A startling discovery beneath Lake Tahoe’s pristine water has regulators and scientists scrambling to size up the potential threat to the local ecosystem. Two days after Labor Day, divers discovered the lake’s first new invasive species in 15 years: the New Zealand mudsnail. “I’m worried because this is something we’ve tried to keep out,” said Dennis Zabaglo, the Aquatic Invasive Species Program Manager for the Tahoe Regional Planning Agency…