Despite being only one-tenth of 1 percent of Earth’s oceans by area, coral reefs are home to about a quarter of all marine species. They are biodiversity hotspots, providing food and shelter to marine creatures and countless benefits to the people who live nearby.

But coral reefs have been under tremendous pressure from threats like overfishing, pollution and climate change. Warmer temperatures cause coral bleaching. (NOAA offers a great explanation of what coral bleaching is and why it matters.) Carbon dioxide in the atmosphere gets absorbed by the ocean, making it more acidic, which is also detrimental to corals. If current trends continue, scientists predict that up to 70 percent of coral reefs could be lost by mid-century.

This is what a mostly lifeless, bleached coral reef looks like. A sad sight.This is what a mostly lifeless, bleached coral reef looks like. (Photo: Acropora/Wikipedia)

Thankfully, it’s not all bad news

If you find most of the recent news on coral reefs depressing, you’re not the only one. But not all of the headlines are bad. A recent study (pdf) by the Scripps Institution of Oceanography reports that while many reefs are doing poorly, some are thriving. Better understanding that difference might help us figure out how to protect these underwater treasures.

Over the past decade, the researchers have studied 450 coral reefs spread around 56 islands in the central Pacific, including Hawaii, the Phoenix islands, the Mariana Archipelago and American Samoa. This massive undertaking has revealed how much of an impact proximity to humans has on the coral reefs. Those near remote islands are dramatically healthier than those near populated areas. The remote corals host more species including sea turtles, jellyfish, manta rays, sharks, as well as more colorful algae.

Near inhabited islands, the cumulative impact of human activities seems to be causing a reduction in the presence of the calcifying, reef-building organisms that are essential to the health of the reefs. The researchers have also noticed an increase in fleshy algae, which can take over reefs and displace healthy corals.

A variety of colorful corals.A variety of colorful corals. (Photo: Toby Hudson/Wikipedia)

“There are still coral reefs on this planet that are incredibly healthy and probably look the way they did 1,000 years ago,” Jennifer Smith, lead author of the study and a professor at Scripps’ Center for Marine Biodiversity and Conservation, told The San Diego Union-Tribune. “The scientists were practically in tears when we saw some of these reefs,” she added. “We’ve never experienced anything like it in our lives. It was an almost religious experience.”

This discovery should help bolster the efforts of conservationists. Some groups were getting discouraged because it was believed that climate change and ocean acidification were all but dooming coral reefs, but it turns out that good, old-fashioned conservation makes a difference. It might be able to keep the sea’s most fertile areas healthier and more resilient while we deal with these other global issues.

Michael Graham Richard ( @Michael_GR ) Michael writes for MNN and TreeHugger about science, space and technology and more.