The primary threat to coral reefs is their relative proximity to the activities of man. While most oceanic communities exist outside the immediate reach of events happening on land, coral inhabits shallow waters that border land masses of every size and description.
When man acts upon the land, those actions generally result in the generation of sand, soil, and other materials that get washed away into local streams. Some of this runoff is light enough to remain suspended in water and is carried into the shallow seas that surround land masses.
Coral is a filter feeder, straining water and extracting food from the plankton and microscopic life forms that are carried by the ocean currents to and around the reef. While there are a significant number of deep water coral communities, those that inhabit shallow water around inhabited land masses draw most concern from ecologists and environmental scientists, and with good reason
Unfortunately, sediment follows the same path. Sediment harbors a number of structures common to the land it came from including minute sand particle and the colloids that constitute clay. By their size and their propensity to resist being broken up into smaller particles that might pass through the feeding mechanisms of coral and its many filter-feeding cousins, the sediment will literally plug the "mouths" of coral, starving it to death.
It has happened before than areas of the ocean, through natural progressions and currents, have become unsuitable for coral causing , eventually, the death of a reef. Some have recovered with a subsequent return of favorable conditions, but usually the recovery never takes place.
The reefs destroyed by land based sediment do not come back.
Along with the reef, the entire ecosystem soon fails as fishes and other fauna relocate to avoid dying off with the coral. Beyond these obvious changes, the reef supports an enormous number of epiphytes and symbiots that contribute to the clarity and water chemistry around the reef. They die with the coral, adding to the detritus that fogs the water along with the sediment.
Regional fisheries also suffer because a coral reef is the foundation of an immense food chain which, once disrupted, can turn previously rich fishing grounds into vast oceanic deserts. Without a place to live, breed, and raise their young safely among the coral branches, those species upon whom game fishes depend for food must move out of the area in order to survive and reproduce.
Recent attempts to use old tires as a substitute for a natural coral reef have been largely unsuccessful. It seems there is more to the coral reef than just the complexity of its structure that causes so many species essential to the region's ecological balance to gather and propagate there.
There are many waterways that brought sediment to the ocean long before man plowed under his first forest, and there will always be places where coral will not thrive, but man cannot continue to add to those places indiscriminately and expect the ecological health of the oceans to remain unchanged. The death of the coral reefs is a symptom of a general sickness that will seriously and permanently alter the basis of life on this planet: the seas.
Every effort to preserve the reefs will positively impact the oceans that hold them. Something as simple as controlling the effluence of sediment from man's activities will go a long way toward keeping the reefs alive.