Rising ocean temperatures kill a third of Guam’s coral reefs

Multiple bleaching events driven by above-average sea temperatures have led to the largest known mass mortality of coral reefs on the U.S. territory in the last five centuries, says a new study.

Rising ocean temperatures killed off more than one-third of all coral reefs on the island of Guam and up to 60 per cent along its eastern coast from 2013 to 2017.

The study conducted by University of Guam researchers described the situation caused by multiple bleaching events as “the largest known mass mortality of coral reefs on the U.S. territory to be recorded in the last five centuries.”

Bleaching is the process by which corals become too stressed from environmental changes and expel the essential symbiotic algae living in their tissues, causing them to turn completely white and sometimes die, explains a statement.

“Our reefs are undergoing very dramatic changes — very suddenly — that haven’t been seen in the last 500 years,” commented first author Laurie J. Raymundo. “We need to protect what remains and rehabilitate where we can using whatever means we can because they are essential to the island, both ecologically and economically. We depend on our coral communities for significant coastal protection, fish habitats, and tourism.”

Guam is home to one of the most diverse coral reef systems in the United States, with more than 350 known coral species. Human factors, like pollution and coastal development, continue to threaten coral, but rising sea temperatures from a changing climate have greatly exacerbated the problem.

The study used photo transect surveys at 46 coral reef sites in the wake of a severe bleaching event in 2013, with repeat surveys on a subset of these sites in 2014, 2016 and 2017, according to the statement.

The team found a 34 per cent decline in island-wide coral cover at the shallow seaward slope sites and a 37 per cent decline at shallow reef flats along the western coast. Shallow seaward slope communities along the eastern, windward coast were particularly devastated, with an estimated 60 per cent of live coral cover lost.

Ongoing documentation and analysis of coral stressors, like bleaching, low tides and diseases, for example, as well other studies to better understand coral’s response and resilience to climate change, are critical to developing effective reef management strategies for Guam and beyond, say the researchers.

Photo credit: UFSWS – Pacific Region/ CC BY-NC 2.0

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