Copépodes planctoniques du lagon de Nouvelle-Calédonie: facteurs écologiques et associations d'espèces

In the southwestern part of the lagoon of New Caledonia (South Pacific Ocean), a plankton sampling programme was conducted from February 1978 to April 1979. During 11 cruises, 5 stations in the open sea and various bays were sampled at approximately monthly intervals. A transect of 3 stations (mid lagoon, near-reef and barrier-reef channel) visited each fortnight, completed the sampling programme. The copepods collected were identified to the specific level, counted and the 52 most abundant species analysed for seasonal and regional variations. Cruise and transect data, dealt with separately, were analysed as follows: (a) Correspondence analyses (reciprocal averaging) of qualitative and quantitative copepod counts were made; these revealed the relationships between species and between samples by means of factorial design. After computation of factorial axes, the barycentres of different sets of samples were projected as reference points (zero weight) in the factorial space; these reference points, characterizing different stations and sampling times, facilitate ecological interpretation. (b) Species partition was achieved by two successive methods: non-hierarchical, followed by hierarchical classification. Between-species distance was computed from their co-ordinates on the factorial axis. (c) The clusters of species obtained were plotted in the factorial planes to assess ecological preference. Then, clusters from cruise and transect data sets were compared to improve the copepod classification. The main ecological factors appear to be (i) spatial patterns, (ii) seasonal temperature cycle, (iii) changes in wind force and direction. Different populations inhabit the open sea, near-reef, mid-lagoon, shallow and deep-bay waters. Acartia australis outnumbers all other species in the reef vicinity. A. amboinensis is the most abundant in the deep, fjord-like bay. Canthocalanus pauper, Paracalanus parvus, Bestiola sp., Centropages orsinii, A. bispinosa are characteristic of shallow bays. In the factorial structure produced by correspondence analysis, certain seasonal barycentres appear close to some station barycentres: summer close to mid-lagoon, winter close to open sea, and spring close to near-reef barycentres, respectively. This may be explained by the seasonal dynamics of the lagoon water. After the heavy summer rainfalls, freshwater runoff carries midlagoon plankton towards the open sea; conversely, during the winter westerly gales, oceanic species enter the lagoon through the barrier-reef channel or above the reef with the swell breakers. In October, the surface-layer current induced by the strong trade wind carries large swarms of A. australis out to the open sea. Finally, the variations of plankton populations and biomass in the lagoon seem to be governed by the direction of water flow across the reef channels. Enrichment factors are terrestrial sediment wash-out after rainfall and, probably, trade wind-induced upwelling. Therefore, the more or less steady state of the lagoon's plankton biomass may result from the fact that enrichment factors are also exportation factors. Reciprocally, the entry of plankton-poor oceanic water cannot increase the plankton biomass in the lagoon.