| Summary: | During three cruises in April, May and November 2014 in Porsangerfjord, northern Norway, the role of hydrography and phytoplankton composition in shaping zooplankton vertical distribution was studied. Simultaneous collection of biological (phytoplankton and zooplankton distribution/abundance) and environmental data (temperature, salinity, density, Chlorophyll a) was sampled using a high-resolution autonomous VPR. Phytoplankton species composition was also determined using cell counts. Two stations representing two different hydrographic conditions were sampled, one in the inner part and the other in the outer part of the fjord. The phytoplankton bloom started in May. The outer part was dominated by Phaeocystis pouchetii and the inner part by diatoms (Chaetoceros spp.). The inner part of the fjord was dominated by small copepods species like Pseudocalanus spp., Microsetella norvegica, and Oithona spp. whereas the outer part was dominated by large copepods like Calanus finmarchicus. Zooplankton distribution changed over season, in early bloom they were spread over the water column, during the bloom they were linked to phytoplankton vertical distribution and in the winter situation they were at depths. Hydrography was not the only factor responsible for herbivorous zooplankton distribution. Herbivorous zooplankton was affected by phytoplankton species composition. C. finmarchicus and Pseudocalanus spp. avoided the dense layer of P. pouchetii while M. norvegica was observed grazing on P. pouchetii. Zooplankton vertical distribution was therefore linked to both abiotic (hydrography) and biotic (phytoplankton composition and distribution) factors which varied in importance throughout the season.
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