Black Reefs–When the Ship Hits the Reef

The first time I dived at the remote Kingman Reef, in 2005, I thought I found paradise. In the middle of the Pacific Ocean, almost 2000 km south of Hawaii, lies a pristine coral reef, covered with colorful corals and a carpet of giant clams with unbelievable electric blues and greens. When I returned in 2007, I thought I had entered the dark land of Mordor.

The healthy corals of the windward side of Kingman Reef, permanently washed by the breaking waves, had died, and the reef shifted into a carpet of dark slime – filamentous algae and microbes. The former crystal clear waters were now murky like a swimming pool after turning off the filtration system. We called it the ‘black reef.’

What was the reason for such a shift – from pristine to degraded? We found the answer right away: the wreck of a teak-hulled fishing vessel filled with iron-rich compressors, engines and unidentifiable machinery. We will never know who the ship belonged to, and what happened to her crew. All we know is that that vessel wasn’t there two years earlier. The ship is a ghost, killing the reef around it little by little.

A scientific study published today at The ISME Journal shows that the shipwreck is releasing iron slowly into the surrounding waters, thus fertilizing the iron-poor waters of Kingman Reef and causing a population explosion of algae, and microbes. The result is the killing of one km of reef in less than three years.

Linda Wegley of San Diego State University (SDSU) and lead author of the study says that ” the black reefs show that a very small amount of some pollutant (in this case iron) can kill a large area of a pristine reef.”

The science team found similar black reefs in other coral atolls and islands in the central Pacific. “The differences between the surrounding reefs and the black reefs are truly amazing. The former are some of the most beautiful in the world, whereas the black reefs are some of the most dead and dark reefs we have ever seen” said Forest Rohwer, professor and director of the marine microbiology lab at SDSU.

Pristine vs 'black reefs' on the Line Islands. Left column (top to bottom): Kingman Reef, Millennium Atoll, Malden Island (photos by Enric Sala). Right column (top to bottom): Black reef at Kingman, black reef at Millennium, and anchor chain killing corals at Jarvis Island (photos by Gareth Williams, David Obura and Mark Vermeij).


To add insult to injury, a number of these ships sank on reefs that are pristine and protected by the U.S. as Marine National Monuments. In particular, the wrecks are within the 12-mile Wildlife Refuge managed by the US Fish and Wildlife Service. The ecological value of these reefs is global and irreplaceable.

Knowing that these shipwrecks pose a significant threats to coral reefs here and in multiple regions where iron is a limiting nutrient, what’s the solution?

William Chandler, Marine Conservation Institute’s Vice President for Government Affairs, believes that “shipwrecks located in iron-poor regions of the Pacific must be removed immediately to protect the integrity and viability of coral reef ecosystems. The U.S. Fish and Wildlife Service’s failure to remove two shipwrecks in the Pacific Remote Islands Marine National Monument is undermining the very purpose of the monument.”

Our own shipwreck expert, my fellow Explorer-in-Residence Bob Ballard, tells me that “I never thought it would have been that destructive, but now knowing what we know, it is clear iron needs to be removed from a reef environment as fast as possible.”

Marine ecologist Dr. Enric Sala is a National Geographic Explorer-in-Residence who combines science, exploration and media to help restore marine life. Sala’s scientific publications are used for conservation efforts such as the creation of marine protected areas. 2005 Aldo Leopold Leadership Fellow, 2006 Pew Fellow in Marine Conservation, 2008 Young Global Leader at the World Economic Forum.
  • […] The National Geographic Society’s News Watch: Black Reefs-When the Ship Hits the Reef by Enric… reports on a newly investigated threat to coral reefs in the Pacific Ocean— iron. […]

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    At beginning, I thought the ship was filled with petroleum.
    And now, I really want to know how the iron-rich water can be a killer of the environment.

  • Lois Carrington Hildum

    Need to read more of such a world of wonder, under the sea. It must be as it is writen a paradise. Your eyes must tell us what you see so we can understand a place we can never go. That place that is a place of wonder. What you do is real.

  • Andrew Barnes

    @FEI.ZHAO… Iron is an essential nutrient for life. In those waters iron is a limiting nutrient, meaning the amount of life supported there is limited to the amount of iron. Once the area is flooded with unnatural amounts of iron, algae, which was once limited by low amounts of iron, can flourish. This may seem like a good thing, but the algae eventually depletes the water of oxygen and the entire ecological system is destroyed, hence the reef looks dead.

  • John Cruce

    This just goes to show how extremely fragile our precious environment really is.It needs a concerted effort by individuals,companies, and governments to do all they can to conserve and protect. This is terrible !

  • F Rupach

    I guess,evrybody know that the iron always melting whrn
    they met their enemies;,equal with Fer+NaCl=Chlor minus Natrium,it means the water is smeel noy good !

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