Remarques sur la structuration des populations zooplanctoniques dans la zone de l'émissaire de Marseille-Cortiou

Abstract

The impact of a polluted water-layer on zooplanktonic populations was studied in the sewage discharge zone at Marseille-Cortiou throughout 1977 and a modification of specific diversity was observed: diversity was instable near the outfall, but increased slightly and became more stable further away in non-polluted stations. The population composition did not seem to change. Species rank varied slightly and the number of species increased with increasing distance from the outfall. Specific “dominance” remained relatively unchanged, and essentially consisted of only 5 species: Clausocalanus acuicornis, Evadne spinifera, Oithona helgolandica, Temora stylifera and Centropages typicus. The population structure, as illustrated by “Frontier” diagrams, was higher during or just preceding winter. Population organisation was the same close to or far away from the outfall, although the total population counts and number of species were smaller at the most polluted stations. The taxons exhibiting the largest decreases in density were: euphausids, nauplii, larvae of various brachyurans and gastropods, fish eggs, the pteropod Cavolinia inflexa, the copepod Calanus minor and several Coryceidae. The greatest disturbance of the population structure at all stations was observed during the summer. The composition of the zooplanktonic population seems to depend much more on seasonal factors than on the level of pollution. In was not possible to distinguish a cluster of species which were sufficiently dominant to be considered as really characteristic; perhaps the rather high exposure of the sewage site to winds and waves prevents the formation of a local facies.