Reporting from Barataria Bay, La., and Los Angeles — There's a destructive liquid flowing into the Gulf of Mexico — and it's not oil. It's the muddy freshwater of the Mississippi River, which has been released from southern Louisiana's vast levee system and into estuaries in greater quantities than usual. The goal has been to use the rush of freshwater to keep sticky oil from reaching the sandy shores of the state. » Don't miss a thing. Get breaking news alerts delivered to your inbox. The tactic has proved moderately successful in some areas, but the extra freshwater creates lower-than-normal salinity levels in Barataria Bay and Breton Sound, which flank the southeast portion of Louisiana that juts out into the gulf. Some biologists are worried. Mass oyster deaths have been reported in those two areas, early evidence suggesting that the freshwater could be shaking up a delicate ecosystem and a struggling seafood industry — both already threatened by the largest offshore oil spill in U.S. history. Though the extra flow of freshwater is a temporary measure, biologists are inundated with questions regarding the long-term effect on the area's once-productive oyster beds and the complex web of animals, such as clams, acorn barnacles and tube worms. "You're removing part of the food chain," said Ed Cake, an oyster biologist based in Mississippi. Biologists also worry the dropping salinity levels could displace shrimp, speckled trout and southern flounder. The young of all three species live and grow in the tepid, nursery-like conditions of the bayous and inlets of Barataria Bay and Breton Sound. But the principal concern is the oyster, one of the building blocks of marine life and a key indicator of environmental conditions. "Oysters become that initial sentinel," said Earl Melancon, a biological sciences professor at Nicholls State University, about 60 miles southwest of New Orleans. "You don't know their impact until you look at their annual life cycle." On a recent morning, Melancon set out on a forensic mission in Barataria Bay. He was looking for dead oysters and wondering whether next year would see a crop. He knew dead mollusks had been found northeast of here, around Breton Sound. "I'm fairly confident that what we're seeing out there is a freshwater event that's killing everything, he said before the trip. Much like a fisherman harvesting a catch, graduate student Dan O'Malley, his blue T-shirt sleeves rolled up to his shoulders, plunged a dredge into the 90-degree brownish green waters of the bay. He pulled up a muddy heap of hard shells and plopped it onto a silver metal sheet. "All right," Melancon said, standing at the helm of the boat. "We got enough there for a sample." Oyster shells were wide open, the inner white mother of pearl layer exposed — a sign the oysters hadn't been dead for long. Hooked mussels, which can tolerate lower salinity levels, were still alive and attached to some of the dead oysters. There was no oil in sight. "To me, that's an indication of freshwater mortality," Melancon said. For the Louisiana native, the scene was heartbreaking. His family migrated here from Nova Scotia 240 years ago. "Oysters and other animals can't take this degree of heat and this salinity," Melancon said. Each sample yielded the same results. "Are you finding anything alive?" Melancon asked Justin Sancho, another graduate student, at the next stop.