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Hourly, year‐round flow cytometry has made it possible to relate seasonal environmental variability to the population dynamics of the smallest, most abundant phytoplankton on the Northeast US Shelf. To evaluate whether the insights from these data extend to Synechococcus farther from shore, we analyze flow cytometry measurements made continuously from the underway systems on 21 cruises traveling between the Martha's Vineyard Coastal Observatory (MVCO) and the continental shelf break. We describe how seasonal patterns in Synechococcus, which have been documented in detail at MVCO, occur across the region with subtle variation. We find that the underlying relationship between temperature and division rate is consistent across the shelf and can explain much of the observed spatial variability in concentration. Connecting individual cell properties to annual and regional patterns in environmental conditions, these results demonstrate the value of autonomous monitoring and create an improved picture of picophytoplankton dynamics within an economically important ecosystem. [ABSTRACT FROM AUTHOR]

Picophytoplankton are a ubiquitous component of marine plankton communities and are expected to be favored by global increases in seawater temperature and stratification associated with climate change. Eukaryotic and prokaryotic picophytoplankton have distinct ecology, and global models predict that the two groups will respond differently to future climate scenarios. At a nearshore observatory on the Northeast US Shelf, however, decades of year‐round monitoring have shown these two groups to be highly synchronized in their responses to environmental variability. To reconcile the differences between regional and global predictions for picophytoplankton dynamics, we here investigate the picophytoplankton community across the continental shelf gradient from the nearshore observatory to the continental slope. We analyze flow cytometry data from 22 research cruises, comparing the response of picoeukaryote and Synechococcus communities to environmental variability across time and space. We find that the mechanisms controlling picophytoplankton abundance differ across taxa, season, and distance from shore. Like the prokaryote, Synechococcus, picoeukaryote division rates are limited nearshore by low temperatures in winter and spring, and higher temperatures offshore lead to an earlier spring bloom. Unlike Synechococcus, picoeukaryote concentration in summer decreases dramatically in offshore surface waters and exhibits deeper subsurface maxima. The offshore picoeukaryote community appears to be nutrient limited in the summer and subject to much greater loss rates than Synechococcus. This work both produces and demonstrates the necessity of taxon‐ and site‐specific knowledge for accurately predicting the responses of picophytoplankton to ongoing environmental change. [ABSTRACT FROM AUTHOR]

Because of its temperate location, high dynamic range of environmental conditions, and extensive human activity, the long-term ecological research site in the coastal Northeastern US Shelf (NES) of the northwestern Atlantic Ocean offers an ideal opportunity to understand how productivity shifts in response to changes in planktonic community composition. Ocean production and trophic transfer rates, including net community production (NCP), net primary production (NPP), gross oxygen production (GOP), and microzooplankton grazing rates, are key metrics for understanding marine ecosystem dynamics and associated impacts on biogeochemical cycles. Although small phytoplankton usually dominate phytoplankton community composition and Chl a concentration in the NES waters during the summer, in August 2019, a bloom of the large diatom genus Hemiaulus, with N 2 -fixing symbionts, was observed in the mid-shelf region. NCP was 2.5 to 9 times higher when Hemiaulus dominated phytoplankton carbon compared to NCP throughout the same geographic area during the summers of 2020–2022. The Hemiaulus bloom in summer 2019 also coincided with higher trophic transfer efficiency from phytoplankton to microzooplankton and higher GOP and NPP than in the summers 2020–2022. This study suggests that the dominance of an atypical phytoplankton community that alters the typical size distribution of primary producers can significantly influence productivity and trophic transfer, highlighting the dynamic nature of the coastal ocean. Notably, summer 2018 NCP levels were also high, although the size distribution of Chl a was typical and an atypical phytoplankton community was not observed. A better understanding of the dynamics of the NES in terms of biological productivity is of primary importance, especially in the context of changing environmental conditions due to climate processes. [ABSTRACT FROM AUTHOR]

In marine ecosystems, most physiological, ecological, or physical processes are size-dependent. These include metabolic rates, uptake of carbon and other nutrients, swimming and sinking velocities, and trophic interactions, which eventually determine the stocks of commercial species, as well as biogeochemical cycles and carbon sequestration. As such, broad scale observations of plankton size distribution are important indicators of the general functioning and state of pelagic ecosystems under anthropogenic pressures. Here, we present the first global datasets of the Pelagic Size Structure database (PSSdb), generated from plankton imaging devices. This release includes the bulk particle Normalized Biovolume Size Spectrum (NBSS) and bulk Particle Size Distribution (PSD), along with their related parameters (slope, intercept, and R2) measured within the epipelagic layer (0-200 m) by three imaging sensors: the Imaging FlowCytobot (IFCB), the Underwater Vision Profiler (UVP) and benchtop scanners. Collectively, these instruments effectively image organisms and detrital material in the 7-10,000 µm size range. A total of 92,472 IFCB samples, 3,068 UVP profiles, and 2,411 scans passed our quality control and were standardized to produce consistent instrument-specific size spectra averaged in 1x1° latitude/longitude, and by year and month. Our instrument-specific datasets span all major ocean basins, except for the IFCB which was exclusively deployed in northern latitudes, and cover decadal time periods (2013-2022 for IFCB, 2008-2021 for UVP, and 1996-2022 for scanners), allowing for a further assessment of the pelagic size spectrum in space and time. [ABSTRACT FROM AUTHOR]

Ocean production and trophic transfer rates, including Net Community Production (NCP), Net Primary Production (NPP), Gross Oxygen Production (GOP), and microzooplankton grazing rates are key metrics for understanding marine ecosystem dynamics and impacts on biogeochemical cycles. Because of its temperate location and high dynamic range of environmental conditions and long-term human utilization, the long-term ecological research site in the coastal Northeastern U.S. Shelf (NES) of the Northwestern Atlantic Ocean offers an ideal opportunity to understand how productivity shifts in response to changes in planktonic community composition. While small phytoplankton usually dominate in the NES waters during the summer, a bloom of the large diatom genus Hemiaulus, with N2 fixing symbionts, was observed in the mid-shelf region during the summer of 2019. NCP was 2.5 to 9 times higher when Hemiaulus dominated compared to NCP throughout the same geographic area during the summers of 2020--2022. The Hemiaulus bloom in summer 2019 also coincided with higher trophic transfer efficiency from phytoplankton to microzooplankton, higher GOP and NPP, and higher sea surface temperatures than summers 2020-2022. This study shows that the presence of an atypical phytoplankton community that alters the typical size distribution of the primary producers can greatly influence productivity and trophic transfer, highlighting the dynamic nature of the coastal ocean. Notably, summer 2018 NCP levels were also high although no atypical phytoplankton community was present. A better understanding of the dynamics of the NES in terms of biological productivity is of primary importance, especially in the context of changing environmental conditions due to climate processes. [ABSTRACT FROM AUTHOR]