AME 78:187-200 (2017)  -  DOI: https://doi.org/10.3354/ame01814

ABSTRACT: Phylogenetically diverse pico- (cells <2 µm diameter) and nanoplanktonic (2–20 µm) microbial eukaryotes are ubiquitous in coastal ecosystems; however, there is little understanding of the intrinsic (biological) and extrinsic (abiotic) factors that influence species composition in these communities. We investigated the microbial eukaryotes in the Havre-aux-Maisons Lagoon (Magdalen Islands, Gulf of St. Lawrence), which lacks riverine inputs and has little exchange with the surrounding gulf. We hypothesized that intrinsic successional processes and nutrient drawdown in the lagoon would lead to a decrease in average cell size over the summer. Samples for nutrients, cell concentrations from flow cytometry (FCM), and taxonomic identity using high throughput amplicon sequencing of the V4 region of the 18S rRNA gene were collected on 10 occasions between June and October 2009 and analysed in the context of physical and climatological data. Ratios of pico- to nanophytoplankton indicated no decline in average cell size over the summer, with picophytoplankton concentrations more variable. The microbial eukaryotic communities formed 3 major clusters based on phylogeny and UniFrac analysis. Taxonomically, dinoflagellates were dominant in the largest cluster, while picophytoplankton and nanoflagellates dominated the 2 other clusters. The concentrations of eukaryotic picophytoplankton increased in early summer, but decreased following periods with lower average wind speeds and increased following higher average winds. During periods with lower average wind speeds, dinoflagellates dominated. The clear impact of wind was consistent with climatological events indirectly influencing biotic interactions and picophytoplankton species composition.

KEY WORDS: Dinoflagellates · Coastal processes · Microbial community selection · 18S rRNA gene