Primary production drives varied zooplankton migration strength and twilight‐zone particle dynamics across ecological gradients in the western North Pacific

Abstract

Diel vertical migrations (DVM) of zooplankton play a crucial role in transferring organic matter efficiently to the deep ocean. However, the spatial regulatory determinants of DVM strength, including migrant biomass and migration amplitude, remain understudied. We conducted 34 stratified trawls and 85 Underwater Vision Profiler 5 casts across latitudes 12.5°N to 41.5°N in the western North Pacific and developed a structural equation model to explain DVM variability relative to measured and remotely sensed environmental data. Migrant biomass was mainly determined by net primary productivity (NPP), being two orders of magnitude greater in high NPP waters due to higher zooplankton biomass and larger migrating individuals. Migration amplitude was also larger at high NPP stations but was determined by a direct negative correlation to euphotic zone depth and an offsetting indirect positive interaction with water clarity. Migration patterns reflect the classic tradeoff between predation risk and reduced daytime feeding for regular DVM and likely avoidance of mesopelagic visual predators at night for reverse migrants. In high‐resolution profiles, particle abundances and large particle contributions increased in the twilight zone during the daytime, generally aligning with biomass distributions and respiratory fluxes of migratory zooplankton. Migrants likely contribute to mesopelagic particle dynamics with pulsed fecal matter delivery from the euphotic zone and direct interactions with deep particle fields. Mesopelagic particle dynamics might also be linked to deep migrant mortality, the least quantified component of the biological carbon pump.