| Summary: | Knowledge of the seasonal dynamics within the Arctic marine microbial network is scarce, particularly during the polar night period. The abundances of different microbial groups in Adventfjorden, Svalbard were investigated by the use of flow cytometry from weekly samples collected from November 2012 to June 2013. Quantitative reverse transcrip- tion PCR (RT-qPCR) assays were carried out to determine the amounts of of rbcL and 18S rRNA transcripts for two important autotrophic picoeukaryotes, Micromonas pusilla and Phaeocystis sp. from January to June 2013. With additional data from chlorophyll a measurements and hydrographic conditions, the dynamics of the microbial network were assessed during the sampling period. The results showed that both M. pusilla and Phaeocystis sp. existed in an active state, even during the dark period, may be an adaption to life in the Arctic marine environment, which is characterized by extreme annual light changes. The spring bloom dynamics followed a common pattern with dominance of small autotrophs before and after the main chlorophyll a peak, which was found to occur on April 24th. When the nutrients available were depleted, autotrophic pico- and nanoeukaryotes and Cryptophyceae were found to be the most efficient competitors. Bacteria and virus abundances increased after the peak of the spring bloom, but decreased quickly, probably due to grazing by heterotrophic nanoflagellates. A small population of Synechococcus was also found, which was thought to have been brought in with Atlantic water in the beginning of the sampling period. Increasing temperatures and changes in water masses as a result of climate change may alter the dynamics of the arctic marine microbial network in favor of other, more temperate species connected to Atlantic water. Shifts in the spring bloom dynamics has the potential to affect organisms in higher trophic levels which are adapted to the strong seasonality found in the Arctic environment. MAMN-MAR MAR399
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