The present study addresses the ecology of two dominant copepod species in the Bay of Morbihan, Kerguelen Archipelago. The biomass of the herbivore Drepanopus pectinatus (from 2 mg m)3 in winter up to 500 mg m)3 in summer) is tightly coupled to seasonal changes in chlorophyll a concentration in the region, whereas the biomass of the predatory euchaetiid Paraeuchaeta antarctica increases during two distinct periods over the year: 250 mg m)3 in early summer, with the recruitment of the annual generation, and 100 mg m)3 in autumn, with the deposition of lipids as energy reserves in C5 stages and adults. The juvenile growth rates predicted by temperature-dependent models (0.09 day)1) closely approximate those observed in D. pectinatusin summer, but are much greater than those observed in P. antarctica (from 0.001 to 0.04 day)1 depending on developmental stages). This difference can be explained by the reproductive strategies and trophic positions of the two species and may also result from the dependence of larval growth on energy reserves in P. antarctica. The production rates are five- and tenfold greater in juvenile stages than in adults, respectively, for D. pectinatus and P. antarctica. The secondary production by D. pectinatusis insufficient to support P. antarctica during winter, when the predatory species probably shifts to alternate prey. In summer the predation by P. antarctica accounts for only a minor part of the mortality estimated for D. pectinatus (from 20% to 60% depending on the examined station). At two of the three stations examined in the Bay of Morbihan, the production of P. antarctica could potentially support the dietary requirements of planktivorous seabirds in the region (2,000 kg prey day)1 for common diving petrels, Pelecanoides urinatrix, and 90 kg prey day)1 for rockhopper penguins, Eudyptes chysocome filholi).