PLANKTON BIOMASS AND PRODUCTION IN AN OPEN ATOLL LAGOON - UVEA, NEW-CALEDONIA

Uvea lagoon is an atoll-type one with a discontinuous belt of small is lets on its western part and the main island to the east. Its depth in creases steadily from east to west. A 2 week cruise in September 1992 aimed to study the ways in which these morphological features influenc e the functioning of the lagoon pelagic ecosystem. Hydrological parame ters present a fair homogeneity, both horizontally and vertically over the whole lagoon, which is due to an efficient mixing and important e xchanges with the oligotrophic open ocean. Lack of significant nutrien t concentrations (NO3, NO2, NH4, PO4, SiO3) in the water mass is in ag reement with low planktonic biomasses: Chlorophyll a (Chl a) concentra tion is 0.233 mg m(-3), and ash-free dry weight is 5.25 and 7.55 mg m( -3) for [35-200 mu m] and [200-2000 mu m] size fractions respectively. These biomass levels are more than twice the concentration of the sur rounding open ocean. Total Chl a is dominated by the >1 mu m size-frac tion, thus contrasting with the dominance of small cells (<1 mu m) in the open ocean. Phytoplankton prevails in the [35-200 mu m] size-class , indicating the occurrence of microphytobenthos brought by mixing of the water column. The [200-2000 mu m] fraction is made up primarily of copepods (61% of the dry weight), appendicularians and radiolarians. Planktonic predators, such as chaetognaths are almost absent. Three di fferent methods dealing with carbon production, i.e., C-14 fixation, i n-bottle O-2 production, and natural O-2 variations, lead to a coheren t estimate of pelagic primary production: 27.5 mg C m(-3) d(-1). Half of this production is achieved by <1 mu m cells. Zooplankton productio n, which was assessed by the C/N/P ratios method, is equal to 10.4 mg C m(-3) d(-1) and its P:B ratio is 114%. On the whole, Uvea lagoon app ears to be oligotrophic compared with other ones, because it is wide-o pen. (C) 1997 Elsevier Science B.V.