Dogwhelks feed on barnacles on the shores of Swan’s Island in Maine. A new study documents the decline of these and three other tidal species – at least in part due to climate change. Photo credit: Jonathan AD Fisher
The decline in shellfish species on rocky coasts corresponds to climate-related changes.
Two decades of data from a study on Maine’s Swan Island document a slow and steady decline in mussels, barnacles and snails.
The waters of the Gulf of Maine are warming faster than the oceans almost everywhere on earth. And when the level of carbon dioxide in the atmosphere increases, it is absorbed by the oceans, causing the pH to drop. Ocean acidification makes it difficult for shellfish to thicken their clams – their primary defense against predators.
In a new study in the journal Communication biologyResearchers Peter Petraitis, a retired professor of biology at Penn’s School of Arts & Sciences, and Steve Dudgeon, a professor of biology at California State University in Northridge who completed a postdoctoral fellowship with Petraitis in Penn in the 1990s, show that the climate changes takes a toll on Maine’s marine life. A data set collected over two decades, which includes the numbers of five species of mussels, barnacles, and snails, shows that all – some slowly, others faster – have declined in part due to climate change.
“These species are often overlooked because they are so common,” says Petraitis. “They’re just everywhere on the rocky shores. People don’t think anything is going to happen to them. If they’re going down about 3% per year, that’s a relatively small change so you might not notice them for a while. But one year people will suddenly look around and say, ‘Where are all the snails, clams and barnacles?’ ”
These species “form the core of an iconic food web” in the Gulf of Maine, says Dudgeon. “The simultaneous decline in five species, including both indigenous and non-indigenous, is proportionally large and can lead to profound changes in the ecology of the territorial seas in the region.”
In 1997, Petraitis and Dudgeon conducted a long-term experiment on Swan Island in the Gulf of Maine to investigate the ecological principles of several stable states. The concept is a focus of Petraitis’ research and the subject of his 2013 book “Several stable states in natural ecosystems”. It contains the idea that given the right environmental disturbances, an ecosystem can quickly switch between completely different compositions of organisms.
Such disruption occurs in shellfish on Swan’s Island when intermittent severe winter storms cause sea ice to scrape off any organisms clinging to rocks on the shore, forcing communities to rebuild from scratch over the next year.
In 1996, Petraitis and Dudgeon simulated a single massive ice clearing event by scraping the rocks to see what would happen if the shore repopulated. Since then, the researchers have made an annual trip to their 60 study areas on Swan’s Island, counting the abundance of organisms that live not only in the scraped areas, but also in areas that remain in their natural state, the control areas.
The current work used these control plots to examine five common shellfish species: the tortoiseshell limpid (Testudinalia testudinalis), the periwinkle (Littorina littorea), the dog snail (Nucella lapillus), the mussel (Mytilus edulis) and the barnacle (Semibalanus balanoides) ).
“We did not expect that the control areas would change much,” says Petraitis, “but we were surprised that these populations were declining.”
Using frequency data from 1997 to 2018, the researchers found that very young clams were in the sharpest free fall, declining nearly 16% annually, while the other four species declined by 3 to 5% each year. During this period, the total number of limpets, evergreens, and dog snails decreased by 50%, contractions that the researchers describe as “sobering”.
To answer the question of why, the researchers examined data on ocean temperature and chemistry. They found that the downward movement of clams and evergreens coincided with rising summer ocean temperatures collected from a nearby buoy.
Meanwhile, the decline in limpets and dog snail populations corresponded with an increase in aragonite saturation, a measurement that is tracked with ocean pH. This was unexpected as lower aragonite saturation is associated with more acidic ocean waters, making it more difficult for shellfish to build their clams. “This may indicate other conditions in coastal areas that vary with the saturation state of the aragonite,” says Petraitis.
Changes in the numbers of barnacles did not correspond to changes in sea temperature, pH, or aragonite saturation, suggesting that other factors play a role in their decline.
All five species play important ecological roles in the Gulf of Maine.
As filter feed, mussels and barnacles remove phytoplankton from the water column, “digest it, poop it and fertilize the bank,” says Petraitis. Limpets and periwinkles feed on algae and algae, so a lower number can lead to algal blooms and “greener” coastal areas.
With all five species serving as prey for a wide variety of animals, shrinking populations will echo the food chain and affect humans as well.
“Without animal consumption, which transfers organics into the food web,” says Dudgeon, “production in coastal seas is increasingly being shifted directly to decomposition pathways by microbial organisms rather than supporting populations of species that people fish and on which coastal economies depend.” ”
Petraitis also notes that the common periwinkle that is now a coastal landmark was introduced to the Gulf of Maine from Europe sometime in the mid-19th century. “Now it is the most common pasture on the banks – they eat like goats,” he says. “Before 1860 the bank probably looked a lot greener without the evergreen than it does now. If they sink, the bank may return to its state in the 1850s. ”
While presenting these results at conferences in recent years, Petraitis said he had heard anecdotes from other scientists about the similar disappearance of mussels across the North Atlantic, suggesting the phenomenon is not limited to the Gulf of Maine.
Reference: October 20, 2020, Communication biology.
DOI: 10.1038 / s42003-020-01326-0
Peter Petraitis is Professor of Biology at the University of Pennsylvania in the Department of Biology of the School of Arts & Sciences.
Steve Dudgeon is a professor of biology at California State University at Northridge.
The study was supported by the National Science Foundation (grants OCE-9529564, DEB-0314980, DEB-1020480, and DEB-1555641).
