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JOURNAL OF PLANKTON RESEARCH | VOLUME 20 | NUMBER 8 | PAGES 1599-1614 | 1998
© Oxford University Press
research-article |
Seasonal abundance of copepod assemblages and grazing pressure in the Kerguelen Island area (Southern Ocean)
OOB, UMR7621, Laboratoire Arago 66 650 Banyuls/mer 1UMR 39, OMP, 14 Ave E.Belin, 31400 Toulouse, France
Received on May 30, 1997; accepted on April 15, 1998 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 m–3 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, 1.2–2.5 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, seasonal changes in the estimated grazing rate (27–1299 ng of Chl 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 Chl a day–1 m–3 (or 38 µg C day–1 m–3), 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 phytoplankton: the carbon required for the resting metabolism alone (94 µg C day–1 m–3 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.
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