By TODD McLEISH/ecoRI News contributor
JAMESTOWN, R.I. — When Jen Scranton went swimming in a protected
cove at Fort Wetherill last month, she felt like she was swimming “in a giant
wet ball pit.” She said she was surrounded by thousands of jellybean-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
southern New England have reported unusually large numbers of jellyfish in
recent years. And while this year isn’t 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 professor at Providence College who
studies jellyfish, said 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.”
Those jellyfish species that are struggling 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 can’t 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,” Costello said. 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 of 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,” Costello said. “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 sea nettles, a small bell-shaped jellyfish
with foot-long tentacles, may also be increasing in numbers in Rhode Island
because of warming waters. The supervising environmental scientist at the Rhode
Island Department of Environmental Management noted that sea nettles are
becoming increasingly common in South County salt ponds, 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 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,” Deacutis 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 noted 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’s a signal of doom.
“If sea nettles started exploding in the salt ponds, that would be
a bad thing for people,” Deacutis 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.”
Rhode Island resident and author Todd McLeish runs a wildlife blog.