TROPHIC STRUCTURE AND PRODUCTIVITY OF THE LAGOONAL COMMUNITIES OF TIKEHAU ATOLL (TUAMOTU ARCHIPELAGO, FRENCH-POLYNESIA)

Carbon standing stocks and fluxes were studied in the lagoon of Tikehau atoll (Tuamotu archipelago, French Polynesia), from 1983 to 1988. The average POC concentration (0.7-2000-mu-m) was 203 mg C m-3. The suspended living carbon (31.6 mg C m-3) was made up of bacteria (53%), phytoplankton < 5-mu-m (14.2%), phytoplankton > 5-mu-m (14.2%), nanozooplankton 5-35-mu-m (5.7%), microzooplankton 35-200-mu-m (4.7%) and mesozooplankton 200-2000-mu-m (7.9%). The microphytobenthos biomass was 480 mg C m-2. Suspended detritus (84.4% of the total POC) did not originate from the reef flat but from lagoonal primary productions. Their sedimentation exceeded phytobenthos production. It was estimated that 50% of bacterial biomass was adsorbed on particles. The bacterial biomass dominance was explained by the utilisation of 1) DOC excreted by phytoplankton (44-175 mg C m-2 day-1) and zooplankton (50 mg C m-2 day-1) 2) organic compounds produced by solar-induced photochemical reactions 3) coral mucus. 50% of the phytoplankton biomass belongs to the < 5-mu-m fraction. This production (440 mg C m-2 day-1) exceeded phytobenthos production (250 mg C m-2 day-1) when the whole lagoon was considered. The zooplankton > 35-mu-m ingested 315 mg C m-2 day-1, made up of phytoplankton, nanozooplankton and detritus. Its production was 132 mg C m-2 day-1.