A national treasure, the Chesapeake Bay is the largest estuary in North America and the third largest in the world. A body of water where fresh and salt water mix, estuaries are rich habitats for marine organisms, providing abundant resources for the inhabitants of the watershed and the Bay's shoreline. The Bay receives half its water volume from the Atlantic Ocean, the rest from an enormous 64,000-square-mile watershed. All streams, ponds, swamps, and marshes within the park are part of this watershed, providing lush habitat for a multitude of birds, fish, mammals, aquatic invertebrates, and plants. It is estimated that more than 500 million pounds of seafood is commercially harvested annually from the Bay.