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A long-term survey of the monthly variations of the copepod assemblages was carried out off Kerguelen Island (Kerfix Station) from February 1992 through to January 1995. Copepods were clearly dominant in the mesozooplankton (>90% of the total), with population abundances ranging from <100 individuals m~3 in winter to 700 individuals nr3 in summer. Calanus simillimus, Rhincalanus gigas, Metridia lucens, Oithona frigida, Oithona similis and Calanoides acutus were the most abundant species. From mid-1993, incubation experiments were performed with copepods in order to estimate the grazing pressure on the phytoplankton standing stock. In addition, estimations of their metabolic cost (oxygen comsumption) were also made. All experimental measurements were made using four size groups (Gr) defined by the cephalothorax length of the copepods. These are: Gr I, 0.2-1.1 mm; Gr II, 1.2-23 mm; Gr III, 2.6-33 mm; Gr IV, 3.7-63 mm. The average grazing rates ranged between 0.7 and 2.2 ng of chlorophyll (Chi) a cope pod"1 day1, according to the size class of the individuals. For the whole copepod assemblage, seasonal changes in the estimated grazing rate (27-1299 ng of Chi a m~3 day"1) were due to changes in the abundance of the copepod species, not to apparent intrinsic seasonal trends in their physiological rates. The average ingestion reached a maximum level of 356 ng Chi a day1 m~' (or 38 ug C day1 nr3), representing 0.12% of the mean phytoplankton standing stock. In the summer period, up to 2% of the primary production was consumed, but the carbon needs, expressed by the respiratory metabolism, showed a slight discrepancy with the contribution of phyto- plankton: the carbon required for the resting metabolism alone (94 ug C day1 nr3) representing -0.31% of the algal standing stock. In summer, this metabolism could have been equivalent to 13% of the primary production. These results strongly suggest that the food of the mesozooplanktonic copepods off the Kerguelen Islands must include a large proportion of non-phytoplankton material.

Seasonal abundance of copepod assemblages and grazing pressure in the Kerguelen Island area (Southern Ocean)