Scientists say that brown tide is back and threatening marine life on Long Island's South Shore.

Brown tide first occurred in Long Island waters in 1985. Marine scientists and baymen say that since then, it's become worse.

"The levels this year, over a million cells per milliliter, are the worst we've seen since 2012," says Dr. Christopher Gobler, of Stony Brook University. "That is a level that is very harmful to marine life."

Scientists say that the only way to combat brown tide is to update antiquated septic systems and to stop lawn fertilizers from flowing into Long Island waterways.

Brown tide is a harmful algae bloom that thrives on nitrogen those systems leech into the water. It is toxic to shellfish and eel grass, among other types of marine life, but not humans.

Suffolk County, the state Department of Environmental Conservation and other scientists have been exploring new technology that would cut back the nitrogen load that is entering Long Island's waterways.