 The river is part of the larger Indian River Lagoon system, the most diverse estuarine environment in North America with more than 4,000 plant and animal species, including manatees, oysters, dolphins, sea turtles and seahorses.Historically, the St. Lucie was a freshwater river with no connection to either the Atlantic Ocean or Lake Okeechobee.Beginning in the late 19th century, the river and its watershed underwent a series of modifications for navigation, flood control and water supply purposes.