from that of petroleum.

In the water or just on top? Algal blooms are mostly in the water column although often in the upper layers, but they may have a floating layer. Oil slicks are generally only floating on the surface.

Nighttime Bioluminescence. Oil isn't bioluminescent—it doesn't produce light—but some of the algae that form surface blooms do. The light comes from chemical reactions in the algal cells. So when it's dark out, the water may glow—especially when waves break. During the day, you can take some water into a very dark room, let your eyes adapt to the darkness for several minutes, and then swirl the container. A blue glow or individual flashes of light indicate bioluminescent algae. Keep in mind, though, that some red tide algae do not produce light.

Unsure? Sometimes it's best to leave it to the experts. If you think you see oil on the water, report it to the National Response Center at 1-800-424-8802.