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The Northern California Current (NCC) system is a productive coastal ecosystem with a mosaic of temporal and spatial features. The phytoplankton community plays a crucial role in supporting the rich ecosystem and economically important fisheries of the NCC. Our study integrates data across two years (2022-2023) and multiple transects to investigate the community composition of two major phytoplankton groups in the NCC: picocyanobacteria and photosynthetic picoeukaryotes (PPE). The abundances and cell sizes of the phytoplankton were measured using flow cytometry. We found PPE present at similar concentrations in both summer and winter, while picocyanobacteria were much more abundant in the summer than the winter. The relationship between the picocyanobacteria and PPE varied across on- to off-shore transects with different coastal bathymetry. Abundances of both picophytoplankton increased with distance from shore. Cell size also varied along these gradients. Sampling during a marine heatwave in summer 2023 revealed a shift towards smaller picophytoplankton. Overall, these data reveal a dynamic microbial community underlying a productive coastal system, which could inform management decisions and future ecosystem models in the context of climate change and marine heat waves. 

Abstract The Laurentian Great Lakes provide economic support to millions of people, drive biogeochemical cycling, and are an important natural laboratory for characterizing the fundamental components of aquatic ecosystems. Small phytoplankton are important contributors to the food web in much of the Laurentian Great Lakes. Here, for the first time, we reveal and quantify eight phenotypically distinct picophytoplankton populations across the Lakes using a multilaser flow cytometry approach, which distinguishes cells based on their pigment phenotype. The distributions and diversity of picophytoplankton flow populations varied across lakes and depths, with Lake Erie standing out with the highest diversity. By sequencing sorted cells, we identified several distinct lineages ofSynechococcalesspanning Subclusters 5.2 and 5.3. Distinct genotypic clusters mapped to phenotypically similar flow populations, suggesting that there may not be a clear one‐to‐one mapping between genotypes and phenotypes. This suggests genome‐level differentiation between lakes but some degree of phenotypic convergence in pigment characteristics. Our results demonstrate that ecological selection for locally adapted populations may outpace homogenization by physical transport in this interconnected system. Given the reliance of the Lakes on in situ primary production as a source for organic carbon, this work sets the foundation to test how the community structure of small primary producers corresponds to biogeochemical and food web functions of the Great Lakes and other freshwater systems. 

Abstract Pyrosomes are widely distributed pelagic tunicates that have the potential to reshape marine food webs when they bloom. However, their grazing preferences and interactions with the background microbial community are poorly understood. This is the first study of the marine microorganisms associated with pyrosomes undertaken to improve the understanding of pyrosome biology, the impact of pyrosome blooms on marine microbial systems, and microbial symbioses with marine animals. The diversity, relative abundance, and taxonomy of pyrosome-associated microorganisms were compared to seawater during a Pyrosoma atlanticum bloom in the Northern California Current System using high-throughput sequencing of the 16S rRNA gene, microscopy, and flow cytometry. We found that pyrosomes harbor a microbiome distinct from the surrounding seawater, which was dominated by a few novel taxa. In addition to the dominant taxa, numerous more rare pyrosome-specific microbial taxa were recovered. Multiple bioluminescent taxa were present in pyrosomes, which may be a source of the iconic pyrosome luminescence. We also discovered free-living marine microorganisms in association with pyrosomes, suggesting that pyrosome feeding impacts all microbial size classes but preferentially removes larger eukaryotic taxa. This study demonstrates that microbial symbionts and microbial prey are central to pyrosome biology. In addition to pyrosome impacts on higher trophic level marine food webs, the work suggests that pyrosomes also alter marine food webs at the microbial level through feeding and seeding of the marine microbial communities with their symbionts. Future efforts to predict pyrosome blooms, and account for their ecosystem impacts, should consider pyrosome interactions with marine microbial communities.