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

This paper deals with the variations on feeding activities and diel migrations 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 dissolved oxygen taken each 12 hours showed a stratification of the water column in two distinct layers: (1) a superficial layer with higher temperature, moderate salinity, and high oxygen concentrations; (2) a colder, more saline and almost anoxic deep layer. In situ chlorophyll a measurements were made at -1, -3 and -5 m; concentrations were relatively homogeneous 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 out clearly a nocturnal migration, with individuals concentrating in both superficial layers; thus an unimodal migratory pattern can be inferred. Gut fluorescence measured following the Mackas and Bohrer technique (1976), showed higher values during night time, and values for females were the highest with wider day-night variations. Similar results were found in laboratory experiments with copepods fed with a culture of Dunaliella tertiolecta. Gut evacuation rate was measured in two laboratory experiments either mixing or separating males and females. Evacuation rate was 17.92 and 27.25 min for males and females respectively. Phytoplankton daily ration for A. tonsa calculated by gut fluorescence and gut evacuation rate was particularly low, for it represents only 10% of the individual carbon weight. Moreover, grazing impact on phytoplankton is very restricted, it represents less than 1 % of the daily phytoplankton stock.