Single-use plastic strikes again.A juvenile leatherback sea turtle was found dead at Shinnecock East County Park in Southampton on Friday, according to Rachel Bosworth of the Atlantic Marine Conservation Society. And while a definitive cause of death could not be determined, Ms. Bosworth surmised that it may have drowned due to plastics found in the mammal’s intestines. During a necropsy on Saturday morning, staff biologists found a 13-gallon plastic bag, as well as smaller black garbage bags and food wrappers, in the animal’s gastrointestinal tract.“That could have led to the animal’s death,” Ms. Bosworth said on Tuesday. “The lungs looked collapsed, as if the animal had drowned.”It could take up to several months for pathologists working with the society to confirm the cause of death, she said. Ms. Bosworth explained that marine debris is an overwhelming problem on the East End. Last year, approximately 14 of the 80 or so sea turtles serviced by responders from the organization died from ingesting marine debris or suffering from entanglement.On average, plastic pollution affects nearly 700 marine species per year, according to a 2017 study by Plymouth University.Americans produce nearly 300 million tons of plastic every year, 50 percent of which is for single use, like straws, plastic bags and disposable bottles—which account for approximately 14 percent of plastic pollution, according to Plastic Oceans, a California-based nonprofit organization. Many of the items can resemble food sources to some marine creatures. To help combat the rise of plastic pollution, both the Southampton and East Hampton town boards and several East End villages banned the use of single-use plastic shopping bags in 2014, and Suffolk County enacted a similar, more inclusive ban on January 1 of this year.On June 28, biologists from the society responded to four dead marine mammals—a bottlenose dolphin, a loggerhead sea turtle found off the coast of Montauk, and two gray seals—according to the group’s chief scientist, Robert DiGiovanni Jr. “We've had quite a large number of animals—a lot more than what we would normally anticipate. That’s why we’re trying to engage public outreach lectures,” Mr. DiGiovanni said.Among those lectures include screenings of a short documentary, “Straws” by Linda Booker, which discusses the dangers of single-use plastics, at local East End sites such as Downs Farm Preserve in Cutchogue and the Greenport Theatre.Ms. Bosworth stressed that sick, injured or deceased marine mammals should be reported to the New York State Stranding Hotline at 631-369-9829.“Marine debris poses a serious threat to marine life,” the organization posted on its social media sites. “Conservation starts with you.”