Evolution d'une population de copépodes dans le système des courants équatoriaux de l'Océan Pacifique. Zoogéographie, écologie et diversité spécifique

During the cruise “Alizé” along the Equator in the Pacific Ocean from 86° 20 W to 151° 01 E (Fig.1), 33 night tows were made with a 1 m plankton net, in the 0 to 200 m layer. The samplings show, 3 types of copepod distribution (Figs. 2 to 5); the main type presents the well-documented decrease from east to west. The different distributions seem controlled by: suface phytoplankton abundance in the case of Eucalanus sublenuis temperature and the thermocline in Euchaeta media and Pleuromamma quadrungulata; type and availability of prey in Euchaeta marina; upwelling and the Peru current in Eucalanus elongatus hyalinus and Rhincalanus nasutus. The 3 warm-water species Rhincalanus cornutus, Eucalanus attenuata and Eucalanus subcrassus are more abundant in the western part of the Pacific Ocean, where temperature is higher. The 2 cosmopolitan species Pleuromamma xiphias and P. abdominalis do not show any clear relationship with the biotope. Probably the presence of some species in the upper water layer in the zone of intense divergence and upwelling emphasizes the westward decrease. It has been possible to define 4 zones by phytoplankton distribution, currents, vertical circulation, and the physical and chemical properties of the biotope; these zones agree with the zonation of the copepods in accordance with the results of a rank test. Within these limits, the 4 indices of specific diversity I=-Σ pi log₂ pi and the equitibility have been calculated. The diversity variations account for the ecosystem evolution after Margalef's theory. The lower specific diversity in the oriental zone is related to the well developed divergence, the high rates of nutrients, the intense phytoplankton production, the instability in time and the southern water inflow; in this zone, the ecosystem is in an early stage of succession with a percentage of herbivores such as Eucalanus subtenuis (45%) present. The diversity index confirms the changes introduced by the trade winds along the Equator, mainly in the eastern part of the Pacific Ocean.