A Chaetoceros bloom in the Western North Atlantic.

This article focuses on the composition and dynamics of the North Atlantic Spring Bloom in the Western subpolar North Atlantic during May 2010, analyzed through phylogenetic read placement of 18S rRNA gene metagenomes from the Geotraces GA02 transect. The study identified a bloom dominated by Chaetoceros diatoms, particularly related to the Phaeoceros group, alongside significant Chlorophyta (green algae) presence, contrasting with the 2016 NAAMES study that found picoeukaryotic Chlorophyta dominance without a diatom bloom. The phytoplankton community varied notably between stations, with diatoms prevalent in northern, nutrient-rich waters and coccolithophores and picocyanobacteria more abundant further south. Additionally, diverse microzooplankton groups—including ciliates, dinoflagellates, Cercozoa, Amoebozoa, Euglenozoa, and symbiotic Radiolarians such as Acantharea and Collodaria—were detected, highlighting complex trophic interactions often underrepresented in traditional microscopy. The findings suggest spatial and temporal variability in spring bloom composition and underscore the need for continued, detailed sampling to understand potential regime shifts in the Western North Atlantic ecosystem.