sized osmotrophs during a nutrient induced phytoplankton bloom

Abstract. We report the transient population dynamic re-sponse of the osmotrophic community initiated by a nu-trient pulse in mesocosms exposed to different pCO2 lev-els. Differences in phytoplankton and heterotrophic bacte-ria abundances associated with the CO2 treatment are also described. Coastal seawater was enclosed in floating meso-cosms (27 m3) and nutrients were supplied initially in or-der to stimulate growth of microbial organisms, including the coccolitophorid Emiliania huxleyi. The mesocosms were modified to achieve 350µatm (1×CO2), 700µatm (2×CO2) and 1050µatm (3×CO2) CO2 pressure. The temporal dy-namics was related to nutrient conditions in the enclosures. Numerically small osmotrophs (picoeukaryotes and Syne-choccocus sp.) dominated initially and towards the end of the experiment, whereas intermediate sized osmotrophs bloomed as the initial bloom of small sized osmotrophs ceased. Maximum concentrations of E. huxleyi were approx-imately 4.6×103 cells ml−1 whereas other intermediate sized osmotrophs reached approximately twice as high concentra-tions. The osmotrophic succession pattern did not change, and neither were we able to detect differences with regard to presence or absence of specific osmotrophic taxa as a conse-quence of altered pCO2. Towards the end of the experiment we did, however, record significantly higher picoeukaryotic-and lower Synechococcus-abundances in the higher CO2 treatments. Slightly increased cell concentrations of E. hux-leyi and other nanoeukaryotes were also recorded at elevated pCO2 on certain days