INFLUENCE OF SOME CORAL-REEF COMMUNITIES ON THE CALCIUM-CARBONATE BUDGET OF TIAHURA REEF (MOOREA, FRENCH-POLYNESIA)

The calcium carbonate budget of coral reefs is the result of the inter action of the processes of calcification and biological degradation, a nd is reflected in the chemical properties of the seawater overlying t he reefs. A series of experiments at Moorea Island (French Polynesia) in 1988 monitored the diurnal and nocturnal variations in the chemical properties of seawater under field and laboratory conditions. Our res ults revealed that in the study area (Tiahura barrier reef flat), the calcium carbonate budget varied over space and time as a function of l ocation in the water current. Two in-situ sites were investigated, one was situated 100 m from the algal crest of the barrier reef. the othe r 300 m further downstream. As a result of cumulative upstream events, the daily net calcification was ten times higher at the downstream (5 .22 gm-2 d-1) than at the upstream (0.45 gm-2 d-1) site. The carbonate uptake by in situ Porites lobata in enclosures (8 kg m-2 yr-1) was te n times higher than the uptake by the whole community in the Surroundi ng water (0.8 kg m-2 yr-1) and five times higher than that recorded fo r P. lobata in laboratory experiments (1.4 kg m-2 yr-1), where illumin ation levels were 10 % of in situ levels. In laboratory experiments, t he planktonic fraction of the seawater had no perceptible influence on the calcium carbonate budget. In the absence of bioeroders, living co ral totally depleted the carbonate content of the seawater (3.7 gm-2 d -1). Bioerosive organisms played an important role in restoring this c alcium carbonate; e.g. sea urchins grazing on algal turf covering dead coral ingested CaCO3 and released this as a carbonate powder (1.26 gm -2 d-1); a form of carbonate which is extremely accessible to chemical dissolution.