In the Gulf of Maine, water temperatures are rising faster than anywhere else on the planet. And, like most everywhere else in the ocean, Maine's waters are also getting more acidic.

The combination of ocean warming and acidification is driving declines among five species of mussels, barnacles and snails along the coast of Maine, according to new research, published Tuesday in the journal Communications Biology.

"These species are often overlooked because of how common they are," lead study author Peter Petraitis said in a news release.

"They're just everywhere across the rocky shores. People don't think anything is going to happen to them. If they decline by about 3 percent a year that's a relatively small change so you might not notice it for a while," said Petraitis, a retired professor of biology at the University of Pennsylvania. "But one year, people are going to suddenly look around and say, 'Where are all the snails, mussels and barnacles?'"

In 1997, Petraitis and his research partner Steve Dudgeon, a biology professor at California State University, Northridge, who worked as a postdoctoral fellow under Petraitis in the 1990s, set out to study how snails, mussels and barnacles recolonize and reorganize themselves in the wake of significant environmental perturbations.

On the Maine coast, powerful winter storms can cause ice to scour the rocks, stripping the shellfish away and forcing communities of snails, mussels and barnacles to rebuild in the spring.

More than 20 years ago, Petraitis and Dudgeon simulated an ice scouring event among a handful of test plots on Maine's Swans Island. They also marked off a variety of control plots for monitoring.

For the latest study, the research duo took advantage of data collected from the control plots over the last two decades.

Petraitis and Dudgeon tracked the abundance and distribution of five common shellfish species: the tortoiseshell limpet, Testudinalia testudinalis; the common periwinkle, Littorina littorea; the dogwhelk, Nucella lapillus; the blue mussel, Mytilus edulis; and the barnacle, Semibalanus balanoides.

"We didn't expect to see much change in the control plots, but we were surprised to see these populations declining," Petraitis said.

Between 1997 and 2018, researchers measured a 16 percent annual decline in the number of young mussels in the control plots. The other four species declined at an annual rate of between 3 and 5 percent. Over the course of the two decades, the abundance of limpets, periwinkles and dogwhelks in the control plots decreased by 50 percent.

When researchers compared the annual species data with ocean temperature and acidity data, they found warming increases tracked most closely with downward trajectory of mussels and common periwinkles.

Declines in the number of limpets and dogwhelks corresponded with increases in the aragonite saturation state, a measure that tracks closely with changes in ocean pH. Typically, increases in ocean acidity cause decreases in aragonite saturation, so the finding was unexpected.

"This may be indicative of other conditions at nearshore areas that vary with aragonite saturation state," Petraitis said.

Periwinkle declines weren't associated with changes in ocean temperature, acidity or aragonite saturation state.

All five of the monitored species are integral to Maine's coastal ecosystems. Because limpets and periwinkles feed on algae and seaweed, researchers suspect declines could lead to larger coastal algal blooms. Meanwhile, mussels and barnacles work as filter feeders -- siphoning phytoplankton from the water column, digesting and depositing them back into the ocean, fertilizing the shore.

All five shellfish species also serve as important prey for birds and mammals farther up the food chain.

"Without animal consumption transferring organic matter up the food web, production in coastal oceans will be increasingly shunted directly through pathways of decomposition by microbial organisms, rather than to support populations of species that humans fish and on which coastal economies depend," Dudgeon said.

Global warming has caused harmful levels of ecological disruption across the globe, and the Maine coast is no exception.

Petraitis and Dudgeon, however, suggest declines in periwinkle numbers could leave Maine looking a bit more like it was in first half of the 19th century, before periwinkles were introduced from Europe and became the primary shelled grazer on the rocks of New England.

"Before 1860, the shore without periwinkles probably looked a lot greener than it does now," Petraitis said. "As they decline we may see the shore revert back to its state in the 1850s."

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