Detection, classification and ecology of acoustic scattering layers Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9c67wq23s

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  • Acoustic scattering layers of biological origin have been observed for nearly 70 years across the world’s oceans. The organisms that comprise these layers are known to be important features in most oceanic ecosystems, providing a vital trophic link between small phytoplankton grazers and larger species. There are many aspects of their ecology, however, that remain poorly described. To address this research gap, an automatic layer detection algorithm was designed to identify scattering layers based on their acoustic structure and their horizontal continuity. It classifies layers into broad, unstructured background layers, and high-energy, consistently organized, horizontally contiguous internal strata. The method was rigorously tested on data collected in three disparate ecosystems: the Gulf of California, Monterey Bay and the Bering Sea. With this new tool, scattering layer boundaries were identified in data from the Gulf of California. The bottom of the background layer in this region averaged 463 m depth during the day and 434 m at night, the bottom of the main migrating stratum averaged 333 m during the day and 54 m at night, and a consistent near-surface layer averaged 43 m during the day and 61 m at night. Average maximum rates of diel vertical migration were 8.6 cm/s when ascending and 6.9 cm/s when descending. Using CTD profile data in conjunction with layer boundary information, the hypothesis that scattering layers in the Gulf of California are associated with specific oxygen values was tested. Several methods were employed to separate the influence of oxygen from other parameters including simple regressions, contingency tables, and logistic models. Suggestive evidence was found that shallow scattering layers were associated with the thermocline, and midwater and deeper layers were more associated with light, temperature or density than with oxygen, but no results were conclusive and a consistent physiological explanation of scattering layer depths below the thermocline could not be supported. I concluded that scattering layer depths and migration rates in the Gulf of California were broadly consistent with prior reports; however, evidence that scattering layer depths in the Gulf of California were associated with any one environmental parameter was lacking. Great care should be exercised when attempting to explain scattering layer depths through the lens of a single parameter, as covariates may offer alternate explanations and scattering layer depths are by nature complex features composed of individual organisms with a variety of physiological and non-physiological needs. The new tools I developed to describe the characteristics of scattering layers, to automatically identify layers based on their internal structure, and to analyze their relationship to the surrounding habitat contributed to our understanding of scattering layers in the Gulf of California, and can contribute generally to future studies of these widespread and ecologically significant features.
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  • description.provenance : Submitted by David Cade (caded@onid.orst.edu) on 2014-03-26T18:42:42Z No. of bitstreams: 2 license_rdf: 1527 bytes, checksum: d4743a92da3ca4b8c256fdf0d7f7680f (MD5) CadeDavidE2014.pdf: 32248482 bytes, checksum: 9409d074dc1759e529f71136a9dab18d (MD5)
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