Plankton biomass and production in an open atoll lagoon: Uvea, New Caledonia

Abstract

Uvea lagoon is an atoll-type one with a discontinuous belt of small islets on its western part and the main island to the east. Its depth increases steadily from east to west. A 2 week cruise in September 1992 aimed to study the ways in which these morphological features influence the functioning of the lagoon pelagic ecosystem. Hydrological parameters present a fair homogeneity, both horizontally and vertically over the whole lagoon, which is due to an efficient mixing and important exchanges with the oligotrophic open ocean. Lack of significant nutrient concentrations (NO 3, NO 2, NH 4, PO 4, SiO 3) in the water mass is in agreement with low planktonic biomasses: Chlorophyll a (Chl a) concentration is 0.233 mg m −3, and ash-free dry weight is 5.25 and 7.55 mg m −3 for [35–200 μm] and [200–2000 μm] size fractions respectively. These biomass levels are more than twice the concentration of the surrounding open ocean. Total Chl a is dominated by the >1 μm size-fraction, thus contrasting with the dominance of small cells (<1 μm) in the open ocean. Phytoplankton prevails in the [35–200 μm] size-class, indicating the occurrence of microphytobenthos brought by mixing of the water column. The [200–2000 μm] fraction is made up primarily of copepods (61% of the dry weight), appendicularians and radiolarians. Planktonic predators, such as chaetognaths are almost absent. Three different methods dealing with carbon production, i.e., 14C fixation, in-bottle O 2 production, and natural O 2 variations, lead to a coherent estimate of pelagic primary production: 27.5 mg C m −3 d −1. Half of this production is achieved by <1 μm cells. Zooplankton production, 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 appears to be oligotrophic compared with other ones, because it is wide-open.