The Inner Harbor's no place to swim anyway, but now you can add another reason not to go in the water downtown: jellyfish.
Softball-sized, milky white and bell-shaped, with long tentacles trailing, the gelatinous animals could be seen moving slowly about Thursday in the murky water by the Constellation.
Scientists identified them as Chrysaora quinquecirrha — the most common of sea nettles in the Chesapeake Bay. Usually, though, they hang out farther south, where they sting unwary bathers and swimmers.
But the researchers said the lack of rainfall this summer likely triggered the harbor invasion by making the water here just salty enough to attract them. It's been abnormally dry on both sides of the bay, according to the U.S. Drought Monitor, with moderate to extreme drought gripping the western end of the state and the lower Eastern Shore.
"What apparently has happened is that the optimal salinity range has shifted up the bay," said Raleigh Hood, a biological oceanographer at the University of Maryland's Horn Point Environmental Laboratory near Cambridge. "Normally, down here, we're sea nettle heaven."
Hood said he's not surprised by their northward migration this year. He and Christopher Brown, a scientist at the National Oceanic and Atmospheric Administration, developed a system for predicting and mapping their abundance and spread, based on a variety of factors like water salinity and temperature. The computer model shows little likelihood of finding nettles in the Patapsco River, but it does show that salinity levels in the harbor and just outside it are elevated now, right around what sea nettles find most comfortable.
"They're happy as clams in that range," Hood said.
Sea nettles can be found year-round in the middle and lower bay and its rivers, from around Annapolis south, according to the Chesapeake Bay Program's field guide. They're at their peak in July and August, typically in the moderately salty middle of the bay, according to Denise Breitburg, senior scientist with the Smithsonian Environmental Research Center in Edgewater. But they can remain abundant as late as October, she added in an e-mail.
While the harbor influx of sea nettles is just one more weather-related oddity in a year of extremes, it's hard to say whether it has any broader relevance, Hood says.
"Global jellyfish populations are increasing," he explained. "That's a pretty good indication the world is going out of kilter." Some have attributed the jelly surge to global warming, others to the degradation of coastal waters worldwide.
Here in the Chesapeake Bay, though, it's not clear whether sea nettles are increasing or declining, Hood said. The average water temperature has increased slightly — possibly an indication of climate change — and water quality generally is considered poor throughout much of the estuary.
But Breitburg, who's been studying the bay's jellyfish for years, found that sea nettle densities have actually declined since the late 1980s, according to an article published last year by the Maryland Sea Grant program. She suggested that the dropoff may be related to the swoon of the bay's oysters, despite the bay's pollution and signs of climate shift.
Jellyfish lay eggs in the water, which settle to the bottom and attach to hard surfaces like rocks, pilings and oyster shells. But as oysters have dwindled, their shell-covered reefs have been smothered in silt, depriving jellyfish polyps of places to spend the winter.
Even if the numbers are down a bit, there are still plenty out there to nail folks who spend time in or on the water.
"I got stung by one just the other day," Hood said. "It's annoying but not life-threatening, unless you're allergic to it."
Of course, that shouldn't be an issue in the harbor — Baltimore health authorities warn against swimming there because of potentially disease-causing bacteria in the water.
But the ghostly looking nettles are safe to watch, as long as you stay out of the drink. And if it's any consolation — or motivation to get out to see them — this is likely their last hurrah. The nettles farther south are already starting to die off, reports Breitburg, and these will, too, once it rains enough to lower the salinity level again, or it gets colder.