(CN) – A reef at a popular Hawaiian tourist destination is in peril despite state protection, as the 2014 global coral bleaching event continues into a third year.
Warming sea temperatures have bleached 47 percent of coral reefs overall and nearly 10 percent have died at Hanauma Bay during the ongoing global bleaching event, according to a study published Tuesday in the journal PeerJ.
Researchers found temperature is by far the largest factor in coral bleaching at Hanauma Bay, where fish, coral and other organisms are protected.
"Global climate change poses a direct threat to the biological sustainability of the protected reefs of Hanauma Bay Nature Preserve and a clear economic and cultural threat to the state of Hawaii," said lead author Ku’ulei Rodgers, a researcher at the University of Hawaii at Manoa.
Despite past and current efforts to protect the coral reef ecosystem, carbon will continue to be absorbed by the ocean and water temperatures will continue to rise if current global carbon emission levels maintain, the team warns.
Some 70 percent of coral reefs across the world have been affected by bleaching, which the National Oceanic and Atmospheric Administration considers the most obvious visual sign of climate change in the marine environment. Bleaching occurs as photosynthetic algae called zooxanthellae – which provides the reef with oxygen and food – die off from exposure to warmer ocean temperatures. Without zooxanthellae, the coral decay and lose their vibrant colors.
The new research stems from late University of Hawaii researcher Paul Jokiel’s Coral Reef Assessment and Monitoring Program, which has been studying the Hanauma Bay Nature Preserve since 1999. The program’s surveys revealed a significant decrease in shallow coral cover beginning in 2002.
Reef-building coral at the Hanauma preserve form a barrier across the entire bay, separating the shallow nearshore from the deeper seaward regions. Along with its two channels, this barrier influences how cool water is carried from the open ocean to shore and how it then exits the bay.
These flow patterns were a primary focus of the study as the team wanted to determine why bleaching occurred more severely in certain areas of preserve. The researchers found the most severe bleaching and mortality in areas where water typically warms up and pools for extended periods of time. Other areas benefit from a circulation pattern that shifts cool water in and warm water out more quickly.
While harvesting pressure and a lack of direct human fishing have aided an ecosystem that appears more diverse than other locations on Oahu, coral cover continues to decrease. The absence of living coral tissue causes the physical framework of the reef to continue to erode, which will cease to provide countless services – such as food and shelter – to reef-dwelling organisms including fish species.
Co-author Keisha Bahr, another researcher at the University of Hawaii, said coral will not get a break in 2017, as the global bleaching event of 2014 – the most severe on record – continues.
"Warmer seawater temperatures are again predicted for the Hawaiian Islands in 2017, with the grave possibility of more coral bleaching and mortality," she said.
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