For the first time, NOAA and partner scientists have connected the concentration of human-caused carbon dioxide in waters off the U.S. Pacific coast to the dissolving of shells of microscopic marine sea snails called pteropods.

“This is the first time we’ve been able to tease out the percentage of human-caused carbon dioxide from natural carbon dioxide along a large portion of the West Coast and link it directly to pteropod shell dissolution,” said Richard Feely, a NOAA senior scientist who led the research appearing in Estuarine, Coastal and Shelf Science. “Our research shows that humans are increasing the acidification of U.S. West Coast coastal waters, making it more difficult for marine species to build strong shells.”

The global ocean has soaked up one-third of human-caused CO2 emissions since the start of the Industrial Era. While this reduces the amount of this greenhouse gas in the atmosphere, it comes at a cost to the ocean. CO2 absorbed by seawater increases its acidity, reducing carbonate ions, which are building blocks used by shellfish to grow their shells. 

The pteropod, a sea snail the size of the head of a pin, is found in the Pacific Ocean. It has been the focus of research in recent years because its shell is affected by how much CO2 is in seawater and it may be an indicator of ocean acidification affecting the larger marine ecosystem.

A key piece of the new research was determining how much human CO2  emissions have added to naturally occurring CO2 in seawater off the U.S. West Coast. Using several decades of measurements from the Pacific Ocean taken through the U.S. Global Ocean Carbon and Repeat Hydrography Program and new data from four NOAA West Coast research cruises conducted between 2007 and 2013, the research team developed a method to estimate additional CO2 from human-caused emissions since the start of the Industrial Era as compared to CO2 from natural sources.

The analysis shows that concentrations of human-caused CO2 are greatest in shallow waters where the atmosphere gives up large amounts of its CO2 to the sea. The researchers also estimated that CO2 concentrations from fossil fuel emissions make up as much as 60 percent of the CO2 that enriches most West Coast nearshore surface waters. But the concentrations dropped as they measured deeper. It drops to 21 percent in deeper waters of 328 feet or 100 meters, and falls even lower to about 18 percent in waters below 656 feet or 200 meters. Concentrations vary depending on location and seasons as well.

Once researchers created a detailed map of the human-generated CO2 concentrations, they  looked at how pteropod shells fared in areas with varying seawater CO2 concentrations. They found more than 50 percent of pteropod shells collected from coastal waters with the high CO2 concentrations were severely dissolved. An estimated 10 to 35 percent of pteropods taken from offshore waters showed shell damage when examined under a scanning electron microscope.

“We estimate that since pre-industrial times, pteropod shell dissolution has increased 20 to 25 percent on average in waters along the U.S. West Coast,” said Nina Bednaršek of the University of Washington. Earlier research by Bednaršek and others has shown that shell dissolution affects pteropod swimming ability and may hamper their ability to protect themselves from predators.

“This new research suggests we need a better understanding of how changes in pteropods may be affecting other species in the food chain, especially commercially valuable species such as salmon, sablefish, and rock sole that feed on pteropods,” Bednaršek added.