In this paper, measurements of high-frequency\udbroadband (160–590 kHz) acoustic backscattering from surface\udtrapped nonlinear internal waves of depression are presented.\udThese waves are ideal for assessing the contribution from oceanic\udmicrostructure to scattering as they are intensely turbulent.\udAlmost coincident direct microstructure measurements were performed\udand zooplankton community structure was characterized\udusing depth-resolved net sampling techniques. The contribution to\udscattering from microstructure can be difficult to distinguish from\udthe contribution to scattering from zooplankton using a single\udnarrowband frequency as microstructure and zooplankton are\udoften colocated and can have similar scattering levels over a range\udof frequencies. Yet their spectra are distinct over a sufficiently\udbroad frequency range, allowing broadband backscattering measurements\udto reduce the ambiguities typically associated with the\udinterpretation of narrowband measurements. In addition, pulse\udcompression signal processing techniques result in very high-resolution\udimages, allowing physical processes that are otherwise\udhard to resolve to be imaged, such as Kelvin–Helmholtz shear\udinstabilities. In this study, high-resolution acoustic observations of\udmultiple nonlinear internal waves are presented and regions with\uddistinct scattering spectra are identified. Spectra that decrease in\udlevel across the available frequency band were highly correlated\udto regions of intense turbulence and high stratification, and to\udKevin–Helmholtz shear instabilities in particular. Spectra that\udincrease in level across the available frequency band were consistent\udwith scattering dominated by small zooplankton. Simple\udinversions for relevant microstructure parameters are presented.\udLimitations of, and improvements to, the broadband system and\udtechniques utilized in this study are discussed.
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