Phytoplankton dynamics in contrasting early stage North Atlantic spring blooms: composition, succession, and potential drivers

摘要

The spring bloom is a key annual event in the phenology of pelagicecosystems, making a major contribution to the oceanic biological carbonpump through the production and export of organic carbon. However, there islittle consensus as to the main drivers of spring bloom formation,exacerbated by a lack of in situ observations of the phytoplankton communitycomposition and its evolution during this critical period.We investigated the dynamics of the phytoplankton community structure at twocontrasting sites in the Iceland and Norwegian basins during the early stage(25 March–25 April) of the 2012 North Atlantic spring bloom. The planktoncomposition and characteristics of the initial stages of the bloom weremarkedly different between the two basins. The Iceland Basin (ICB) appearedwell mixed down to >400 m, yet surface chlorophyll a(0.27–2.2 mg m?3) and primary production(0.06–0.66 mmol C m?3 d?1) were elevated in the upper 100 m.Although the Norwegian Basin (NWB) had a persistently shallower mixed layer(<100 m), chlorophyll a (0.58–0.93 mg m?3) and primaryproduction (0.08–0.15 mmol C m?3 d?1) remained lower than inthe ICB, with picoplankton (<2 μm) dominating chlorophyll abiomass. The ICB phytoplankton composition appeared primarily driven by thephysicochemical environment, with periodic events of increased mixingrestricting further increases in biomass. In contrast, the NWB phytoplanktoncommunity was potentially limited by physicochemical and/or biologicalfactors such as grazing.Diatoms dominated the ICB, with the genus Chaetoceros(1–166 cells mL?1) being succeeded by Pseudo-nitzschia(0.2–210 cells mL?1). However, large diatoms (>10 μm)were virtually absent (<0.5 cells mL?1) from the NWB, with onlysmall nano-sized (<5 μm) diatoms (i.e. Minidiscusspp.) present (101–600 cells mL?1). We suggest microzooplanktongrazing, potentially coupled with the lack of a seed population of bloom-forming diatoms, was restricting diatom growth in the NWB, and that largediatoms may be absent in NWB spring blooms. Despite both phytoplanktoncommunities being in the early stages of bloom formation, differentphysicochemical and biological factors controlled bloom formation at the twosites. If these differences in phytoplankton composition persist, thesubsequent spring blooms are likely to be significantly different in terms ofbiogeochemistry and trophic interactions throughout the growth season, withimportant implications for carbon cycling and organic matter export.