Much of the coast and the inner estuaries are built of mangroves; there is no border between the coastal marshes and the bay. Thus the marine ecosystems in Florida Bay are considered to be a part of the Everglades watershed and one of the ecosystems connected to and affected by the Everglades as a whole. More than 800 square miles (2,100 km2) of Florida Bay is protected by Everglades National Park, representing the largest body of water in the park boundaries.[79] There are approximately 100 keys in Florida Bay, many of which are mangrove forests.[80] The fresh water coming into Florida Bay from the Everglades creates perfect conditions for vast beds of turtle grass and algae formations that are the foundation for animal life in the bay. Sea turtles and manatees eat the grass, while invertebrate animals, such as worms, clams and other mollusks eat the algae formations and microscopic plankton.[81] Female sea turtles return annually to nest on the shore, and manatees spend the winter months in the warmer water of the bay. Sea grasses also serve to stabilize the sea beds and protect shorelines from erosion by absorbing energy from waves.