It could be the plot of a horror movie: humans wake up one day to discover that chemical changes in the atmosphere are dissolving away parts of their bodies. But for small marine life known as sea butterflies, or pteropods, this is what’s happening off the West Cost of the U.S. Increased carbon in the ocean is melting away shells of sea butterflies, which are tiny marine snails that underpin much of the ocean’s food chain, including prey for pink salmon, mackerel, and herring.
“We did not expect to see pteropods being affected to this extent in our coastal region for several decades,” said William Peterson, Ph.D., an oceanographer at National Oceanic and Atmospheric Administration (NOAA)’s Northwest Fisheries Science Center who co-authored the findings in a paper for the journal, Proceedings of the Royal Society B.
Sampling sea butterflies in the species Limacina helicina off California, Washington, and Oregon in the summer of 2011, researchers found that over 50 percent of onshore sea butterflies suffered from “severe dissolution damage,” according to the paper. Offshore, 24 percent of individuals showed such damage.
Sea butterfly in the Limacina helicina species. Photo by: Russ Hopcroft, University of Alaska, Fairbanks/NOAA.
The shells of sea butterflies are dissolving due to increased acidification in the oceans caused by society’s CO2 emissions. While emissions from burning coal, gas, and oil are pumped into the atmosphere, the oceans eventually soak up nearly a third of the global cumulative emissions. This increase in CO2 in the oceans leads to a decrease in the availability of calcium carbonate and its crystal form, aragonite, which sea butterflies use to make their shells. Many other key species require calcium carbonate, such as corals, crustaceans, mollusks, and some plankton species.
The extent of ocean acidification varies across regions, depth, and seasons, but is particularly acute where seasonal upwellings occur. This is when deep, cold water is pushed to the surface, a phenomenon that further depletes aragonite. In fact, the worst of the impacts were seen off the coast of California, where low aragonite levels had increased sixfold in the top 100 meters of the water column.
“Limacina helicina from onshore regions showed dissolution that was evenly spread over entire surface of shells, while in offshore regions only the first whorl…showed evidence of dissolution. This suggests that less corrosive offshore conditions only affected pteropods during early stages, while prolonged exposure to more severe undersaturated conditions in onshore regions resulted in dissolution covering the whole shell,” write the scientists.
Prior to the industrial revolution—and the resultant massive influxes of CO2 from burning fossil fuels—the researchers estimated that around 20 percent of sea butterflies would have seen some shell dissolution. Today’s percentage is more than twice that—53 percent—near the coastline, and the scientists predict that 70 percent of sea butterflies will be impacted in the region by 2050.
“Acidification of our oceans may impact marine ecosystems in a way that threatens the sustainability of the marine resources we depend on,” said Libby Jewett, Director of the NOAA Ocean Acidification Program. “Research on the progression and impacts of ocean acidification is vital to understanding the consequences of our burning of fossil fuels.”
Limacina helicina. Photo by: Russ Hopcroft, University of Alaska, Fairbanks/NOAA.
This is not the first time scientists have found evidence of worsening ocean acidification dissolving shells of sea butterflies. Two years ago, researchers reported that they discovered sea butterflies of the same species, Limacina helicina, were losing their shells in Antarctica’s Southern Ocean.
“The snails do not necessarily die as a result of their shells dissolving, however it may increase their vulnerability to predation and infection, consequently having an impact to other parts of the food web,” lead author, Geraint Tarling of the British Antarctic Survey said in 2012.
In fact, the most recent paper notes that the shell is key to sea butterfly reproduction: “the shell is of particular importance…during the reproductive stage, when sperm are exchanged between individuals and need to be stored before fertilizing an egg.”
Since sea butterflies provide a base food source for many fish species, scientists say their decline could hurt all sorts of marine animals, including whales and seabirds.
The current acidification of the ocean is occurring at a faster rate than anytime in the last 50 million years, and the future impacts could be so catastrophic that former NOAA head, Jane Lubchenco, dubbed ocean acidification “climate change’s equally evil twin.” Yet ocean acidification still garners little media attention and global greenhouse gas emissions continue to rise.
- Bednarsek N, Feely RA,
Reum JCP, Peterson B, Menkel J, Alin SR, Hales
B. 2014 Limacina helicina shell dissolution as
an indicator of declining habitat suitability due
to ocean acidification in the California Current
Ecosystem. Proc. R. Soc. B 20140123.
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