Zooplankton of the Kara Sea in early spring: Abundance, demography, feeding and reproduction of dominant copepods Calanus spp. and Metridia longa in relation to ice retreat

Most dramatic environmental changes in the Arctic seas occur in spring, with timing of ice melt, increase of water temperature and food concentration decisive for the reproduction and recruitment success of the large copepods. In the Kara Sea, most of the studies on zooplankton community were performed at the end of the productive season. The main goal of this study is to gain knowledge on distribution, demography, grazing and reproduction of the dominant zooplankton species in the Kara Sea in early spring soon after ice retreat. Zooplankton were collected, feeding and reproduction rates studied along the retreating ice edge over the Kara Sea shelf and in the St. Anna Trough from 22 to June 30, 2021. All studied copepod species, despite different overwintering and life cycle strategies, responded similarly to the timing of ice retreat. An earlier ice melt led to higher abundance and an advanced demographical structure of the populations of dominant copepods, Calanus glacialis, C. finmarchicus, C. hyperboreus and Metridia longa. Egg production of C. glacialis and C. finmarchicus was high in the areas where the ice has recently melted. Spawning of C. finmarchicus females and presence of early developmental stages over the Kara Sea shelf suggests successful local recruitment of this basically boreal species. Daily rations of three Calanus species were high irrespective of phytoplankton concentration which indicates considerable exploitation of alternative food sources during spring post-bloom situations. Fairly low grazing impact on phytoplankton suggests that copepods community does not control spring development of phytoplankton and most part of the newly produced organic matter would be available for benthos.