Diel vertical migration and frontal variability of acoustic backscatter in the Balearic Sea

Abstract

Acoustic Doppler current profilers (ADCPs) use active sonar to measure current velocities by measuring the sound returned by scatterers (most often zooplankton) in the water column. The volume of scatterers, or echo intensity, has been used to measure the abundance of zooplankton and characterize diel vertical migration (DVM). DVM is the mass vertical movement of zooplankton and fish between the surface waters where they feed at night, and the mesopelagic zone where they avoid predators during the day; it is considered the largest migration of biomass on Earth, happens in every ocean, and is important to the global carbon cycle. This thesis uses a combination of data that I helped acquire during the Office of Naval Research-funded CALYPSO 2022 field campaign in the Balearic Sea. Acoustic backscatter from a 38kHz ADCP and a 150kHz ADCP is translated into mean volume backscattering strength (MVBS) to characterize the sound scattering layers (SSLs) in the Balearic Sea. WireWalker data is used to model subsurface light. The MVBS is compared to measurements of temperature, salinity, chlorophyll concentration, and dissolved oxygen (DO) from the EcoCTD, a towed instrument that simultaneously measures hydrographic and biological parameters. The analysis reveals one permanent scattering layer at 300m – 600m and two migrating scattering layers in the top 50m and between 100m – 300m. The layers are likely made up of zooplankton like krill and pteropods and pelagic fish. The speed of vertical migration ranges from 1 – 11cms−1, and migrators are follow isolumes during migration times. DVM has the strongest effect on backscatter anomalies, but during daytime and nighttime, DO is most correlated with the backscatter anomaly. We demonstrate that ADCPS can be used to characterize SSLs and DVM. The uniquely co-located EcoCTD data from CALYPSO enables us to compare the frontal variability in scatterers to variability in biological and physical parameters. Characterizing the SSLs, DVM, and frontal variability of acoustic backscatter furthers understanding of the global carbon cycle.