The waters around Sanya, China, in the northern South China Sea, are murky at best. The city has a population of 700,000 and welcomes more than 16 million tourists each year. Industrial and agricultural pollutants and sediment runoff flowing into Sanya Bay have made the seafloor muddy with turbid, nutrient-saturated waters thought inhospitable for animal growth. But until around 5,000 years ago, this bay was home to a vibrant coral reef. Now, new research shows that a warming and rising sea is causing coral to return—albeit in a limited state.
During the mid-Holocene, between 7,000 and 4,500 years ago, the water temperature in this part of the South China Sea was up to 2 °C warmer than it is today. Combined with the higher sea levels at the time, the warmer water caused the reefs around the bay to thrive, creating a kaleidoscope of corals dominated by branching Acropora. As the sea level dropped after the end of the mid-Holocene warm period, the corals were exposed to more solar radiation and had less space to grow. Eventually, they died out, leaving only fossils behind.
Today, as the sea rises and warms once more, new life is being breathed into this old reef. Over the past few years, scientists surveying and sampling the area have discovered a resurgence of corals—most of them hardy ones known as Porites compressa.
Photo: Yuheng Chen/Unsplash
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