Coral reefs are among the earth's most productive ecosystems, but coral-reef species also consume most of this productivity. Consequently, very little energy is lost from a coral reef before it is consumed and much of the energy and nutrients are recycled in the reef in an effective manner. Equally surprising is that to maintain this productivity little or no nutrients are consumed from seawater currents passing over the reef. This has been shown by measuring the quantity of nutrients such as phosphates and nitrates in seawater on the upstream and downstream sides of the reef. Studies have shown that there is no net difference in the phosphorus concentrations on either side of a current passing the reef and that nitrates are higher on the down than the upstream side of the reef. Although nutrients in seawater may be consumed by reef organisms, organisms are releasing nutrients at or greater than the rate of nutrient consumption. In the case of nitrogen, lawns or turfs of nitrogen-fixing benthic algae are converting nitrogen gases dissolved in seawater into the forms required by organisms. This nitrogen is finally lost to the passing seawater. Consequently, productivity and nutrient studies, to date, present a picture of the coral reef as a highly productive, nutrient self-sufficient ecosystem. (Carpenter R. C., Hackney J. M., Adey W. H. 1991)
Unlike many terrestrial ecosystems that require freshwater the coral reef is more limited by light and the rate of uptake of necessary elements such as nitrogen. Light is the largest single energy source at 7.12 x 109 Joules/m2/year but the energy of water motion created by waves, currents, and tides combined is also significant at 1.76 x 109 Joules/m2/year, or 25% of the sunlight's energy. Water motion is necessary in the renewal and loss of necessary elements such as oxygen, carbon dioxide, and nitrogen and plays an important role in production.........
