Abundance Distribution Pattern of Zooplankton Associated with the Eastern Arabian Sea Monsoon System as Detected by Underwater Acoustics and Net Sampling

The abundance distribution pattern of zooplankton associated with the pre-upwelling and late-upwelling phase is assessed for the eastern Arabian Sea (EAS) summer system, using vessel-mounted moving Acoustic Doppler current profiler (ADCP) and the in situ zooplankton samples collected using plankton nets. The distribution pattern of zooplankton is observed to be regulated by physical factors such as coastal upwelling, circulation patterns, mesoscale eddies, regional stratification, the presence of subsurface chlorophyll-a maximum, etc. during different phases of the upwelling cycle. The volume backscattering strength, a proximate factor for the zooplankton biomass, is computed after deriving the appropriate sound absorption coefficient, slant range, and backscatter noise. The linear relation derived by enumerating the backscatter-to-zooplankton biomass relationship was stronger during the pre-upwelling phase (r = 0.58) but weaker during the late-upwelling phase (r = 0.25). The findings underscore the potential of ADCP backscatter as a reliable indicator of zooplankton biomass within the mixed layer depth of the EAS, especially in the stable, calm, early summer season. The derived equations for estimating biomass are log(B) = 5.39 + 0.05  Sv for pre-upwelling and log(B) = 3.10 + 0.02  Sv for late-upwelling. The reduced correlation later suggests that environmental changes, such as zooplankton size and composition shifts, may affect ADCP’s detection threshold, necessitating careful interpretation. The study shows fish larvae act as dominant scatterers due to their gas-bearing properties, reliably indicating proxies for zooplankton abundance across both upwelling phases. Fluid-like and elastic-shelled scatterers vary between phases, reflecting shifts in zooplankton composition and their impact on acoustic backscatter. The analysis of ADCP backscatter data tracks diel vertical migration (DVM) of zooplankton with significant concentrations at depths of up to approximately 80 m during night-time. This study identifies distinct vertical migration velocities with zooplankton ascending in the range of 7.2 cm/s during dusk and descending at 7.7 cm/s during dawn.