Basin-scale zooplankton composition in the Eastern Subarctic Pacific in the winter-spring

Knowledge of zooplankton distributions in the Eastern Subarctic Pacific (ESP) is limited and hinders our ability to assess ecosystem level change. To provide baseline information and increase our understanding of zooplankton ecology in the ESP, large-scale winter to early spring epipelagic zooplankton dynamics were investigated as part of three expeditions. Bongo tows (0.6-m mouth diameter, 250 µm mesh) were collected in 2019 (19 February – 15 March) and 2020 (12 March – 4 April) north of 47.5°N and east of 147.5°W and during a third survey in 2022 (6 February – 17 April), which expanded coverage westward to 172°W. General Additive Models identified a region of low zooplankton abundance and low biomass of small zooplankton in the Alaska Gyre. Cluster analysis of genus abundances separated samples largely along a temperature gradient. Clusters were broadly grouped into cool (<7 °C) and warm (>7 °C) assemblages. Cool assemblage clusters were dominated by subarctic taxa and their composition suggested a broad coherence in zooplankton structure north of the bifurcation of the North Pacific Current (NPC), with variation driven by the circulation of the gyre and inputs from neighbouring current systems. Warm assemblage clusters were primarily associated with the NPC bifurcation and were characterized by a mix of Subarctic, California Current, and Transition Zone taxa. Differences in species composition and biomass across the Alaska Gyre and between the northern ESP and the NPC bifurcation, including increased biomass of small zooplankton associated with the bifurcation, give insight into the foraging landscape that supports planktivores in the ESP.