Acoustic Observations Unveil Seasonal Fluctuations and Regulation of Zooplankton Biomass and Its Vertical Distribution at a Mid‐Latitude Long‐Term Ocean Observatory

Zooplanktonic organisms are considered a key link between different trophic levels. The ecosystem structure and dynamics are affected by changes in their population and phenology. In this work, the causes and timing of changes within the seasonal cycle of zooplankton biomass vertical distribution in oceanic waters of the Bay of Biscay were investigated. For this purpose, long‐term oceanographic time‐series from zooplankton nets and high‐frequency Acoustic Doppler Current Profiler backscattering data, employed as a proxy for zooplankton biomass, were used. This combination enables the observation of the concurrence of a strong seasonality in zooplankton biomass and its daily oscillation tightly linked to phytoplankton stock and its vertical distribution in relation to water column stability and air–sea forcing. During the spring bloom, when food availability is guaranteed, an increase in zooplankton biomass is observed and it is concentrated at the ocean surface, with a reduction in its diel vertical migration. Later in the year, zooplankton biomass is concentrated above the deep chlorophyll maximum (DCM), and the depth range that it occupied is increased or reduced as the DCM deepens or shoals.