Phytoplankton community composition and size structure vary considerably between oligotrophic and eutrophic regions (areas of low or high macro and micronutrients (e.g., iron)) (Hill et al., 2005; Martin et al., 1989; Strom et al., 2006; 2016), between surface and subsurface depths (Hill et al., 2005), and with season (Moran et al., 2012) and climatic conditions (Batten et al., 2021). Phytoplankton represent the base of the food web providing energy for zooplankton, which in turn support the growth of juvenile and adult salmon populations. Certain phytoplankton, like many diatom species, are particularly important food items in the GOA (Odate, 1996, Strom et al., 2007). We aim to broaden understanding of phytoplankton dynamics in the GOA/NPO by investigating spatial and temporal patterns in community structure and biomass and exploring environmental (physics and nutrients) drivers of taxonomic variability that may lead to variation in the quality of phytoplankton biomass available to primary consumers during winter. The spatial variations in phytoplankton biomass, and taxa and community size structure were characterized through measurements of total chla, flow cytometry, and HPLC (analyzed by Canadian PIs Konik, Costa) collected at all IYS stations. In zone 4 (US Ship Shimada) flow cytometry samples were collected from 5, 25, and 50 m for assessment of microbial community cell sizes (Moran et al. 2012), and HPLC samples from 5 m for assessment of phytoplankton functional groups; both data sets will be used to help ground-truth the information obtained from the satellite ocean color measurements. Flow cytometry and HPLC samples were also collected at 5 m depths in zone 5 (on Canadian ship Franklin) and in zones 2-3 (on Russian ship TINRO), so we have samples over the entire area surveyed. The flow cytometry analysis deliverables include tabulated counts (cells/ml) and estimated carbon content (C/cell and C/population) for the following 4 phytoplankton pico- and nanoplankton: Synechococcus, Cryptophytes, picoeukaryotes, nanoeukaryotes (excluding Cryptophytes). Water samples for total chlorophyll a (Chla), were collected from Niskin bottles at six depths (5, 25, 50, 75, 100, 150 m) at all stations surveyed.