Protist community composition during spring in an Arctic flaw lead polynya

The overwintering deployment of an icebreaker during the Canadian Flaw Lead study provided an opportunity to evaluate how protist communities (phytoplankton and other single-celled eukaryotes) respond to changing spring irradiance conditions in flaw lead polynyas, where open water persists between the central pack ice and land fast ice. We combined microscopic analysis of the protist communities (all cell sizes) with clone libraries of 18S rRNA genes and 18S rRNA (from RNA converted to cDNA) of size-fractionated seawater (0.2–3.0 μm) from 10 to 12 m depth in the surface mixed layer. The rRNA gene analysis provided information on the presence of organisms, while the rRNA analysis provided information on the most active members of the community. There was little overlap between the two types of clone libraries, and there were large community shifts over time. Heterotrophic dinoflagellates and ciliates were the most common sequences recovered. The relative proportion of photosynthetic protist sequences increased in March and April, and there was greater representation of Bacillariophyta, Prasinophyta, Haptophyta, and Cryptophyta in the rRNA compared to rRNA gene libraries. Microscopy indicated that large-celled diatoms dominated the community in May, when chlorophyll concentrations were greatest. However, the RNA sequencing showed that heterotrophic and putative parasitic protists were proportionately more active, and the concomitant decrease in nutrients suggested that the spring phytoplankton bloom had begun to decline by this time. These observations provide evidence of substantial changes in protist community structure and function during the spring transition.