Winter-spring transition in the subarctic Atlantic: microbial response to deep mixing and pre-bloom production

Abstract

In temperate, subpolar and polar marine systems, the classical perception is that diatoms initiate the spring bloom and thereby mark the beginning of the productive season. Contrary to this view, we document an active microbial food web dominated by pico-and nanoplankton prior to the diatom bloom, a period with excess nutrients and deep convection of the water column. During repeated visits to stations in the deep Iceland and Norwegian basins and the shallow Shetland Shelf (26 March to 29 April 2012), we investigated the succession and dynamics of photosynthetic and heterotrophic microorganisms. We observed that the early phytoplankton production was followed by a decrease in the carbon:nitrogen ratio of the dissolved organic matter in the deep mixed stations, an increase in heterotrophic prokaryote (bacteria) abundance and activity (indicated by the high nucleic acid:low nucleic acid bacteria ratio), and an increase in abundance and size of heterotrophic protists. The major chl a contribution in the early winter−spring transition was found in the fraction <10 µm, i.e. dominated by pico-and small nanophytoplankton. The relative abundance of picophytoplankton decreased towards the end of the cruise at all stations despite nutrient-replete conditions and increasing day length. This decrease is hypothesised to be the result of top-down control by the fast-growing population of heterotrophic protists. As a result, the subsequent succession and nutrient depletion can be left to larger phytoplankton resistant to small grazers. Further, we observed that large phytoplankton (chl a > 50 µm) were stimulated by deep mixing later in the period, while picophytoplankton were unaffected by mixing; both physical and biological reasons for this development are discussed herein.

The Arctic picoeukaryote &lt;i&gt;Micromonas pusilla&lt;/i&gt; benefits synergistically from warming and ocean acidification

“…Thus, our findings suggest higher picoplankton contribution to future Arctic phytoplankton assemblages under non-limiting conditions, e.g. early in the growing season when picoeukaryotes can already contribute quite substantially to the phytoplankton standing stocks (Marquardt et al, 2016;Paulsen et al, 2015). How such competition between diatoms and picoeukaryotes would manifest under nutrient-depleted conditions that strongly favour M. pusilla is currently unknown.…”

“…Even though picoeukaryotes seem to contribute more to the downward export of organic matter than previously assumed (Waite et al, 2000;Richardson and Jackson, 2007), in comparison to diatoms for example, they are less efficient vectors for carbon export to depth and have a lower energy transfer along trophic levels (Sherr et al, 2003). Consequently, Arctic food webs dominated by picoeukaryotes would look very different from those fuelled by diatom production (Sherr et al, 2003;Paulsen et al, 2015). Due to its motility and capability to grow mixotrophically, M. pusilla is characterized by an exceptionally high cellular C : N ratio compared to other Arctic phytoplankton (Table 2; Halsey et al, 2014;McKie-Krisberg and Sanders, 2014).…”

High export via small particles before the onset of the North Atlantic spring bloom

“…Details of the microbial community and microzooplankton grazing during our cruise are discussed, respectively, by Paulsen et al . [] and Morison and Menden‐Deuer [].…”

Heterotrophic nanoflagellate grazing facilitates subarctic Atlantic spring bloom development

“…Due to their small size (1.1 ± 0.4 μm diameter in the subarctic Atlantic; Paulsen et al, 2015), Synechococcus cells are largely grazed by HNF and microzooplankton (Christaki et al, 1999. This implies that their biomass production will be largely recycled in the microbial food web and thus be of minor contribution to higher trophic levels in the grazing food web.…”