The Trump administration's plans to revive oil and gas exploration off the Atlantic coast moved forward earlier this month with a proposal to allow five energy companies to use seismic air guns. Environmental groups immediately spoke out against the proposal, citing evidence that the devices, which send blasts of compressed air down through the water and ocean bed to provide a view of oil and gas deposits hidden beneath the sea floor, can cause serious harm to marine mammals.
But it's not just whales and dolphins that take a hit; a new study finds that seismic testing appears to wipe out zooplankton, tiny animals that spend much of their lives drifting through the sea.
Researchers have long known that air gun blasts can disrupt hearing in whales and dolphins, which rely on acoustic signals for everything from hunting to mating. But until now, no one had looked into what effect seismic surveys were having on plankton, despite the outsized role these tiny drifters have in marine ecosystems.
"Plankton is the base of the food chain," says Jayson Semmens, an associate professor at the University of Tasmania and lead author on the new study. "It all starts with them, so if they're affected, that affects everything—including whales." Semmens and his colleagues realized it was impossible to determine the full effect of seismic testing on megafauna like whales without understanding how it influenced the abundance and behavior of the organisms that make up the base of marine food chains.
The team tested a single air gun off the coast of Tasmania, Australia. Within an hour of the air gun blasts, the abundance of zooplankton in their samples of ocean water fell by 64 percent, while the number of dead zooplankton present increased threefold. "We were quite surprised at the size of the effect," Semmens says.
"Maybe it is time to start changing how we think about the animals at the bottom of the food chain."
Previous models suggested that any effects air guns might have on this microscopic form of marine life would be limited to a 10-meter range around the devices, but the authors saw significant effects on plankton out to the edge of their survey range—a full 1,200 meters from the air gun. And the fact that these effects were seen at the furthest point the team took measurements suggests they go even beyond 1,200 meters.
The true impact of air guns on zooplankton may be even more significant given the scale of real-world seismic tests: The team was using just one air gun along a single transect, but oil and gas survey ships tow an array of multiple guns, which fire in tandem every few seconds, 24 hours per day, for weeks at a time.
The researchers still don't know exactly how air gun blasts are killing off zooplankton, or what the long-term implications might be for the entire population of these animals, which already have a high turnover rate. But Semmens describes the findings as a "wake up call."
"We can't ignore this," he says. "Maybe it is time to start changing how we think about the animals at the bottom of the food chain."