PROCESSES OF ORGANIC PRODUCTION ON CORAL REEFS

1. The first quantitative studies of production on coral reefs were those of Sargent & Austin who showed that productivity on reefs was considerably higher than in surrounding waters. This high production occurred in spite of nutrient limitation and low productivity of offshore waters. Their conclusions have since been confirmed by numerous other workers in both the Atlantic and the Pacific. 2. Primary production on reefs has been studied by flow respirometry, measuring changes in oxygen or carbon dioxide concentrations in water flowing over reefs. Production of benthic organisms has also been measured in situ by light and dark bottle methods and by radioactive tracer techniques. Production values obtained by the various methods are not identical but their use in combination is to be recommended. 3. Rates of gross primary production on reefs vary between 300–5000 gC/m²/yr. These rates are higher than general oceanic values and as high as those of the most productive marine communities. 4. Sources of primary production include fleshy macrophytes, calcareous algae, filamentous algae on the coral skeletons or calcareous rock, marine grasses and the zooxanthellae within coral tissue. Production values from the various sources fall within the range of production of reefs as a whole. 5. Concentrations of nitrogen and phosphorus in waters flowing over reefs are consistently low. There is evidence to suggest that both these nutrients are recycled rapidly on the reef and that nitrogen is fixed by bacteria and primary producers. 6. In many instances the mass of detritus over coral reefs exceeds the biomass of zooplankton. While the quantitative significance of detritus as food for corals and other benthic organisms has not been evaluated, there is a growing body of evidence to show that this may be the key to understanding secondary production. 7. Opinions differ on the adequacy of zooplankton in satisfying the food requirements of corals and other benthic invertebrates on reefs. The weight of evidence suggests that while there is a removal of zooplankton by benthic organisms, the total biomass carried over the reef is too small to support the energy needs of secondary production. 8. Bacteria are a potential source of energy for secondary production on reefs and are implicated in nitrogen fixation, decomposition and biogeochemical cycling. 9. There is an abundance of sessile invertebrates other than corals on reefs but there are few quantitative data on their importance in secondary production. 10. The biomass of fish on reefs may be very high but the quantitative significance of grazing and predation is not fully established. 11. Studies on the growth of corals themselves have been based on measurements of skeletal accretion. These methods do not lead directly to estimates of reef organic production. Growth rates of corals vary considerably between and within species. 12. Estimates of reef growth have been made from measurements of coral growth and from the flux of calcium carbonate. There is less quantitative information on erosion caused by mechanical damage, by boring organisms and by human pollution. 13. Hydrographic factors influence growth and form of reefs and there is some evidence to show that production is enhanced by conservation of water in lagoonal areas.