Dataset (2003-2018) for southeast Bering Sea used for analyses in the manuscript: Phytoplankton communities in the Bering Sea respond to environmental shifts, with implications for consumer species

Phytoplankton community structure changes in response to environmental conditions, potentially impacting zooplankton through altered trophic interactions. In the Bering Sea, recent stanzas of warmer and colder years coincided with changes in community structure of zooplankton and fish. However, attempts to link these trends to primary producers based on patterns in total chlorophyll a concentration (Chl) have not been successful. Using Chl as a proxy for food availability ignores potential consequences of community shifts in phytoplankton, which span a vast range of cell shapes, sizes, and biochemical composition that do not contribute equally to the diet of primary consumers. We analyzed size and taxonomic composition of phytoplankton with environmental variables from 2003 – 2018, and examined relationships to abundance of two copepods (Calanus and Pseudocalanus) important to age-0 pollock diets. Three distinct environmental stanzas were identified: Warm1 (2003-2005) with high temperature and salinity and low nutrients favoring dominance of flagellates, ciliates, and dinoflagellates, and low abundances of Calanus copepods; Cold (2006-2012) with low temperature and salinity and high nutrients favoring the cryptophyte Rhodomonas and small centric diatoms, and high abundance of Calanus copepods; and Warm2 (2014-2018) with high temperature, salinity, and nutrients (including high SiO4), supporting high abundance of elongated rod-like diatoms, and low abundance of Calanus copepods. Calanus abundance was significantly negatively correlated with biomass of large (>10 µm) phytoplankton, and the taxa Leptocylindrus, Thalassionema, Pseudo-nitzschia, and Dictyocha, and positively correlated with Rhodomonas and small disc diatoms (<10 µm), highlighting links between phytoplankton and zooplankton communities in response to stanzas.