Souring seas caused by carbon dioxide emissions are dissolving a vital part of the ocean food web.
It didn’t take long for researchers examining the tiny sea snails to see something amiss. The surface of some of their thin outer shells looked as if they had been etched by a solvent. Others were deeply pitted and pocked.
These translucent sea butterflies known as pteropods, which provide food for salmon, herring and other fish, hadn’t been burned in some horrific lab accident. They were being eaten away by the Pacific Ocean.
For the first time, scientists have documented that souring seas caused by carbon dioxide emissions are dissolving pteropods in the wild right now along the US West Coast. That is damaging a potentially important link in the marine food web far sooner than expected.
“What we found was just amazing to us,” said Richard Feely, a scientist with the US National Oceanic and Atmospheric Administration’s (NOAA) Pacific Marine Environmental Laboratory, who helped collect the live samples. “We did the most thorough analysis that’s ever been done and found extensive impacts on marine life in the field from ocean acidification.”
This is the broadest and most detailed indication ever that acidification is already damaging native creatures in the wild. It raises many new questions about whether other sea life, too, might already be harmed – directly by acidifying seas, or by subtle shifts in parts of the food chain.
“These changes are happening years earlier than we had projected,” said Nina Bednarsek, a research fellow with NOAA who inspected the pteropods to identify shell scarring. “It is really a first indication of what is going on in our ecosystem.”
Feely and others already had documented that sea chemistry in many areas off the West Coast, particularly in the Pacific Northwest and Northern California, was changing far faster than initially expected as oceans absorb ever more CO2 from fossil fuels. They also had shown that this chemical change already has killed non-native Northwest oyster larvae.
Now, they’ve found severe shell damage on more than half of the pteropods they collected from waters near shore between central California and the Canadian border. The findings were published in the British Journal Proceedings Of The Royal Society B.
The shell damage corresponds so precisely to where chemical changes have hit the marine world hardest – specific coastal hot spots in Washington and Oregon, where water wells up from the deep on windy days – that NOAA scientists said they could clearly pinpoint the cause: atmospheric CO2. As human activity emits more CO2 from cars and power plants, about a quarter of it gets absorbed by the oceans, which lowers the pH of marine water. That change reduces the availability of carbonate ions, which creatures like oysters, mussels and pteropods need to build their shells.
These chemical changes have struck particularly hard in the Northwest. Here, when heavy winds blow in the right direction, deep, cold water wells up from the bottom and gets drawn toward the beach. This water already contains high CO2 from natural processes. The addition of CO2 from humans helps make it some of the most corrosive water found anywhere.
Because of this upwelling phenomenon, some pteropods near shore almost certainly saw shells dissolve even before the industrial revolution, the study says. But those incidents have doubled in the past several hundred years and could triple by 2050.
In fact, the amount of water in the top 100m of West Coast ocean that may be inhospitable to some shelled organisms has increased sixfold since the industrial revolution.
To calculate the shell damage, Feely and others back in 2011 gathered live pteropods from 17 locations during a research cruise and preserved them. Then Bednarsek, who had developed an elaborate technique for examining the tiny shells, spent months detailing the corrosion. It was Bednarsek a few years earlier who first showed that pteropods in parts of Antarctica already were being damaged by acidification. But that was in an isolated region in a frigid environment, where CO2 is more readily absorbed.
Today’s study examined a vast section of water up and down the coast, from close to the beach to many miles out to sea, and found damage to marine snail shells here across a far greater area of water. Outside experts said the discovery was quite significant.
“This is a surprising result,” said Mark Ohman, a zooplankton ecologist who oversees a long-term research program examining the West Coast marine system for California’s Scripps Institution of Oceanography. “Effects of this magnitude were not anticipated this early in the 21st century.”
Dave Mackas, a scientist who recently retired from Canada’s Department of Fisheries and Oceans after years of surveying pteropods off Vancouver Island, said Bednarsek’s findings were disturbing.
“I think she’s got a pretty iron-clad case that damage to the shells and the extent of the damage is proportional to how corrosive the water is that they’re caught from,” he said after reading the study. “It’s a bit scarier than you might have expected. It’s an outcome you expected to happen eventually, but it’s surprising how severe it is so soon.”
Jan Newton, co-director of the Washington Ocean Acidification Centre, housed at the University of Washington, said, “This is a very important revelation. These are not cultured animals, but native organisms living in their native environment.”
Most troubling, scientists said, were the implications for the future. Bednarsek and Feely projected that by 2050 at least 70% of pteropods close to shore up and down the West Coast will see severe shell damage.
Harder to unravel, however, is what these changes mean for the marine system as a whole. For starters, pteropods can repair or patch their shells. But scientists presume that doing so has a cost; the more damage the shell sees, the more energy the animal wastes, potentially leading to premature death.
“This has a huge impact on that animal’s ability to swim, on the animal’s ability to avoid being eaten by predators, and it creates a stress for the animal that they have to find some way to overcome,” Feely said.
The exact link between shell damage and when pteropods can no longer survive is not well understood. But, Ohman said, “It seems unlikely that deteriorating shells would be beneficial to the pteropods’ growth, survivorship and reproduction.”
Food web disruptions
Still, researchers don’t really know the precise role pteropods play in marine ecosystems. In part that’s because we don’t know who all of their predators are. Fifteen years ago, University of Washington researcher Janet Armstrong carved up salmon from the Bering Sea, the Gulf of Alaska and the open sea, to see what the fish ate. Some years, pteropods made up 60% of the diet of juvenile Alaskan pink salmon. Other years they made up just a fraction.
“Can (fish) move to other organisms for their food or can’t they?” Bednarsek said. “We need to understand that.”
In some parts of the North Pacific Ocean, pteropods may make up only 2% to 10% of plankton species, while copepods and krill – which are more nutritious – play a greater role, Mackas said. But in a few isolated spots pteropods can make up half or more of plankton species. Herring, mackerel and some seabirds eat pteropods, as do other pteropod species. In the open oceans, some small fishes, squids and large shrimp eat them. Some of those animals then become important in the diet of tuna, salmon and walleye pollock, the centrepiece of a US$1bil (RM3.2bil) industry based in Seattle and Alaska.
“The ocean food web is connected in a lot more ways than we understand,” Mackas said. “Even if there’s not a hugely strong direct effect on coastal baby fish, there could be a strong two-steps-removed effect on slightly older life stages of important commercial fish.” If pteropod numbers ultimately decline, Mackas said, it’s clear “there will be a consequence of some kind.”
And if pteropods already are being harmed, that suggests other marine life might be impacted already, too. That’s why Newton’s acidification centre is tracking several plankton species in the wild and paying for research on still others. “I think this is a huge flashing light for us that we need better observations and monitoring of our natural environment,” she said. “We need better information.” – The Seattle Times/McClatchy Tribune Information Services