Few places on Earth rival the abundance and splendor of life on the coral reef. A reef visitor can spot living things in almost every size, shape, and color; however, some of the most important reef inhabitants cannot be seen with the naked eye. These invisible organisms live on the reef floor or float in the water column, the huge expanse of water below the surface.
The organization of living things on coral reefs is unique. In most oceans, upper regions of the water teem with plankton, communities of tiny, drifting organisms. The plantlike members of this community, the phytoplankton, are able to carry out photosynthesis. The rest of the community is zooplankton, and it is made up of very small living things that cannot photosynthesize. In seas where the populations of plankton are substantial, waters are also rich in minerals and nutrients. The waters around coral reefs are low in nutrients and have very small populations of plankton. It is this very lack of nutrients and plankton that make the waters of reefs so beautiful. Their vivid blue color is a reflection of the sky, and their crystal-clear transparency is due to the absence of living things in the water column.
Despite low levels of nutrients, coral reef waters are extremely productive parts of the oceans. Productivity refers to the amount of photosynthesis that takes place in an ecosystem, and therefore the amount of food created. Productivity on reefs is 50 to 100 times greater than in nearby ocean waters.
Several kinds of organisms contribute to the elevated productivity
on reefs. Some of the primary producers are large
algae, sea grasses, and sizable populations of microscopic
algae. Many of these green, one-celled organisms live in the
tissues of corals and a few other types of simple animals.