| Summary: | Monthly sampling was conducted in the ice-free Adventfjorden in West Spitsbergen, Svalbard, from March to November 2018 to assess the seasonal development in the Arctic zooplankton community and to investigate the seasonally shifting importance of meroplankton and holoplankton. Special focus was put on the reproduction strategy of three taxa of Copepoda: Calanus spp. Leach, 1816, Pseudocalanus spp. Boeck, 1872 and Oithona similis Claus, 1866. Additionally, three ice-covered fjords, Van Mijenfjorden in West Spitsbergen and Inglefieldbukta and Agardhbukta in East, were sampled in March and April to investigate the impact of sea ice on zooplankton species composition and abundance. Regarding the physical and biological environment in Adventfjorden, chlorophyll a was found in highest concentrations in May although the water column was still unstratified. Simultaneously with the high chlorophyll a concentrations, Cirripedia larvae (meroplankton) greatly dominated the zooplankton community. Throughout the remaining sampling period in Adventfjorden, holoplankton dominated. Of these, Copepoda was the most dominating order whereof Oithona similis was found to be the most numerically important copepod, followed by Calanus spp. and Pseudocalanus spp. The stage distribution of Calanus spp. suggested a dominance of individuals completing their life cycle within one year, with reproduction timed so as the nauplii could take advantage of the spring bloom in May. For Pseudocalanus spp. however, the stage distribution indicated a continuous reproduction with hatching occurring from the end of March until the end of August. Identification of the Pseudocalanus spp. by species-specific PCR and Illumina sequencing suggested a dominance of P. acuspes (Giesbrect, 1881) and P. moultoni Frost, 1989 amongst the reproducing population in Adventfjorden, while older stages of P. minutus (Krøyer, 1845) that were found might have been advected into the fjord. Abundance of small (<150 µm) Copepoda nauplii and presence of young Oithona spp. ...
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