Direct threats are generally isolated incidents involving boats, divers and fishermen on the reef. Propellers and anchors can break apart and crush coral, destroying years, if not decades, of coral growth.
Poorly informed and negligent divers also harm corals by touching and standing on them. Even if a coral is not visibly harmed, human touch can make them more vulnerable to death and disease.
The more traffic on a reef the greater the risk of this type of direct damage. This means that it is very important to monitor and limit the number of divers on any given reef. Failure to keep tourism at bay has been one of the leading causes of coral death, especially in popular dive locations such as the Florida Keys.
Lobster and stone crab traps are another prominent direct impact on many reefs. Commercial traps are small enough to be moved by storms, though large and durable enough to cause substantial damage to reefs. The traps themselves break apart coral, while the lines entangle around all forms of marine life.
Indirect threats aren’t always so easy to pinpoint, as they are generally associated with water quality and clarity.
Larger debris, coming from both the land and the sea, can break apart coral and strangle and suffocate marine life. Plastics are an especially large threat, as they often wrap around smaller branching corals, entangle marine life and kill animals such as sea gulls and turtles.
Water quality is jeopardized on many reefs due to heightened pollutant and nutrient levels associated with inadequate waste water and storm water treatment.
Storm water is rain water that enters the ocean after running off of the land. Impervious surfaces, such as roads, parking lots, roofs and other residential and commercial spaces, magnify storm water by preventing absorption of water into the land. Storm water often includes harmful pollutants such as oil and gasoline, pesticides and fertilizers and other land based chemicals. Fertilizers are particularly harmful, as they cause wide spread algae blooms that out compete sea grasses and corals for sunlight. All storm water runoff alters the chemical composition of near shore waters and causes increased turbidity and sedimentation. This is especially harmful to corals, as they thrive in clean, clear nutrient free waters.
One of the largest challenges in managing storm and waste water is identifying the sources. The near shore waters of the Florida Keys, for instance, contain runoff from South Florida and the everglades. Large channels and currents bring nutrients, pollutants, fertilizers and pesticides from Florida Bay across the islands of the Keys and out to the reef tract. This highlights how long term marine conservation depends not only on local efforts, but on cooperation amongst various stakeholders in multiple regions.
NOAA’s Corals Tutorial http://oceanservice.noaa.gov/education/tutorial_corals/welcome.html