Quantifying plankton dynamics in the internal tide using swarms of buoyancy-controlled robots

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Coastal waters are among the most heavily used and threatened systems on the planet. Successful prediction and management of coastal resources can only come from a deep understanding of the dynamics of the species in these regions. Fluctuations of coastal invertebrate and vertebrate populations are often driven by the supply of planktonic larvae to the adult habitat by ocean currents. Numerous studies have associated the cross-shelf transport of plankton - including the larvae of economically valuable species - with the internal tide: a wave in the ocean's interior that oscillates at the tidal frequency. Though the interactions of plankton with internal waves have been studied for decades, it has not been possible to track individual plankton underwater. Thus, the dynamics underlying the cross-shelf transport of plankton in internal waves and internal tides remain conjectural. This project will use undersea swarms of novel, autonomous plankton-mimicking drifting robots to quantify, in situ, the cross-shore transport of plankton driven by high-frequency internal waves and the internal tide. This research will significantly enhance our understanding of the distributions, settling patterns, and population connectivity of coastal species. One PhD student will be supported and educational outreach in collaboration with the Ocean Discovery Program in San Diego will support curricula creation, after-school programs, and teacher development.These researchers have recently gained the capability to deploy swarms of plankton-mimicking, autonomous, drifting robots in the ocean. These robots, Autonomous Underwater Explorers (AUEs), are 1.5-liter cylinders with temperature and pressure sensors, a hydrophone, and a piston that regulates buoyancy. Subsurface three-dimensional localization is accomplished through an acoustic long-baseline navigation system. The three-dimensional position of each AUE is obtained every 12 seconds with ~1 m horizontal and 1 cm vertical accuracy with a range of ~5 km. This high spatial and temporal resolution represents a major advance over traditional neutrally buoyant floats. Swarms of 20 AUEs will be programmed with either depth-keeping or isotherm-following behaviors, and deployed in the internal tide on the shelf to quantify their transport, accumulation, and vertical movement over a tidal cycle. The swarms will move through a mooring array consisting of a vertically profiling Wirewalker, a thermistor chain, and two bottom-mounted Acoustic Doppler Current Profilers. Data from these deployments will be combined with process studies using a numerical model to test long-standing hypotheses concerning the effects of plankton behavior on transport and accumulation in internal waves and the internal tide. This research will increase the operational capacity of AUEs, advancing the state of the art in studying cross-shelf transport due to internal waves, and lead to new insights into the physical and biological interactions controlling larval transport across the shelf.