GEOTHERMAL ENDOUPWELLING - A SOLUTION TO THE REEF NUTRIENT PARADOX

Coral reef ecosystems have long been regarded as paradoxical because their high biomass and gross primary productivity far exceed that expected for ecosystems in tropical oligotrophic waters. Previous authors have explained the paradox by emphasizing efficient recycling, conservation and storage of nutrients within the reef ecosystem. However, the fact that reefs are net exporters of nutrients and organic matter means that for sustained productivity new nutrients must be imported. Comparisons of the chemical properties of interstitial waters from shallow boreholes in atoll, barrier and lagoonal pinnacle reefs in French Polynesia indicate that their nutrient concentrations are similar or superior to those in Antarctic Intermediate Water (A.I.W.) at 500-1000 m depth. By the geothermal endo-upwelling process, A.I.W. enters the porous reef framework, is driven upward by the local geothermal gradient and emerges at the reef crest to provide nutrients to the flourishing algal-coral ecosystem. Continuing research on the reef nutrient controversy suggests that there are several paths presently converging upon it's solution: among them the endo-upwelling model seems an adequate explanation for barrier reefs located in clear oligotrophic waters such as the Polynesian ocean.