The Florida Keys National Marine Sanctuary is a U.S. National Marine Sanctuary in the Florida Keys. It includes the Florida Reef, the only barrier coral reef in North America and the third-largest coral barrier reef in the world. It also has extensive mangrove forest and seagrass fields. The Florida Keys National Marine Sanctuary, designated in 1990, is the ninth national marine sanctuary to be established in a system that comprises 13 sanctuaries and two marine national monuments. The Florida Keys National Marine Sanctuary protects approximately of coastal and ocean waters from the estuarine waters of south Florida along the Florida Keys archipelago, encompassing more than 1,700 islands, out to the Dry Tortugas National Park, reaching into the Atlantic Ocean, Florida Bay and the Gulf of Mexico. The mission of the sanctuary is to protect the marine resources of the Florida Keys while facilitating human uses that are consistent with the primary objective of resource protection. Sanctuary waters and habitats support high species diversity due to the presence of both tropical and subtropical species, including the largest documented contiguous seagrass community in the northern hemisphere and extensive coral reef habitat. The sanctuary is also home to maritime heritage resources that encompass a broad historical period. Human impact Population and urban development Population growth and urban development may impact the health of coral reefs. Development activities may cause erosion resulting in the runoff of sediments which eventually reach the coral reefs. Storm water runoff may carry fertilizers into the ocean causing damage to the coral reefs, and an increase of nutrient concentrations in the reef may cause an increase of algae which may smother the corals. Sizable foreign objects such as sunken boats and planes provide rich micro-sanctuaries for a plethora of sea life that otherwise would not exist. Fishing activities The harvest of resources from the sea is ever growing. Overfishing has changed the ecological dynamics of marine communities allowing some organisms to dominate reefs that were once controlled by large reef fish populations. Tourism Tourism dollars help to fund scientific research and environmental remediation activities. Tourism may contribute to reef damage. Divers and snorkelers may harm the corals by touching the polyps, and boats may damage it by running aground and dragging anchors. Coral bleaching Global warming coupled with an intense El Niño has led to higher sea water temperatures throughout the area, which can cause coral bleaching. "Minor paling"