Spring phytoplankton bloom dynamics in Norwegian coastal waters: Microbial community succession and diversity

Most studies of spring bloom succession in Norwegian waters have employed light microscopy and accounted for species composition of phyto- and zooplankton. Flow cytometry and molecular tools enable us to extend such investigations to include smaller organisms like bacterio- and virioplankton. Here, we describe succession and diversity of algae, bacteria, and viruses in relation to environmental changes from 15 February to 27 April. The spring succession started with an increase in autotrophic picoeukaryotes and Synechococcus sp. The diatoms bloomed around the middle of March and caused nutrient depletion in the upper part of the water column. Upwelling in the beginning of April gave rise to a second bloom, consisting of diatoms and Phaeocystis pouchetii. Numerically, autotrophic picoeukaryotes and Synechococcus sp. dominated the periods between and after these two major blooms. Heterotrophic bacterial abundance increased throughout the experimental period and reached peak values during and after phytoplankton blooms. These bacteria were succeeded by viruses having low DNA fluorescence, whereas viruses with medium DNA fluorescence bloomed during or after blooms of autotrophic picoeukaryotes. High-DNA fluorescence viruses reached maximum concentrations during and after the diatom and Phaeocystis blooms. The diversity of the bacterial community remained relatively stable, whereas viral diversity varied more and increased after major phytoplankton blooms. Our investigation thus demonstrates how virioplankton are important elements of the total microbial diversity and how they are intimately linked to the rest of the microbial community and possibly act as an internal driving force in spring bloom successions.