Phytoplankton communities respond to environmental shifts in the eastern Bering Sea: Low abundances of Calanus copepods correlate with larger but potentially adverse phytoplankton taxa during warm stanzas Limnology and Oceanography

Phytoplankton communities change in response to environmental conditions, potentially impacting zooplankton through altered trophic interactions. In the Bering Sea, warm and cold stanzas in recent decades correlated with changes in community structure of zooplankton and fish. Attempts to link these trends to primary producers based on patterns in total chlorophyll a (Chl a ) concentration have been unsuccessful. Using Chl a 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 primary consumer diets. We analyzed size structure and taxonomic composition of phytoplankton in relation to environmental variables from 2003 to 2018 and examined relationships to the abundance of two copepod genera ( 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 nutrient concentrations favoring dominance of flagellates, ciliates, and dinoflagellates, and low abundances of Calanus ; Cold (2006-2012) with low temperature and salinity and high nitrogen concentrations favoring the cryptophyte Rhodomonas and small centric diatoms, and high Calanus abundances; and Warm2 (2014-2018) with high temperature, salinity, and nutrients (including high SiO 4 ), supporting high abundances of elongated rod‐like diatoms, and low abundances of Calanus . Calanus abundance (2003-2018) was significantly negatively correlated with biomass of large (> 10 μm) phytoplankton, with moderate‐strong negative correlations with the taxa Leptocylindrus , Ceratium , and Dictyocha , and strong positive correlations with Rhodomonas , highlighting links between phytoplankton and zooplankton community structure in response to alternating environmental stanzas.