High-frequency acoustics and bio-optics in ecosystems research

The propagation of light and sound in the ocean’s interior is modified by the presence of phytoplankton, zooplankton, fish, gas bubbles, and dissolved and suspended material. Information is encoded in the levels and spectral characteristics of acoustic and optical scattering and absorption. Using acoustics and optics allows us to study the distribution of marine life and learn about ecosystem-relevant processes. Two studies are highlighted. In the first, multifrequency, upward-looking echosounders deployed near autonomous, bio-optical profilers were used to track vertical migration and the formation and size structure of ,1 m thick, zooplankton layers in relation to the biomass and size structure of thin phytoplankton layers. In the second, a multifrequency sonar was used to track the temporal (seasonal) evolution of zooplankton biomass and size structure in the Bering Sea at intervals of 20 min. This paper focuses on how advanced technologies are being used to observe processes, distributions, and behaviour of marine life that have, until now, been hidden, as it were, from biological oceanographers.

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