Zooplankton was investigated at fixed site in 24 hours in Kongsfjorden, a glacial fjord situated on the west coast of Spitsbergen (Svalbard) (79°N, 12°E), in order to unveil the level of diurnal variability in community composition and abundance. Parallel to zooplankton study water temperature and salinity were measured while information on local tides and winds was obtained from external sources. Observed changes did not exceed the range of variability regarded intrinsic, resulting from the nature of plankton. Because of this low variability we are of the opinion that the data presented can be regarded a valid measure of the natural heterogeneity of zooplankton communities in hydrologically dynamic Arctic coastal waters in summer. The observed changes in zooplankton were primarily induced by the complex dynamics of the fjord’s water masses. In spite of importance of tidal forcing, the variability in zooplankton did not demonstrate similar temporal fluctuations due to modification of the water movement by other irregular forces (local wind). Also, we have not found any indication of diel vertical migration in coastal water in the Arctic under the condition of midnight sun.

The zooplankton community structure was studied in the Svalbard area at three shelf stations: Billefjorden, Kongsfjorden and Hinlopen (Spitsbergen shelf area), and at two open water stations: Ice West and Ice East (north of Spitsbergen, in the Arctic Ocean). Two different plankton nets – WP-2 and WP-3 – were used to collect a size range of zooplankton. The Bray-Curtis similarity analysis showed differences between sampling stations based on total zooplankton abundance, species composition, and comparison of Calanus spp. development. Total abundance was the highest in Kongsfjorden and Hinlopen. The small omnivorous copepod Oithona similis Claus, 1863 was the dominating species at all localities and the Atlantic copepod Calanus finmarchicus (Gunnerus, 1765) was found at all stations. Calanus spp. development was delayed at the ice stations when compared to the shelf stations. Results are discussed in relation to differences in environmental factors among stations.

Zooplankton community composition, abundance and biomass from two polar localities – Kongsfjorden (Arctic) and Admiralty Bay (Antarctic) is compared. The community composition of zooplankton in both polar regions included similar taxonomic groups and the diversity at the species level was similar. Even though the overall species composition was different, some species were common for both ecosystems, for example Oithona similis, Microcalanus pygmaeus or Eukrohnia hamata. The abundance and biomass of the main zooplankton components (Copepoda) differed greatly between the two ecosystems, both being of an order of magnitude higher in Kongsfjorden than in Admiralty Bay. Kongsfjorden is situated at the border of two regions what induces high productivity with copepods playing an important role, and there is also a strong advection into the fjord. Admiralty Bay is adjacent to the homogenous Antarctic oceanic ecosystem; some advection into the bay occurs as an effect of tide and wind driven processes. Antarctic krill, which was not included in the present study, occupies most of the primary consumers niche and replaces copepods at the second trophic level.