Large numbers of polyps, chemical defenses, and large numbers of offspring all help corals to survive. Unfortunately these defenses are not enough to combat human impacts. Humans may impact the reef in a number of ways, often not realizing the damage being done. Pollution, sedimentation from agriculture and development, coral harvesting, and reef trampling are a few ways we are destroying coral reefs. Pollution can kill reefs with poisons and heavy metals, but it can also kill coral by causing algae blooms.
Sewage contains nitrates and phosphates - fertilizer for plants like algae.
Algae blooms can smother reefs and kill the coral. Sedimentation cuts off sunlight so the zooxanthellae can not photosynthesize. Standing on the reef to rest while snorkeling can crush the coral polyps. Human impacts may take a long time to have an affect on the reef, but they do not go away unless we do something to remove them. When visiting coral reefs make an effort not to stand on or touch the coral since this can crush its tiny polyps.