High-frequency acoustics of marine vegetation – Role of frequencies and imaging angles

Marine vegetation is extremely varied and an essential component of shallow-water habitats. This paper will present two independent strands of work, using a variety of high-frequency acoustic sources to image macrophytes for different purposes. The first series of studies focuses on gas-filled kelp Nereocystis luetkeana, a seaweed ranging along the west coast of North America from California to Alaska, and is an important component of its diverse coastal ecosystems. As part of an investigation into grey whale habitats, we have
measured propagation and attenuation through kelp beds of different densities, using broadband (1-20 kHz) white noise (Wladichuk, 2010, and other works). These field measurements in British Columbia (Canada) were compared with Monte Carlo simulations of sound propagation through kelp beds of increasing densities. The second series of measurements (started in Kruss et al., 2007 and continued through to Kruss et al., 2017) looked at gas-free macrophytes in shallow polar habitats; namely Saccharina latissima (L.) and Laminaria digitata (Huds.) in Svalbard fjords. These were mapped with traditional single-beam echo-sounders (such as the Biosonics DTX, 420 kHz) and with multibeam echosounders (such as the Imagenex 837 Delta-T, 260 kHz), investigating the ranges of angles at which macrophytes could be reliably mapped and what signal processing approaches were the most adapted. These two types of work are combined to show how they can be interpreted in the light of the seminal work done by Jean-Pierre Hermand on seagrass acoustics, for example Hermand et al. (2000) and Hermand (2004), and how his scientific legacy influences future efforts in acoustic mapping of marine vegetation.