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New discoveries about how even small amounts of sediment can severely affect fragile coral ecosystems and suggestions about solutions are presented in a new book written by a team of U.S. Geological Survey (USGS) scientists and their colleagues. Coral reefs are in decline worldwide, and a leading cause of their decline is the runoff of sediment and pollutants from nearby land surfaces.

After a multiyear study of the long fringing coral reef off south Moloka‛i, the scientists' findings have been published as “The Coral Reef of South Moloka‛i Hawai‛i—Portrait of a Sediment-Threatened Fringing Reef.” Using vivid photographs and color illustrations, the book was written, edited, and designed to appeal to a broad audience while maintaining its strong scientific basis. The book begins by explaining the geologic evolution and natural processes that shape the reef and impacts to the reef resulting from human activity on the land. The book concludes by exploring alternative scenarios for the future.

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The reef off south Moloka‛i is the longest fringing coral reef in the Hawaiian chain and one of the best preserved. However, it is also severely threatened by large loads of sediment that wash off land heavily altered by farming, ranching, grazing by wild goats, and other activities. The book sheds new light on the causes and results of the distinct band of muddy water that was first reported by marine explorer Jacques Cousteau decades earlier—a phenomenon that now obscures the reef nearly every day.

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USGS scientists and their colleagues placed instruments on the sea bottom that recorded data on such factors as water temperature, wave height, and suspended-sediment concentration for months at a time. The scientists also mapped coral locations and collected numerous samples of sediment and coral. These data gave scientists an opportunity to monitor in detail how the reef functions. The team studied the process of resuspension of mud on the reef: daily winds and high tides combine to stir the mud particles repeatedly. The effect of this process is that even very small amounts of sediment washed onto the reef from land become suspended nearly every afternoon, blocking light, interfering with photosynthesis of beneficial algae living in the coral, and disrupting many other critical processes that sustain the reef.

Thomas J. Goreau, president of the Global Coral Reef Alliance, wrote the foreword, in which he notes the book's "remarkably integrated approach to the reefs of Moloka‛i, combining geology, oceanography, and biology to provide an in-depth understanding of the processes that have made these reefs grow and that now limit them."

Although the book’s emphasis is on the Moloka‛i reef, the authors believe that their findings provide important information for others entrusted with protecting and managing coral reefs elsewhere in the tropical Pacific and Caribbean. Land-based pollution continues to be a major threat to reefs, along with unsustainable fishing practices and climate change. USGS scientist and lead editor Michael Field observed: "It is now recognized that impacts to coral reefs from climate change may be severe, and so it is all the more important to eliminate, wherever possible, other major stressors to reefs. One of these is very clearly runoff of sediment and pollutants."

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The new book was featured in an article in the December 11, 2008, issue of the Moloka‛i Dispatch, which called it a "landmark report." The article includes an interview with Field and a local perspective on the importance of the new publication.

The volume, published as “The Coral Reef of South Moloka‘i, Hawai‘i—Portrait of a Sediment-Threatened Fringing Reef,” U.S. Geological Survey Scientific Investigations Report 2007-5101, was edited by USGS scientists Michael Field, Susan Cochran, Josh Logan, and Curt Storlazzi.

For more information about USGS coral reef studies, visit the USGS Coral Reef Project web site.

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