Coexistence and competition within Acartia (Copepoda,Calanoida) congeners from Lebanese coastal water: niche overlap measurements

Annual succession and ecological niche formation among six dominant coexisting species of the genus Acartia were studied between 1979 and 1989. The coexisting species: A. discaudata, A. clausi, A. grani, A. italica, A. josephinae and A. latisetosa formed dense populations at different periods in Jounieh harbour (Central Lebanon, eastern Mediterranean). Using numerical data analysis, I defined and measured niche hyperspaces and niche overlaps of these species. Although both spatial and temporal segregation occurred between congeners, the amount of niche overlap was estimated in relation to the availability, use, and abundance of natural resources. The hydrobiological factors prevailing in the port were considered resource states. They include eight parameters: water temperature and salinity, dissolved oxygen, mineral phosphate, nitrate, chlorophyll-a concentration, plankton biomass, and species diversity of the plankton community. Some competition occurred between the co-existing species and an attempt is made to estimate this. As a result of this competition different ecological niches have developed for each species. Niche overlaps, their hyperspaces, and niche breadth between the congeners are estimated and computed using several indices, such as those based on Levin's and Lloyd's formulae. Two groups of Acartia congeners are defined: a group of spring affinity formed by A. clausi, A. discaudata, and A. grani and a second group involving A. italica, A. josephinae and A.-latisetosa, showing a spring-summer tendency. These coexisting species exhibited certain strategies in their opportunistic behaviours, in order to adapt to a highly variable marine environment, such as the one found in Jounieh port.