Rising sea levels will eventually swallow entire islands, and, by extension, much of the world's suitable habitat for sea turtle nests. Already, the encroaching water is re-shaping the planet's low-lying islands and coastal regions, as storms and flooding become more frequent. Now, researchers from Australia have found that intermittent flooding is already impairing hatching in sea turtle nests, which riddle near-shore regions of tropical and temperate beaches around the globe.
The world's largest population of green sea turtles nest on Raine Island, a sandy isle atop the Great Barrier Reef. On any given summer night, upwards of 10,000 female sea turtles come ashore to lay eggs. But the nests on the remote cay are in trouble. Just 12 to 36 percent of eggs on Raine Island actually hatch—well below global averages of 80 percent or more.
"A lot of them die in the nest," says David Pike, a lecturer at James Cook University in Queensland. "We’re not really sure why that happens." The Australian government enlisted Pike and his colleagues to find out.
Submergence in saltwater for six hours reduced hatching success by 30 percent, while submergence for one or three hours lowered success rates by less than 10 percent.
The researchers thought flooding seemed a likely culprit: Scientists have long known that developing sea turtles need to exchange oxygen and carbon dioxide across their leathery shells, but this process is blocked when the eggs are submerged by high tides or storm surges.
To see just how much flooding the nests could take, Pike and his colleagues collected green turtle eggs from Raine Island, which they incubated in a lab. Trays of eggs were then flooded at different stages of development, for one-, three-, or six-hour periods. (Control eggs were not flooded at all.) The team reported yesterday in Royal Society Open Science that submergence in saltwater for six hours reduced hatching success by 30 percent, while submergence for one or three hours lowered success rates by less than 10 percent. In other words, prolonged exposure to ocean waters may be killing at least some sea turtle eggs before they can hatch.
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More work is needed to see if repeated flooding events further decrease hatching success, or if the hatchlings that do make it out of flooded nests have any neurological and behavioral impairments. Still, the authors don't think flooding is entirely to blame for Raine Island's low hatching rates. Even among controls, the hatching rate in the lab was much higher than the rate for eggs that develop in nests on the island. "That suggests that there’s something else happening out there," Pike says.
That "something else" could be the dead eggs themselves: Imagine dozens of decaying eggs left behind in each day's 10,000 nests; multiply that by an entire nesting season. "If you dig up a sea turtle nest to see if the eggs have hatched, and a lot of them haven't, it's nasty, nasty stuff," Pike says. Some scientists think the decaying eggs may have raised the bacterial load of the sand, making it somewhat inhospitable for nests. But for now, the decline in hatching rates on Raine Island remains a mystery.
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