Coral reefs are one of the most fragile, biologically complex, and diversemarine ecosystems on Earth. This ecosystem is one of the fascinatingparadoxes of the biosphere: how do clear, and thus nutrient-poor, waterssupport such prolific and productive communities? Part of the answer lieswithin the tissues of the corals themselves. Symbiotic cells of algae known aszooxanthellae carry out photosynthesis using the metabolic wastes of thecoral thereby producing food for themselves, for their corals, hosts, and evenfor other members of the reef community. This symbiotic process allowsorganisms in the reef community to use sparse nutrient resources efficiently.Unfortunately for coral reefs, however, a variety of human activities arecausing worldwide degradation of shallow marine habitats by adding nutrientsto the water. Agriculture, slash-and-burn land clearing, sewage disposal andmanufacturing that creates waste by-products all increase nutrient loads inthese waters. Typical symptoms of reef decline are destabilized herbivorepopulations and an increasing abundance of algae and filter-feeding animals.Declines in reef communities are consistent with observations that nutrientinput is increasing in direct proportion to growing human populations, therebythreatening reef communities sensitive to subtle changes in nutrient input totheir waters.
