Caribbean Corals Facing Extinction

coralCorals are some of the most amazingly diverse and unique ecosystems on Earth, but they are also some of the most threatened. Global climate change, ocean acidification, and pollution all threaten the health and continued existence of these amazing organisms and the sea life they support. Currently 80% of Caribbean corals have already been lost, and without adequate understanding of the mechanisms leading to coral decline, humans can do little to stop future losses. A new study from OSU provides exciting news about how humans can reduce coral loss by controlling nutrient runoff into the oceans.

As water runs off the land, it picks up pollutants, bacteria, and nutrients and ultimately deposits these materials into the world’s oceans. Nitrogen and phosphorus inputs that result from sewage, agricultural, and other runoff sources are of particular concern. In a three-year, controlled exposure study published in the journal Global Change Biology, researchers from Oregon State University, Florida International University, and the University of Florida revealed the link they discovered between enhanced nutrient levels in seawater and coral disease.

Researchers exposed 1,200 corals at their study site in the Florida Keys to high nitrogen and phosphorus levels in order to determine if such exposure had an effect on coral health. They found that instances of diseased corals increased by 50%, and coral bleaching increased three-fold under nutrient-rich conditions. About half of the diseased corals were infected with black spot syndrome.

The really exciting news is that researchers witnessed an increase in coral health when the nutrient loads were decreased. Within a year after nutrient loading, instances of black spot syndrome decreased to levels consistent with those in corals that had not been exposed to high nutrient levels.

Rebecca Vega-Thurber, an assistant professor in the OSU College of Science who worked on the project, reported that the researchers were surprised at the rapid increase in diseased corals, but added that they were even more surprised by the ability of the corals to recover within 10 months after nutrient enrichment was stopped.

Although the exact mechanism for coral disease remains unknown, researchers theorize that increased levels of nutrients might act directly on corals to decrease health, or might result in increased colonization by bacteria which then infect corals, or a combination of the two. Regardless of the exact cause, their results indicate that a decrease in nutrient discharge from the land into the sea may dramatically increase the survival of corals. Vega-Thurber adds that human control over sewage treatment and use of best management practices to reduce runoff can decrease coral decline.

By Kristen Daly

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