FEEDING-BEHAVIOR AND MIGRATIONS IN A NATURAL-POPULATION OF THE COPEPOD ACARTIA-TONSA

This paper deals with the variations on feeding activities and diel mi grations of Acartia tonsa Dana, the dominant copepod species in Berre lagoon (west Mediterranean French coasts). A 27 hour in situ study was carried out during June 1989, at a station located in the south west of the lagoon. Vertical profiles of salinity, temperature and dissolve d oxygen taken each 12 hours showed a stratification of the water colu mn in two distinct layers: (1) a superficial layer with higher tempera ture, moderate salinity, and high oxygen concentrations; (2) a colder, more saline and almost anoxic deep layer. In situ chlorophyll a measu rements were made at -1, -3 and -5 m; concentrations were relatively h omogeneous through the water column during the whole sampling period. Zooplankton samples were collected every 3 hours with a 200 mu m mesh net, in three strata (0-2 m, 2-4,m, 4-6 m). A complete dominance of A. tonsa was observed in the zooplankton community. Our results point ou t clearly a nocturnal migration, with individuals concentrating in bot h superficial layers; thus an unimodal migratory pattern can be inferr ed. Gut fluorescence measured following the Mackas and Bohrer techniqu e (1976), showed higher values during night time, and values for femal es were the highest with wider day-night variations. Similar results w ere found in laboratory experiments with copepods fed with a culture o f Dunaliella tertiolecta. Gut evacuation rate was measured in two labo ratory experiments either mixing or separating males and females. Evac uation rate was 17.92 and 27.25 min for males and females respectively . Phytoplankton daily ration for A. tonsa calculated by gut fluorescen ce and gut evacuation rate was particularly low, for it represents onl y 10% of the individual carbon weight. Moreover, grazing impact on phy toplankton is very restricted, it represents less than 1 % of the dail y phytoplankton stock.