DECEMBER 6, 2013 -- A warming, more acidic ocean. Grounded ships and heavy fishing nets. Coral reefs face a lot of threats from humans. For these tiny animals that build their own limestone homes underwater, oil spills may add insult to injury.

But how does spilled oil reach coral reefs? And what are the effects?

How an oil spill affects corals depends on the species and maturity of the coral (e.g., early stages of life are very sensitive to oil) as well as the means and level of exposure to oil. Exposing corals to small amounts of oil for an extended period can be just as harmful as large amounts of oil for a brief time.

Coral reefs can come in contact with oil in three major ways:

1. Oil floating on the water’s surface can be deposited directly on corals in an intertidal zone when the water level drops at low tide.
2. Rough seas can mix lighter oil products into the water column (like shaking up a bottle of salad dressing), where they can drift down to coral reefs.
3. As heavy oil weathers or gets mixed with sand or sediment, it can become dense enough to sink below the ocean surface and smother corals below.

Once oil comes into contact with corals, it can kill them or impede their reproduction, growth, behavior, and development. The entire reef ecosystem can suffer from an oil spill, affecting the many species of fish, crabs, and other marine invertebrates that live in and around coral reefs.

As oil spill responders, NOAA's Office of Response and Restoration has to take these and many other factors into account during an oil spill near coral reefs. For example, if the spill resulted from a ship running aground on a reef, we need to consider the environmental impacts of the options for removing the ship. Or, if an oil spill occurred offshore but near coral reefs, we would advise the U.S. Coast Guard and other pollution responders to avoid using chemical dispersants to break up the oil spill because corals can be harmed by dispersed oil.

We also provide reports and information for responders and natural resource managers dealing with oil spills and coral reefs:

• Oil Spills in Coral Reefs: Planning and Response Considerations
• Introduction to Coastal Habitats and Biological Resources for Spill Response (Chapter 3, Sensitivity of Coastal Environments to Oil, includes sensitivity information for coral reef communities)
• Characteristic Coastal Habitats: Choosing Spill Response Alternatives (Particularly, the section Subtidal Habitats: Coral Reefs, PDF)

You can learn more about coral reefs, such as the basic biology of corals, how damaged coral reefs can recover from an oil spill or be restored after a ship grounding, and what we've learned about oil spills in tropical reefs.

For lessons a little closer to home, be sure to find out five more things you should know about coral reefs and listen to this podcast about threats to coral health from NOAA's National Ocean Service.