Scientists studying Long Island's waterways are warning of damage that's being done to bays along the South Shore.According to experts, millions of gallons of treated sewage spew into the bays of western Nassau County each day. They say the sewage is strangling the ecosystems of the marshes, estuaries and bays along the South Shore.Scientists say Nassau's four South Shore sewage plants need to be retro-fitted with technology to remove nitrogen from the sewage. They say although updating the treatment plans will be costly, it's necessary to improve oxygen levels in the water.A comprehensive two-year analysis of the water quality will begin shortly with help from Stony Brook University and the federal government.