UC San Diego

Novel instrumentation plays a crucial role in enabling new observations and providing deeper insights into ocean dynamics. In my thesis, I present methodologies for collecting high spatial-temporal resolution observational data, which allows for a better understanding of physical and biogeochemical processes in the coastal ocean. Specifically, I integrate two novel sensors, the acoustic Doppler current profiler (ADCP) and the Submersible Ultraviolet Nitrate Analyzer (SUNA), onto the Wirewalker (WW) wave-powered profiler. I discuss issues related to fine-scale velocity measurements, biological modulation of nitrate during a harmful algal bloom, and nitrate dynamics over the inner continental shelf.The thesis begins by presenting a new methodology to observe oceanic fine-scale velocity using the combination of velocimeters and the Wirewalker profiler. By correcting vehicle-motion-induced velocity contaminations and removing surface wave signals, a background velocity field with a spectrum wavenumber runoff at a scale of ~3 m over a 100 m vertical range was obtained. This vertical resolution is several times finer than that possible from other velocity-measuring platforms with similar measurement ranges. Next, I present in situ evidence demonstrating the vertical migration of the dinoflagellate that facilitates their nitrate uptake during the 2020 red tide in the Southern California Bight. The vertical migration pattern of Lingulodinium polyedra (the dominant species) and their biological modulations to the subsurface nitrate field were quantified. The loss of nitrate in the nitracline was balanced by proportional increases in phytoplankton concentrations, confirming a 50-year-old hypothesis. The swimming ability of the dinoflagellate allows them to outcompete other non-motile phytoplankton during times of high stratification and nutrient limitation. Finally, an undocumented onshore-directed cross-shore nitrate gradient over the inner shelf of the Southern California Bight was revealed. The nitrate gradient was possibly formed by enhanced vertical nitrate flux due to elevated turbulent diffusivity when isopycnals constantly impinged on a sloping sea floor. Furthermore, the study demonstrated that near-inertial internal waves were responsible for transporting the onshore nitrate offshore, providing a new mechanism for fueling the offshore primary productivity. These findings highlight the need for new instrumentation to better understand complex coastal biogeochemical dynamics.