SEASONAL ABUNDANCE OF COPEPOD ASSEMBLAGES AND GRAZING PRESSURE IN THEKERGUELEN ISLAND AREA (SOUTHERN-OCEAN)

A long-term survey of the monthly variations of the copepod assemblage s was carried out off Kerguelen Island (Kerfix Station) from February 1992 through to January 1995. Copepods were clearly dominant in the me sozooplankton (>90% of the total), with population abundances ranging from <100 individuals m(-3) in winter to 700 individuals m(-3) in summ er. Calanus simillimus, Rhincalanus gigas, Metridia lucens, Oithona fr igida, Oithona similis and Calanoides acutus were the most abundant sp ecies. From mid-1993, incubation experiments were performed with copep ods in order to estimate the grazing pressure on the phytoplankton sta nding stock. In addition, estimations of their metabolic cost (oxygen comsumption) were also made. All experimental measurements were made u sing four size groups (Gr) defined by the cephalothorax length of the copepods. These are: Gr I, 0.2-1.1 mm; Gr II, 1.2-2.5 mm; Gr III, 2.6- 3.3 mm; Gr IV, 3.7-6.5 mm. The average grazing rates ranged between 0. 7 and 2.2 ng of chlorophyll (Chl) a copepod(-1) day(-1), according to the size class of the individuals. For the whole copepod assemblage, s easonal 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 specie s, not to apparent intrinsic seasonal trends in their physiological ra tes. The average ingestion reached a maximum level of 356 ng Chi a day -(1) m(-3) (or 38 mu g C day(-1) m(-3)), representing 0.12% of the mea n phytoplankton standing stock. In the summer period, up to 2% of the primary production was consumed, but the carbon needs, expressed by th e respiratory metabolism, showed a slight discrepancy with the contrib ution of phytoplankton: the carbon required for the resting metabolism alone (94 mu g C day(-1) m(-3)) representing similar to 0.31% of the algal standing stock. In summer, this metabolism could have been equiv alent to 13% of the primary production. These results strongly suggest that the food of the mesozooplanktonic copepods off the Kerguelen Isl ands must include a large proportion of non-phytoplankton material.