A trip to a coral reef off the coast of the Florida Keys, an outdated nautical chart and an argument with a boat captain led biogeochemist Kimberly Yates to make a starting discovery: The seafloor around parts of the continent is breaking away, much more than scientists had previously assumed.
Yates, an oceanographer with the U.S. Geological Survey’s St. Petersburg Coastal and Marine Science Center, was in the region for a research trip. She jumped off the boat and was swimming in the water to look for a spot to place some equipment. When she finally did, she signaled to the captain several times to bring the boat over—but he refused. Finally, she swam back to the boat and asked him why he wouldn’t navigate that way.
“He said he couldn’t take the boat there because the water is too shallow. He had looked at the nautical chart and said that there was only around 2 feet of water there,” Yates recalled. “But it was much deeper—I had to put him in the water with me and show him that there was around 12 feet of water in that spot. We came back thinking either that nautical chart was wrong, or we’ve lost about 10 feet of seafloor since it was made.”
Over the next two years, Yates and her team traveled to five reef tracts in the Florida Keys, the U.S. Virgin Islands and Maui, Hawaii. In all five, they found evidence that the reefs aren’t the only victim of erosion: The seafloor is eroding, as well, increasing the depth of the water in those areas. As this becomes more common, it could pose a risk to coastal communities that are already vulnerable to sea-level rise and lack the protection of natural buffers.
“At current rates, by 2100 sea floor erosion could increase water depths by two to eight times more than what has been predicted from sea level rise alone,” Yates noted in a statement.
Yates and her colleague David Zawada came to this conclusion by studying nautical data stretching as far back as the 1800s. It was collected by the National Oceanic and Atmospheric Administration, USGS, and the Army Corps of Engineers.
“In a very general sense, what we did was basic accounting. We had depth data that was measured decades ago, and then we had measurements in the same regions that were taken relatively recently—we simply computed the difference,” said Zawada.
They documented their findings in a study published in the journal Biogeosciences: Of the habitats they studied, more than three-fourths showed volume losses; the mean seafloor elevation loss was around 0.09 meters. In Maui, they found that the seafloor had lost around 81 million cubic meters of volume—enough sand and rock to fill up 81 Empire State Buildings.
As the climate continues to change, the loss of seafloor will accelerate the rate of sea-level rise. It could also worsen the effects of other climate change consequences, like more frequent storms. The authors of the study note that the actual depth of the water in the spots they studied is roughly equal to predictions made for the year 2100. Their study also found that the rate at which coral is growing back is too slow to allow it to serve as a buffer for the coastline.
“Corals and a shallow seafloor provide a barrier to the coastline, against things like storms, waves and tsunamis,” said Yates. “When you’re starting on the beach, you can see surfers on big waves. But by the time they actually reach the sand, the waves are much smaller—that’s because coral structures and a shallow floor cause the waves to break up. But if the water gets deeper and coral erodes away, the waves can make it to the shoreline and cause erosion.”
While the study does not look specifically at the reasons for this loss, Yates says that scientists have pinpointed several factors: coastal development, pollution, warming ocean temperatures, overfishing, coral disease and ocean acidification, to name a few.
“If things continue on the track they are, then the problem will continue to get worse at the study sites we looked at,” she said.
Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.