Essai d'utilisation de la diversité spécifique dans l'analyse des communautés de copépodes planctoniques du lagon de Nouvelle-Calédonie

Abstract

During a one-year survey of the south-western part of the New Caledonian lagoon (February 1978–April 1979), zooplankton was sampled at eight stations representing different biotopes. This paper attempts to explain the regional dynamics of the copepod community by means of specific diversity (H) and its components: number of species (S) and evenness (H:H _max). For each station, cumulative samples were computed by progressively summing the successive samples. The diversities of these cumulative samples form a temporal-diversity spectrum. This spectrum is characteristic of the stability or variability of the population at each station. It appeared to be closely related to the date of the starting point of the cumulation. Therefore, a mean annual spectrum was calculated for each station, by averaging all the spectra obtained from different starting points. This mean annual-diversity spectrum offset seasonal variations, and seemed to be characteristic of the station's taxocoenose. It tended asymptotically towards the diversity of the yearly cumulated sample (composite sample made up of all samples collected during a year at the same station). In most cases, the spectrum tended towards this asymptote at lower values. However, at some stations, where a single species greatly outnumbered the others, the diversity of the yearly composite sample was lower than that of each individual sample. Thus, for these latter stations, the curve of the spectrum tended towards its asymptote at upper values. Rankfrequency curves were also used to reveal the distribution of individuals among species in the yearly cumulated samples, and proved to be very characteristic of the different stations. According to Legendre and Legendre, the number of species is a function of the stability of the environment and the evenness of the species' distribution is inversely proportional to the biological activity of the environment. A pattern of regional variations is proposed from the interpretation of the yearly averaged or cumulated indexes (H, S, H/H _max) at the different stations. Open-sea and mid-lagoon populations have almost equivalent diversities, but these diversities are not attained in the same manner. The open-sea population benefits from a stable (highly predictable) environment and has a great number of species; high interspecific competition, due to the scarcity of food, leads to a moderate evenness. In the less stable environment of the mid-lagoon, the number of available niches is lower, resulting in less species, but the greater abundance of food induces less interspecific competition and, consequently, high occupation rates of the niches available, i.e., high evenness. At the boundaries of its biotope, the mid-lagoon population suffers two kinds of stress: terrestrial run-off near the cost and turbulence from breakers, and predation by plankton-feeders around the barrier reef. These stresses decrease both species richness and evenness. Elimination of most species results in the dominance of Acartia amboinensis in a deep, fjordlike river mouth, and of A. australis in the reef areas. The mixing of lagoon and open-sea plankton in the vicinity of the channel is reflected by an inflection point in the frequency-rank curves. The outcome of these exchanges is probably an exportation of plankton from the lagoon towards the open sea. This may be viewed as an exploitation of the lagoon ecosystem by the open-sea ecosystem, inhibiting a full maturation of the former.