How safe is it to swim at the Jersey Shore?

Gulls provide the “hah-hah” laugh track for a day at the Jersey Shore, but the fowl excrement they produce is no joke.

All that poop from gulls, geese, and other flapping creatures — as well as humans — can be the determining factor in whether sunbathers become swimmers. Heavy rains wash fecal matter into the water, one of the contributing factors as to why a beach might be forced to close in the days after a storm.

“There’s a fair correlation between rainfall and advisories and closings, mostly on the bay and river side,” said Larry Hajna, spokesperson for the New Jersey Department of Environmental Protection. “And if you have a rainstorm event over several days, that can result in closings and advisories on the beach as well.”

>>READ MORE: Jersey Shore beaches: What’s open, what’s closed

The Jersey Shore’s water quality for swimming is good overall, says Hajna. The DEP monitors water to ensure it is safe for swimmers according to the Cooperative Coastal Monitoring Program. The agency also keeps watch over water-quality issues that might affect aquatic life, like oysters and clams.

Using aerial surveillance and chlorophyll measurements, the DEP checks on any algae blooms or areas with floating trash. Algae blooms typically are not harmful to people, but they can affect oxygen levels for fish and aesthetic issues for people if blooms get close to shore.

Trash also washes in after heavy rainstorms. That’s one way climate change may contribute to water health at the Shore: More storms, heavier downpours, and chronic so-called sunny-day flooding that occurs during high tides all mean a higher likelihood of debris in the water and beach closures.

When it rains, water flows over lawns, parking lots, and streets and into storm drains, catchment basins, and drain pipes. Single-use plastic bottles, fast-food wrappers, cigarette butts, gasoline, motor oil, antifreeze, fertilizers, pesticides, pet droppings, and, yes, gull poop all get swept up in the torrent and end up in waterways. Meanwhile, older sewer systems are prone to overflow, releasing untreated sewage into the water. The pollution can also kill fish and wildlife.

Michael J. Kennish, a Rutgers research professor in the Department of Marine and Coastal Sciences, said runoff from fertilizers containing phosphorus and nitrogen causes algae and other underwater plants to grow. This chokes off oxygen to back bays, including Barnegat and Little Egg Harbor Bays in Ocean County.

Such back-bay areas are more vulnerable to pollution, Kennish said, because they are contained and shallow, like lagoons. Back bays are also prominent in Atlantic and Cape May Counties.

“The water-quality conditions are generally good, but they could be improved,” Kennish said. “The more impacted systems need concerted remedial efforts to improve them. It is important to maintain vigilance on the environmental conditions of our estuarine and coastal marine waters.”

To test if beaches and bays are safe for swimming, officials monitor for Enterococci bacteria, which are produced in the guts of humans, mammals, and birds and can cause infections. When the level of bacteria exceeds a certain threshold, beaches get closed or an advisory is issued. The standard for a safe beach is 104 or fewer colonies of Enterococci per 100 milliliters of sampled water.

Swimming in polluted water can have health risks, usually minor. Symptoms can include nausea, vomiting, diarrhea, and abdominal pain.

But New Jersey beach closings are relatively rare along the state’s 130 miles of oceanfront coastline. Last year, 98 percent of ocean beach samples met the swimming standard. Usually, only a handful of beaches are closed each year due to bacteria; there are many more advisories to alert bathers when water quality is lower but still safe.

There have been 805 ocean beach closures in New Jersey since 2005, according to an Inquirer analysis of state data. The closures were for a range of reasons, such as heavy rainfall, sewage, syringes, and trash. Just under a third were for bacterial presence. Spring Lake in Monmouth County has seen the largest number of closures at 11. Atlantic City was second with nine. Cape May, Ventnor, and Ocean City have had three each.

Reports have surfaced in recent years of tiny clinging jellyfish releasing their powerful stings on swimmers. The dime-size invertebrates, Gonionemus vertens, are native to the Pacific Ocean but began showing up in our waterways in 2016.

The jellyfish tend to live in shallow bays and estuaries among eel grass and other underwater vegetation, “clinging” to it. Some have floated into Barnegat Bay, and two men had to be hospitalized last year after being stung.

While bay swimmers should be cautious — clinging jellyfish have up to 90 tentacles with stinging cells — beachgoers have little to fear. Scientists are watching to see if the jellyfish spread to the ocean in significant numbers and become a more widespread threat.