Coral reefs are beautiful, vibrant ecosystems that house roughly one quarter of all marine species and provide billions of dollars in products and services to humans each year. But they are also vulnerable to human activities – both direct disturbances and small, indirect effects that build up over time. Read about why it's important for coral to be resistant and resilient, in a blog post by Smithsonian marine scientist Dr. Nancy Knowlton.
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Two Views of Coral Reefs: Thriving and Threatened
Hawaii’s Maro Coral Reef
Credit: ©James D. Watt/Ocean StockThis bluefin trevally is lucky to call Hawaii’s Maro Coral Reef, part of the Papahānaumokuākea Marine National Monument, its home. Maro is the largest reef in the Northwest Hawaiian Islands and just one of the many marine ecosystems protected in the 140,000 square miles of Papahānaumokuākea, one of the largest marine protected areas in the world. Learn more about Papahānaumokuākea and other marine heritage sites -- including stunning photos.
Bleached Corals, Pacific Ocean
Credit: Wolcott HenryCompare the healthy coral on the left with the bleached coral on the right. Increased water temperatures caused the bleached coral to lose the microscopic algae that give the coral color and provide it with food. Corals can recover from bleaching if there is time and space for healthy tissues to regrow, but too often other factors such as pollution or fast-growing seaweeds smother their chances. More about coral reef ecosystems can be found in the Coral Reefs section.
Close-up of a Coral Polyp
Credit: © osf.co.uk. All rights reserved.In this close-up photo, you can actually see the photosynthetic algae, or zooxanthellae, living inside a tiny coral polyp. Look for the brownish-green specks in the colorless polyp. Corals depend on these algae for food and for some of their oxygen. To learn more about coral reefs, explore our featured ecosystem Coral Reefs.
Corals Threatened by Acid Seas
Credit: Andy (Flickr user amell)Much of the carbon dioxide we emit into the atmosphere ends up in the ocean. As CO2 levels rise, seawater becomes more acidic. This change in chemistry poses a serious threat to marine organisms including snails, corals, such as in the above photo showing a single bleached polyp, and fish. In the April 2011 issue of National Geographic, writer Elizabeth Kolbert tells the story of how scientists are trying to address ocean acidification before it's too late.
Deep-Sea Coral Habitat
Credit: Jodi Pirtle/Cordell Bank National Marine SanctuaryRockfish, anemones, and other invertebrates inhabit this deep-sea coral reef in Cordell Bank National Marine Sanctuary off the coast of California. Deep-sea corals provide habitat and benefits to a variety of marine life. Explore more in the multimedia feature "Coral Gardens of the Deep Sea."
Dead Coral Overgrown with Algae
Credit: Wolcott HenryThese corals are dead—smothered in sediments and overgrown with algae. The reef is near Komodo Island in Indonesia. More about coral reef ecosystems can be found in our Coral Reefs featured story.
Kingman’s Reef, Line Islands
Credit: Brian Skerry/National GeographicThis photo of Kingman’s Reef in the Line Islands shows what a healthy coral reef should look like. The water is crystal clear. A variety of richly colored corals carpet the seabed. And a native predator, a red snapper, prowls for prey. The reason why this reef is so healthy is that it remains isolated from human impacts such as overfishing and pollution.
Unhealthy Coral Reef, Kiritimi, Line Islands
Credit: Jennifer E. Smith/Scripps Institution of OceanographyAlgae has overtaken this coral reef off heavily populated Kiritimati, or Christmas Island. Few fish swim in the murky waters. The causes include pollution, overfishing, and increased water temperature. More about coral reef ecosystems can be found in our Coral Reefs featured story.
Indonesian Coral Reef
Credit: © Chris Newbert/Minden PicturesA variety of organisms make their home on this tropical coral reef in Indonesia. More about coral reef ecosystems can be found in the Coral Reefs section.
Bleached Corals
Credit: Emma Hickerson/NOAAWhat is coral bleaching? These white corals in the Gulf of Mexico’s Flower Garden Bank National Marine Sanctuary are bleached due to an increase in water temperatures, which causes corals to lose the microscopic algae that provide them with food. Bleaching spells trouble for coral reefs. Learn more in the Coral Reefs section.
Diver Cleaning Up Reef
Credit: Amos Nachoum 2005/Marine PhotobankA diver cares for the reef by cleaning up discarded fishing gear and garbage. Even if you don't have scuba gear, you can help protect the ocean and coral reefs!