Abstract
The North Atlantic phytoplankton spring bloom is the pinnacle in an annual cycle that is driven by physical, chemical, and biological seasonality. Despite its important contributions to the global carbon cycle, transitions in plankton community composition between the winter and spring have been scarcely examined in the North Atlantic. Phytoplankton composition in early winter was compared with latitudinal transects that captured the subsequent spring bloom climax. Amplicon sequence variants (ASVs), imaging flow cytometry, and flow-cytometry provided a synoptic view of phytoplankton diversity. Phytoplankton communities were not uniform across the sites studied, but rather mapped with apparent fidelity onto subpolar- and subtropical-influenced water masses of the North Atlantic. At most stations, cells < 20-µm diameter were the main contributors to phytoplankton biomass. Winter phytoplankton communities were dominated by cyanobacteria and pico-phytoeukaryotes. These transitioned to more diverse and dynamic spring communities in which pico- and nano-phytoeukaryotes, including many prasinophyte algae, dominated. Diatoms, which are often assumed to be the dominant phytoplankton in blooms, were contributors but not the major component of biomass. We show that diverse, small phytoplankton taxa are unexpectedly common in the western North Atlantic and that regional influences play a large role in modulating community transitions during the seasonal progression of blooms.
Highlights
Spring phytoplankton blooms in high-latitude oceanic regions are among the most-prominent natural events in the global ocean and have a profound impact on geochemical cycles [1, 2]
These findings illustrate the dynamism of North Atlantic hydrography and the importance of transport as a factor contributing to phytoplankton community structure
A synoptic view of phytoplankton diversity emerged from the multiple technologies co-deployed on North Atlantic Aerosols and Marine Ecosystems Study (NAAMES)
Summary
Spring phytoplankton blooms in high-latitude oceanic regions are among the most-prominent natural events in the global ocean and have a profound impact on geochemical cycles [1, 2]. A succession of coccolithophores, dinoflagellates, and pico-phytoplankton typically is expected to follow the diatom peak [8, 9]. Polar and tropical regions interact hydrographically while harboring distinctive phytoplankton communities [14] and a large westward gradient in eddy kinetic energy drives longitudinal heterogeneity by stirring and distorting the planktonic environment [15]. This creates a complex ecological landscape of dispersal and biological interactions [16] which is considered highly climate sensitive [17]
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