Midwater Zooplankton Response to Seasonality in Export Flux in the North Pacific Subtropical Gyre

Zooplankton in midwaters have the potential to significantly modify export fluxes to the deep ocean, yet their dynamics remain poorly understood. At Station ALOHA in the North Pacific Subtropical Gyre particle export varies on a seasonal basis, with higher fluxes observed in the late summer. Using compound-specific stable isotope analysis (CSIA), we find that the midwater zooplankton community responds rapidly to this summer export event. ‘Source’ amino acid δ¹⁵N values for mesopelagic and upper bathypelagic zooplankton are lower in late summer than in winter, indicating a summer food web more reliant on labile, surface-derived N. In contrast, we find that refractory midwater suspended particles become a more important food resource for the midwater zooplankton community in winter. This seasonality is primarily observed in the larger (1.0 – 2.0 mm) zooplankton size fraction we studied and in target taxa such as small copepods and chaetognaths. We further evaluate seasonal trends in midwater zooplankton trophic structure and biomass and the stoichiometry of zooplankton metabolic demand in the mesopelagic zone.