Effects of increased atmospheric COsub2/sub on small and intermediate sized osmotrophs during a nutrient induced phytoplankton bloom

摘要

We report the transient population dynamic response of the osmotrophiccommunity initiated by a nutrient pulse in mesocosms exposed to differentpCO₂ levels. Differences in phytoplankton and heterotrophic bacteriaabundances associated with the CO₂ treatment are also described.Coastal seawater was enclosed in floating mesocosms (27 m3) andnutrients were supplied initially in order to stimulate growth of microbialorganisms, including the coccolitophorid Emiliania huxleyi. The mesocosms were modified toachieve 350 μatm (1×CO₂), 700 μatm (2×CO₂) and 1050 μatm (3×CO₂)CO₂ pressure. The temporal dynamics was related tonutrient conditions in the enclosures. Numerically small osmotrophs(picoeukaryotes and Synechoccocus sp.) dominated initially and towards the end of theexperiment, whereas intermediate sized osmotrophs bloomed as the initialbloom of small sized osmotrophs ceased. Maximum concentrations of E. huxleyi wereapproximately 4.6×103 cells ml−1 whereas other intermediatesized osmotrophs reached approximately twice as high concentrations. Theosmotrophic succession pattern did not change, and neither were we able todetect differences with regard to presence or absence of specificosmotrophic taxa as a consequence of altered pCO₂. Towards the end ofthe experiment we did, however, record significantly higher picoeukaryotic-and lower Synechococcus-abundances in the higher CO₂ treatments. Slightly increasedcell concentrations of E. huxleyi and other nanoeukaryotes were also recorded atelevated pCO₂ on certain days.