The decline of mussel species on rocky shores is in line with climate change.
Data from a two-decade study of the Swan Islands in Maine document a slow and steady decline in mussels, locusts and snails.
The waters of the Gulf of Maine are warming faster than the oceans almost anywhere on Earth. And as the level of carbon dioxide in the atmosphere rises, it is absorbed by the oceans, leading to a drop in pH levels. Ocean acidification makes it difficult for crustaceans to thicken their shells – their main defense against predators.
In a new study in the journal Communication biology, researchers Peter Petraitis, a retired professor of biology at Penn School of Art and Science, and Steve Dugen, a professor of biology at California State University, Northridge, who completed a postdoctoral fellowship with Petraitis in Penn in the 1990s, show that changing climate affects Maine marine life. Data collected over two decades, including a number of five species of mussels, ears and snails, show that they have all declined – some slower, others faster – partly due to climate change.
“These species are often overlooked because of how common they are,” says Petraitis. “They’re all over the rocky shores.” People don’t think anything will happen to them. If they decrease by about 3% per year, this is a relatively small change, so you may not notice it for a while. But one year, people will suddenly look around and say, “Where are all the snails, mussels and ears?”
These species “form the core of an iconic food network” in the Gulf of Maine, Dugen said. “The simultaneous reduction of five species, including native and non-native, is proportionally large and may cause profound changes in the ecology of the coastal oceans in the region.”
In 1997, Petraitis and Dugen organized a long-term experiment on Swan Island in the Gulf of Maine to study the ecological principles of many stable countries. The focus of Petraitis’ research and the theme of his 2013 book “Multiple Stabilities in Natural Ecosystems” concept encompasses the idea that an ecosystem can quickly switch between completely different organisms, given the right ecological disturbances.
For crustaceans on Swan Island, such a disturbance occurs when periodic powerful winter storms cause sea ice to scrape off all organisms attached to the rocks on the shore, forcing communities to recover from scratch the following year.
In 1996, Petraitis and Dugen simulated a single massive ice wash, scraping the rocks to see what would happen when the coast re-colonized. Since then, researchers have made an annual trip to their 60 study sites on Swan Island, counting the frequency of organisms living not only in the scraped areas, but also in the areas left in their natural state, the control plots.
The present work benefits from these control counts by looking at five common species of shellfish: the tortoise (Testudinalia testudinalis), the periwinkle (Littorina littorea), the dog (Nucella lapillus), the blue mussel (Mytilus edulis), and the barracuda (Semibalanus).
“We didn’t expect to see many changes in the control areas,” says Petraitis, “but we were surprised to see that these populations were declining.”
Using abundance data from 1997 to 2018, the researchers found that many young mussels are in the sharpest free fall, declining by almost 16% per year, while the other four species are declining by 3 to 5% each year. During this time, lameness, periwinkle and canine declines decreased by a total of 50%, contractions, researchers describe as “sobering.”
To find out why, the researchers looked for data on ocean temperature and chemistry. They found that the downward trajectory of mussels and periwinkles coincided with rising summer ocean temperatures collected by a nearby buoy.
Meanwhile, declining populations of lame and canine roots correspond to an increase in the state of aragonite saturation, a measurement that tracks the pH of the ocean. This was unexpected, as lower levels of aragonite saturation are associated with more acidic ocean waters, making it difficult for crustaceans to build their shells. “This may be indicative of other conditions in coastal areas, which vary depending on the state of saturation with aragonite,” says Petraitis.
Changes in the number of insects do not correspond to changes in ocean temperature, pH or state of saturation with aragonite, which suggests that other factors play a role in their reduction.
All five of these species play a critical ecological role in the Gulf of Maine.
Because filter feeders, mussels and ears remove phytoplankton from the water column, “digesting them, discarding them and fertilizing the shore,” says Petraitis. Limpets and periwinkles feed on algae and algae, so smaller numbers can lead to algae blooms and “greener” coastal areas.
As all five species serve as prey for different animals, shrinking populations will publicize the food chain, affecting humans as well.
“Without animal consumption transferring organic matter up the food web,” Dugen said, “coastal ocean production will increasingly be linked directly through microbial degradation pathways, rather than supporting populations of species that humans hunt and on which coastal economies depend. . “
Petraitis also notes that the common periwinkle, now emblematic of the coast, was introduced to the Gulf of Maine from Europe sometime in the mid-19th century. “Now this is the most common pasture on the shore – they feed like goats,” he said. “Before 1860, the shore without greenery probably looked much greener than it does now. As they dwindle, we can see the coast return to its state in the 1850s. “
While presenting these findings at conferences over the past few years, Petraitis says he has heard anecdotes from other scientists about similar extinctions of mussels in the North Atlantic, suggesting that the phenomenon is not isolated from the Gulf of Maine.
Reference: October 20, 2020, Communication biology.
DOI: 10.1038 / s42003-020-01326-0
Peter Petraitis is a professor of biology at the University of Pennsylvania in the Department of Biology at the School of Arts and Sciences.
Steve Dugen is a professor of biology at Northridge State University in California.
The study was supported by the National Science Foundation (grants OCE-9529564, DEB-0314980, DEB-1020480 and DEB-1555641).