Warming seas and ocean acidification aren't the only hazards facing the world's coral reefs. A new study suggests that the communities can be thrown quickly and seriously out of balance by the iron from sunken ships. Scientists hope the findings will encourage the prompt removal of derelicts before they can damage the fragile ecosystems.
The problem with shipwrecks appears to be a particularly aggressive reef-dwelling creature called Rhodactis howesii, a type of sea anemone. When nutrients are abundant and there are no predators, R. howesii thrives. Unfortunately, it also eats coral, threatening the foundation of the ecosystem.
Several previous studies have linked shipwrecks and reef degradation, but researchers in Hawaii decided to measure the effect in detail. They surveyed a coral reef off Palmyra, an isolated atoll in the middle of the Pacific Ocean. There they found high densities of R. howesii near a longline fishing boat that sank in 1991. Those densities steadily declined with distance from the wreck; and within about 100 meters, they dropped to zero--with a few exceptions. The exceptions, the team reports today in PLoS ONE, involve navigation buoys installed on the atoll in 2001.
Because Palmyra is so isolated, the team concludes, there's no chance the R. howesii population could be spurred by runoff from agricultural or industrial activities that provide its usual sources of nutrients. Instead, the data show, the R. howesii colonies seem to have radiated out from the hull; the iron dissolved in seawater appears to be their source of nourishment. So the longer the source of iron persists, the farther the anemones can spread, until they completely consume the coral and effectively kill the reef, the team concludes.
The research "is an excellent account of the long-term ecological harm resulting from vessel groundings on coral reef ecosystems," says marine biologist William Precht of the U.S. National Oceanic and Atmospheric Administration's Florida Keys National Marine Sanctuary in Key Largo. It underscores the need to remove wrecks from coral reefs quickly, he says.