When Jen Scranton went swimming in a
protected cove at Fort Wetherill in Jamestown last month, she felt like she was
swimming “in a giant wet ball pit.” She said she was surrounded by thousands of
jelly bean-sized jellyfish. And although she didn’t get stung, she did feel a
bit disconcerted by the experience.
She’s not the only one. Many people
throughout Rhode Island and beyond have reported unusually large numbers of
jellyfish in recent years. And while this year is not turning out to be a big
year for jellyfish in Narragansett Bay, some scientists claim that the warming
of the oceans may be creating conditions that benefit jellyfish.
Jack Costello, a biology professorat Providence College who studies jellyfish, said that there is considerable
debate among scientists about whether there are substantial changes in
jellyfish populations worldwide.
“The popular story is that we’re
changing the oceans, getting rid of the fish and causing jellyfish to
multiply,” he said. “There may be some truth to that. But there is also clear
evidence that some species of jellyfish aren’t doing well,” especially those
that spend part of their life cycle on the seafloor.
The only thing scientists seem to
agree on is that there is great variability in jellyfish abundance from year to
year that cannot be explained. Last year jellyfish numbers boomed in
Narragansett Bay. Like algae blooms, some jellyfish species explode in
abundance in regular cycles, while others occasionally get blown into the Bay
by changing currents.
“Most of the jellies that people
know about are the high profile species, the large ones we can see easily,”
said Costello. These include the common moon jelly often seen at beaches in the
summer and the lion’s mane jelly, a large cold-water species most often seen
locally in April and May. The Portuguese Man ‘o War, a southern species familiar
to many people, is perhaps the most dangerous variety, but it very seldom shows
up in Rhode Island waters.
According to Costello, one of the
major periods of jellyfish activity in Narragansett Bay is in late winter and
early spring, when several tiny species are most active. But few people notice
them. That’s when several species of ctenophores are increasingly abundant.
Ctenophores are gelatinous predators
on zooplankton and larval fish, and one species found in Narragansett Bay is
clearly benefitting from global warming.
“They overwinter in shallow inshore
waters, which warm up before the central bay,” explained Costello. “And when we
have a really warm early spring, they produce higher populations, which affect
the population of zooplankton and affect the life cycle of other species in the
bay.”
Chris Deacutis said that sea
nettles, a small bell-shaped jellyfish with foot-long tentacles, may also be
increasing in numbers in Rhode Island due to warming waters. The supervising
environmental scientist at the Rhode Island Department of Environmental
Management said that sea nettles are becoming increasingly common in the salt
ponds in South County, where they are often difficult to see but produce a
painful sting that “feels like someone rubbed a match across your face.”
Deacutis agrees with Costello,
though, that there is little scientific evidence that links temperatures and
jellyfish numbers. But, he said, that doesn’t mean it isn’t happening. He said
that no one is measuring jellyfish densities regularly, and that would be the
only way to determine if changes are really taking place.
“The bay varies significantly
depending on the weather from season to season,” he said. “Heavy rains in
summer cause hypoxia very quickly, and that would probably drive more
gelatinous species. Jellyfish seem to do OK even in low oxygen conditions and
can find enough food in those conditions to survive and reproduce pretty well.”
Deacutis said that in a low oxygen
environment, the food web becomes “short circuited. The sensitive species like
fish swim away or die, while those like jellyfish that can survive on bacteria
and small zooplankton do well.”
But even if jellyfish populations
really are growing due to the warming climate, he doesn’t believe it is a
signal of doom.
“If sea nettles started exploding in
the salt ponds, that would be a bad thing for people,” he said. “But would it
harm the ecosystem? There’s no evidence of that. Climate change is causing
complex reactions to ecosystems like Narragansett Bay. Things are changing.
It’s not necessarily true that things are getting worse as much as they are
simply changing.”
This article first appeared on EcoRI.org on August 10, 2016.
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