| Summary: | The aim of this study was to investigate the microbial planktonic food web in the pre- bloom period of the Subpolar North Atlantic Ocean. Through repeated visits to the Icelandic Basin, Norwegian Basin, and on the Shetland Shelf in the period March 28th to May 1st, we recorded the abundance of all the functional groups of both autotrophic and heterotrophic microbes. At the first visits, all stations were characterized by low concentrations of chlorophyll a (0.1-0.5 µg l^{-1}) and a low abundance of heterotrophic bacteria (2-3.4 × 10^5 cells ml^{-1}), heterotrophic nanoflagellates (22-84 cells ml^{-1}), ciliates (1-2 cells ml^{-1}), and heterotrophic dinoflagellates (0.1-0.3 cells ml^{-1}) within the upper mixed layer. Following the abundance of heterotrophic protists generally increased; 2- fold for bacteria and up to 5-fold for heterotrophic nanoflagellates. An initial dominance of pico eukaryotes within the phytoplankton community was observed in late winter. This was followed, however, by a significant decrease during the pre-bloom period, despite high nutrient concentrations and increasing light intensity. The decrease of pico eukaryote was concurrent with an increase of heterotrophic nanoflagellates, hence grazing pressure. The microbial trophic interactions were analysed further via grazing experiments, performed with water sampled at the Icelandic Basin. These revealed heterotrophic nanoflagellate removal rates of 10-20 % of bacterial standing stock d^{-1} and as high as 30- 50 % of the standing stock of pico phytoplankton d^{-1} in the euphotic zone. We conclude that heterotrophic nanoflagellates in the pre-bloom can satisfy up to half of their carbon demand by herbivory, and thus the strong focus of heterotrophic nanoflagellates' role, as being mainly bacterivorous, should be revised. We document that the pre-bloom is a productive period with carbon entering the ocean food web largely via the microbial food web. Thus, not only the seasonal changes of physical condition, but also the microbial dynamics ...
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