Spatial and Temporal Variability of Zooplankton Thin Layers: The Effects of Composition and Orientation on Acoustic Detection of Layers

Our primary long-term objective is to better understand the physical and biological mechanisms of formation and maintenance of thin layers of zooplankton. Because zooplankton can be strong sound scatterers, acoustic instruments are effective at detecting and describing zooplankton thin layers. Using a combination of instruments (acoustics, image-forming optics, ADCPs, CTDs, and bio-optical sensors) and sampling platforms (a fleet of gliders and a profiling package), we plan to determine the temporal and spatial scales of acoustic backscatter from zooplankton aggregations, the taxonomic and size composition of the zooplankton in such layers, and the associations of zooplankton thin layers with physical parameters. To do this, it is imperative that we understand the factors influencing the frequency dependent backscatter from the organisms. In particular, the orientations of the plankton relative to the acoustic source can have significant effects on the resultant backscatter. Hence, a secondary objective is to improve our understanding of in-situ acoustic backscatter from zooplankton so that moving platforms that change orientation, such as gliders, can provide accurate acoustic survey data on the distribution and composition of scattering features. 1. What is the temporal and horizontal spatial variability of zooplankton scattering layers and the spatially coincident physical and biological parameters associated with these layers? 2. What is the time scale of zooplankton layer generation (aggregation) and destruction (dispersal) and how do these correlate with physical characteristics and phytoplankton thin layer formation? 3. What are the composition of zooplankton scattering layers and the in-situ orientation of the organisms and how does this affect the spectrum and magnitude of measured backscatter? 4. How do physical and biological mechanisms together form and maintain scattering layers?