Biscayne National Park consists largely of expanses of marine and brackish waters. The bodies of water that make up the park include the southern portion of Biscayne Bay, the northern limits of Card Sound, the continental shelf region on the ocean side of the park's barrier islands, and a number of tidal channels that connect the ocean with Biscayne Bay and Card Sound. Biscayne Bay is a shallow sub-tropical estuary located along the southeastern coast of the Florida peninsula. It is the largest portion of the park. The bay serves as a nursery where juvenile marine life resides. The bay is one of the most productive ecosystems in the park. Freshwater flow from inland areas brings nutrients into the bay. Tides, wind, and freshwater input forcing affect water movement in the Bay. Agriculture and urban development threaten the Biscayne Bay watershed and Biscayne Bay ecosystem. Miami-Dade is the second largest agricultural producing county in Florida and the population of the County continues to grow. The major environmental issues facing the South Florida watershed are: mercury contamination, ecological degradation of Florida Bay, water supply conflicts, nutrient enrichment, loss of historic hydropatterns, rapid regional population growth, spread of exotic plants and animals, loss of native populations, and the conversion of remaining wetlands to other land uses.