SPACE AND TIME DISTRIBUTION OF ZOOPLANKTON IN A MEDITERRANEAN LAGOON (ETANG DE BERRE)

In the Berre lagoon, a large brackish and shallow area near Marseille, the environmental factors (temperature, salinity, oxygen, suspended p articulate matter and chlorophyll) generally display strong space and time variations. The rotifer Brachionus plicatilis and the copepod Aca rtia tonsa constitute the bulk of the zooplankton population during al l the year. Their space and time distributions were studied in 23 stat ions distributed all over the lagoon, during four seasonal cruises (Fe bruary, June, October, November), at surface and bottom layers. There is no marked difference in the horizontal and vertical distribution of the two species, (except in November when rotifers were prevailing in surface and copepods at depth) and in their time occurrence. When the four series of data are pooled, correlation analysis show that A. ton sa is positively correlated with temperature, salinity and seston and negatively to oxygen and chlorophyll. B. plicatilis is positively corr elated with temperature and seston, but also with chlorophyll, while s alinity has a negative effect. The specific eggs number of both specie s is chlorophyll dependent. Considering seasonal cruises separately, s ome differences appear in the sense or the significance of these diffe rent correlations. The respective distribution of the two species is o nly partly dependent on the variation of the environmental factors: mo st of the variance remains unexplained, as indicated by the result of a stepwise multiple regression analysis using the most significant fac tors (temperature, salinity and oxygen explain 33 to 42% of the varian ce in Acartia, while temperature and salinity explain 27 to 28% of the variance in Brachionus). Thus, internal behavioral factors could also play a role in the distribution nf organisms, particularly in some ca ses of aggregations of organisms observed during this study. As the tw o species occupied the same space habitat most of the year, they are p otentially in competition for food. A way to optimize the food utiliza tion could be the time separation of their feeding activity, nocturnal in Acartia and diurnal in Brachionus. Another way could be selective feeding upon food particles depending on their size (Brachionus being able to use finer particles than Acartia) or their quality (Brachionus being more herbivorous than Acartia) as demonstrated in some grazing experiments carried out in parallell.