Temporal and spatial variation in stocks of autotrophic and heterotrophic microbes in the upper water column of the central Arctic Ocean

As part of the SHEBA/JOIS drift experiment, we continually analysed abundance and biomass of autotrophic and heterotrophic microbes in the upper 120 m of the water column of the ice-covered Central Arctic Ocean from November 1997 through August 1998. Microbial biomass was concentrated in the upper 60 m of the water column. There were low but persistent stocks of heterotrophic and autotrophic microbes during the winter months. Phytoplankton biomass began increasing when winter snow melted from the ice-pack in early June, after which there was a progressive decline of nitrate and silicate in the euphotic zone. We observed three distinct blooms over the summer. The initial bloom consisted of diatoms and phytoflagellates, mainly 2 μm-sized Micromonas sp.; the two subsequent blooms were dominated by the flagellated (non-colonial) Phaeocystis sp. The carbon:chlorophyll ratio of the phytoplankton was 31±11. Stocks of bacteria and heterotrophic protists approximately doubled during the growing season, increasing in tandem with increase in phytoplankton biomass. Increase in cell abundances of bacteria and of the phytoflagellate Micromonas over 40–50 d periods during the initial bloom period yielded estimates of realised growth rate of 0.025 d−1 for bacteria and of 0.11 d−1 for Micromonas. Heterotrophic protists included flagellates, ciliates, and dinoflagellates, with biomass divided nearly evenly between nanoplankton (Hnano, 0–20 μm) and microplankton (Hmicro, 20–200 μm) size classes.