Diel vertical migration of zooplankton at the S1 biogeochemical mooring revealed from acoustic backscattering strength

We examined the diel vertical migration of zooplankton by using the backscatter strength obtained from moored acoustic Doppler current profilers at mooring site S1 (30^oN,145^oE) in the North Pacific subtropical gyre. The clear relationship between backscatter intensity and bulk zooplankton biomass indicated that the backscatter likely represents the bulk biomass. There was clear seasonal variability in the vertical distribution and migration of the high-backscatter layers in that they became deeper than the euphotic zone (<100 m) in winter and confined above this depth in other seasons. Seasonal changes in daylight hours also affected the timing of the diel migration. We found that lunar cycles and physical events, such as mixed-layer deepening and re-stratification, and the passage of a mesoscale eddy, also affected the diel migration. Our results also indicated that the accuracy of biomass estimates based on net samples could be influenced by the high temporal variability of zooplankton distributions. We suggest, therefore, that a combination of net samplings and acoustic observations were necessary to understand the temporal variability of zooplankton behavior.