Development on land directly affects coral reefs. Deforestation leads to the erosion of tremendous amounts of soil into the sea. Settling on a reef, the soil smothers coral and blocks sunlight from reaching the zooxanthellae. Dredging forchannels further stirs up coral-choking sediment. Pollution is a growing problem, too. Freshwater and soil runoff can pick up toxic agricultural and industrial chemicals that are carried to the reef. And then there is untreated sewage, bilge waste, oil spills, thermal affluents, etc.