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.

北大西洋浮游植物春季开花是由物理、化学和生物季节性驱动的年度循环中的顶峰。尽管它对全球碳循环做出了重要贡献，但在北大西洋几乎没有研究过冬季和春季之间浮游生物群落组成的转变。将初冬的浮游植物组成与捕捉随后春季开花高潮的纬度样带进行了比较。扩增子序列变体 (ASV)、成像流式细胞术和流式细胞术提供了浮游植物多样性的概要视图。浮游植物群落在所研究的地点并不统一，而是以明显的保真度映射到受亚极地和亚热带影响的北大西洋水团上。在大多数车站，小区 < 20 微米直径是浮游植物生物量的主要贡献者。冬季浮游植物群落以蓝细菌和微型植物真核生物为主。这些转变为更加多样化和充满活力的春季群落，其中包括许多 prasinophyte 藻类在内的微型和纳米植物真核生物占主导地位。硅藻通常被认为是水华中的主要浮游植物，它们是贡献者，但不是生物量的主要成分。我们表明，多样化的小型浮游植物类群在北大西洋西部出人意料地普遍，并且区域影响在调节开花季节性进展期间的群落过渡方面发挥了重要作用。这些转变为更加多样化和充满活力的春季群落，其中包括许多 prasinophyte 藻类在内的微型和纳米植物真核生物占主导地位。硅藻通常被认为是水华中的主要浮游植物，它们是贡献者，但不是生物量的主要成分。我们表明，多样化的小型浮游植物类群在北大西洋西部出人意料地普遍，并且区域影响在调节开花季节性进展期间的群落过渡方面发挥了重要作用。这些转变为更加多样化和充满活力的春季群落，其中包括许多 prasinophyte 藻类在内的微型和纳米植物真核生物占主导地位。硅藻通常被认为是水华中的主要浮游植物，它们是贡献者，但不是生物量的主要成分。我们表明，多样化的小型浮游植物类群在北大西洋西部出人意料地普遍，并且区域影响在调节开花季节性进展期间的群落过渡方面发挥了重要作用。