Changes in lipid composition of copepods and Euphausia superba associated with diet and environmental conditions in the marginal ice zone, Bellingshausen Sea, Antarctica

The effect of varying diet and environmental conditions at the Marginal Ice Zone (MIZ) on the fatty acid and hydrocarbon compositions of five species of copepod and krill, Euphausia superba, was investigated. Zooplankton at the MIZ experienced a range of conditions, from a low algal biomass (mainly flagellates) under pack-ice to a spring bloom dominated by diatoms in the open ocean. Principal Component Analysis classified the copepods into three dietary regimes: (i) omnivores or general algal feeders under the pack ice, (ii) dinoflagellate feeders, and (iii) diatom feeders in the open ocean. This classification was supported by the distribution of the diatom marker n-heneicosahexaene (n-C21:6) and a general indicator of herbivory, the isoprenoid pristane. The fatty acid and hydrocarbon composition reflected dietary preferences and availability as the season progressed. Of the copepods under the pack-ice, Oithona spp. was omnivorous whereas Calanus propinquus was feeding preferentially on flagellates. Metridia gerlachei fed on flagellates in all conditions, but also included diatoms in its diet during the bloom. Calanoides acutus and Rhincalanus gigas, which passed the winter in diapause, were feeding almost exclusively on diatoms in the open ocean. Euphausia superba, which were also mainly diatom feeders in the open ocean, were feeding on the sea-ice algae (diatoms) and suspended material from the water column (dinoflagellates) under the pack-ice.