PHYTOPLANKTON BIOMASS AND PRODUCTION IN 2 TUAMOTU ATOLL LAGOONS (FRENCH-POLYNESIA)

Nutrient concentrations, phytoplankton biomass (chlorophyll a, chl a) and primary production (C-14 uptake) were measured over a 5 yr period (1990 to 1994) in the atoll lagoons of Takapoto (4 yr water residence time; with pearl oyster aquaculture) and Tikehau (0.5 yr residence tim e; without pearl oyster but with a fishery), French Polynesia. In both atolls, phosphate and silicate concentrations (0.1 mu M PO4 and 0.8 m u M SiO2) were lower inside the lagoon than in surrounding oceanic sur face waters. Picoplankton <1 mu m dominated phytoplankton biomass (61% ) and productivity (55%) in both lagoons. Average assimilation numbers were high (13 mg C mg(-1) chl a h(-1)) and average chl a doubling rat es of the <1 mu m fraction were estimated to be 1.1 and 1.3 d(-1) in T akapoto and Tikehau, respectively. Average daily primary production du ring the 1991 to 1994 period in Takapoto and Tikehau lagoons was estim ated to be 0.8 and 0.7 g C m(-2) d(-1), respectively.