| dc.description.abstract | Sound-scattering layers are found throughout the global ocean at depths between 200 and 1000 meters, referred to as the deep scattering layers (DSLs). DSLs are biological in nature, primarily formed by organisms such as zooplankton and mesopelagic fish, many of which undertake diel vertical migration (DVM). Despite their recognized ecological and biogeochemical significance, quantitative understanding of DSLs remains limited due to their vast spatial extent, deep habitat, complex biological composition, and dynamic coupling with ocean physics. This thesis leverages long-term broadband acoustic observations and signal processing methods to characterize the DSL from the individual to the population level, with the goal of improving biomass estimates and elucidating DSL dynamics in relation to oceanographic processes.
Over two years, a deep-submerged mooring equipped with a broadband split-beam echosounder (36–44 kHz) was deployed in the slope sea off the New England continental shelf. The system continuously monitored individual organisms in the DSL between ~545 and 575 m depth. Tracking ~820,000 individual organisms, this study reveals a remarkably consistent target strength (TS) distribution across years, dominated by two distinct groups, with mean TS values of –53 dB and –49 dB, associated with migratory and non-migratory species, respectively. Variations in TS distribution at depth were linked to DVM, resident depth, and water masses, highlighting the importance of in situ TS observations for biomass estimation.
A focused case study characterized the DSL within a warm-core anticyclonic eddy occupied by tagged sharks. The DSL inside the eddy exhibited high organism abundance and a unique vertical composition, with an upper layer dominated by fluid-like organisms. Energetic, near-inertial, oscillatory population movements with pronounced vertical shear were observed, likely driven by the "inertial drainpipe" effect of anticyclonic eddies that channel wind-generated near-inertial waves to depth. The superposition of wave and eddy velocity fields may result in a bowl-shaped DSL with persistent finescale vertical structures, i.e., thin layers. The distinct biological and physical environment within the eddy likely contributes to its role as a hotspot for marine top predators.
To enhance and advance the capability of broadband split-beam echosounders in complex environments like the DSL, the thesis further revises and introduces methods for angle estimation of both single and overlapping echoes. A revised split-aperture correlator improves the angle estimation of single echoes from targets with highly frequency-dependent backscattering responses, while a broadband maximum likelihood estimator enables angle and spectral estimation of overlapping echoes from multiple targets, validated through laboratory experiments. | |
