Declines in shellfish species on rocky seashores match climate-driven
changes
Two decades of data from a study of Maine's Swan's Island document a slow
and steady dwindling of mussels, barnacles, and snails

Date:
October 20, 2020
Source:
University of Pennsylvania
Summary:
Mussels, barnacles, and snails are declining in the Gulf of Maine,
according to a new article by biologists. Their 20-year dataset
reveals that the populations' steady dwindling matches up with
the effects of climate change on the region.



FULL STORY ==========================================================================
The waters of the Gulf of Maine are warming faster than oceans almost
anywhere on Earth. And as the level of carbon dioxide rises in the
atmosphere, it's absorbed by the oceans, causing pH levels to fall. Ocean acidification makes it difficult for shellfish to thicken their shells --
their primary defense against predators.


==========================================================================
In a new study in the journal Communications Biology, researchers Peter Petraitis, a retired professor of biology in Penn's School of Arts &
Sciences, and Steve Dudgeon, a biology professor at California State University, Northridge, who completed a postdoctoral fellowship with
Petraitis at Penn in the 1990s, show that the changing climate is taking
a toll on Maine's sea life.

A dataset collected over two decades, including numbers of five species
of mussels, barnacles, and snails, shows that all have been experiencing declines -- some slow, some more rapid -- in part owing to climate change.

"These species are often overlooked because of how common they are,"
Petraitis says. "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% a year that's a relatively small change so you might not
notice it for a while. But one year, people are going to suddenly look
around and say, 'Where are all the snails, mussels and barnacles?'"
These species "form the core of an iconic food web" in the Gulf of Maine,
says Dudgeon. "Concurrent declines of five species, including both native
and non- native, is proportionally large, and may cause profound changes
in the ecology of coastal oceans in the region." In 1997, Petraitis and Dudgeon set up a long-term experiment on the Gulf of Maine's Swan's Island
to study the ecological principles of multiple stable states. A focus of Petraitis's research and the subject of his 2013 book, "Multiple Stable
States in Natural Ecosystems," the concept encapsulates the idea that
an ecosystem can switch quickly between entirely different compositions
of organisms, given the right environmental perturbations.

For shellfish on Swan's Island, one such perturbation occurs when
periodic powerful winter storms cause sea ice to scrape off all the
organisms attached to rocks on the shore, forcing the communities to
rebuild from scratch the next year.



==========================================================================
In 1996, Petraitis and Dudgeon simulated a single massive ice scouring
event by scraping the rocks to see what would happen as the shore
recolonized. Since then the researchers have been making an annual trip
to their 60 study plots on Swan's Island, counting the incidence of
organisms living not only in the scraped areas but also in areas left
in their natural state, the control plots.

The current work took advantage of these control plot counts, looking
at 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," says
Petraitis, "but we were surprised to see these populations declining."
Using abundance data from 1997 to 2018, the researchers found that very
young mussels were in the sharpest free fall, declining almost 16% a year, while the other four species were dwindling by 3 to 5% each year. Over
that time period, limpets, periwinkles, and dogwhelks declined in total
number by 50%, contractions the researchers describe as "sobering."
To get at the question of why, the researchers looked to data on ocean temperature and chemistry. They found that the downward trajectory of
mussels and common periwinkles matched up with increasing summer ocean temperatures collected from a nearby buoy.



========================================================================== Meanwhile declines in populations of limpets and dogwhelks corresponded
with increases in the aragonite saturation state, a measurement that
tracks with ocean pH. This was unexpected, since lower levels of aragonite saturation are associated with more acidic oceanwaters, which make it
harder for shellfish to build up their shells. "This may be indicative of
other conditions at nearshore areas that vary with aragonite saturation
state," Petraitis says.

Changes in barnacle numbers did not correspond with changes in ocean temperature, pH, or aragonite saturation state, suggesting other factors
are at play in their decline.

All five of these species play critical ecological roles in the Gulf
of Maine.

As filter feeders, mussels and barnacles remove phytoplankton from
the water column, "digesting them, pooping them out, and fertilizing
the shore," Petraitis says. Limpets and periwinkles feed on algae and
seaweed, so smaller numbers could lead to algal blooms and "greener"
nearshore areas.

Since all five species serve as prey for a variety of animals, shrinking populations will reverberate up the food chain, affecting humans as well.

"Without animal consumption transferring organic matter up the food web,"
says Dudgeon, "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." Petraitis also notes the common periwinkle,
now emblematic of the coast, was introduced to the Gulf of Maine from
Europe sometime in the middle of the 19th century. "Now it's the most
common grazer on the shores -- they feed like goats," he says. "Before
1860, the shore without periwinkles probably looked a lot greener than
it does now. As they decline we may see the shore revert back to its
state in the 1850s." While presenting these findings at conferences
in the last couple of years, Petraitis says he's heard anecdotes from
other scientists about similar disappearances of mussels across the North Atlantic, suggesting the phenomenon is not isolated to the Gulf of Maine.

The study was supported by the National Science Foundation (grants
OCE-9529564, DEB-0314980, DEB-1020480, and DEB- 1555641).


========================================================================== Story Source: Materials provided by University_of_Pennsylvania. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Peter S. Petraitis, S. R. Dudgeon. Declines over the last two
decades of
five intertidal invertebrate species in the western North Atlantic.

Communications Biology, 2020; 3 (1) DOI: 10.1038/s42003-020-01326-0 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201020081737.htm

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