Sea level rise threatens sea turtles | Science News

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Wild Things

The weird and wonderful in the natural world
Sarah Zielinski
Wild Things

Sea level rise threatens sea turtles

green turtle

A green turtle has to haul herself to shore to lay her eggs. But sea level rise may drown her young, a new study warns.

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Sea levels are rising around the world, and they will continue to do so as glaciers and ice sheets melt and the world’s oceans undergo thermal expansion. How much rise will occur is still unknown — one study published online this week contends it could be as much as three meters in just 50 years — but already coastal areas are being inundated during storms and high tides.

While much of the attention has focused on the effects of such flooding on cities and human habitats, coastal animals and plants will also be affected, as will some ocean species. Sea turtles, for instance, spend most of their lives at sea, but they come onto land to lay their eggs. Those eggs incubate, buried in sand, for six to eight weeks. What happens when water reaches those nests?

There is already evidence that inundation events can wash away sea turtle nests and drown eggs and young turtles. But scientists don’t know how much — or how little — water an egg-bound turtle can withstand and still survive. So David Pike of James Cook University in Townsville, Australia, and colleagues gathered green turtle eggs from Raine Island in the northern Great Barrier Reef. In the lab, the scientists submerged groups of eggs in saltwater for one, three or six hours to simulate the effects of an unusually high tide depending on a nest’s location, with a group kept out of water as a control. They then waited to see how many hatchlings resulted from each group. They report the results of their experiment July 21 in Royal Society Open Science.

After one or three hours of being underwater, egg viability dropped by 10 percent. After six hours, it dropped by 30 percent. Repeated flooding could have an even greater effect, the team warns.

The turtles that hatched appeared normal, but the researchers say that more research is needed to see if the brief period of hypoxia (drowning a nest eliminates the ability for an egg to get oxygen) somehow influences a young turtle’s behavior, learning or ability to spatially orient itself. That could decrease survival later on.

Pike and his colleagues conducted their study with eggs from Raine Island because they wanted to get some insight on a problem there: Whereas most green turtle populations have a hatching success rate of around 80 percent, on Raine Island it has ranged from 12 to 36 percent since 2011. The team was curious whether sea level rise could explain the drop, as there have been episodes of storm overwash and heavy rainfall that have drowned nests.

Based on their experimental results, the researchers conclude that the storms, rainfall and floods probably have had an effect on the Raine Island turtles. But they don’t provide a complete explanation for what is going on. In the experimental eggs that weren’t drowned at all, only about half were viable. Some other factor must be disrupting embryonic development, the researchers say. That factor is currently a mystery, but contenders include issues with turtle moms’ health and contaminants transferred from mom to egg.

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