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.”



Meet the Author
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.