UCLA

Deoxygenation occurs along coastal environments and open ocean that affects ocean biogeochemistry and marine species. The causes of deoxygenation are multifaceted and can be influenced by various physical and biogeochemical properties in the ocean. The California Current System experiences deoxygenation along its coast and continental shelf, and effects sub-basins like the Santa Barbara Channel (SBC), located in the Southern California Bight. Here, oxygen concentrations can become hypoxic and anoxic, influencing the release of trace metals like iron from the sediment and the formation of sulfur-oxidizing bacteria mats in response to sulfide production in the sediment. Quantifying the impacts that deoxygenation has in the Santa Barbara Channel, will provide insights into ocean biogeochemistry locally, and in the California Current System more generally, along with guidance for broader ocean policies to advance regulations to combat ocean deoxygenation. In this dissertation, I will use data from two research expedition on R/V Atlantis in 2019 and 2023 to the Santa Barbara Channel and its basin, to quantify how benthic Fe flux affects phytoplankton growth along with the spatial and temporal distribution of oxygen in the basin; and extrapolate the impact of hypoxia and anoxia on basin-wide benthic Fe flux. Lastly, I developed a framework to determine the factors that can contribute to the implementation of deoxygenation policy into the Biodiversity Beyond National Jurisdiction Treaty. Since deoxygenation can affect marine biodiversity, fisheries management, and blue economies, it is important to determine the social-human environmental factors for governance and management as it relates to deoxygenation. Chapter 2. To investigate the influence of benthic Fe release from the oxygen-deficient deep basin on surface phytoplankton production, we combined benthic Fe flux measurements with numerical simulations using the Regional Ocean Model System coupled to the Biogeochemical Elemental Cycling model (ROMS-BEC). For this purpose, we updated the model Fe flux parameterization to include the new benthic flux measurements from the Santa Barbara Basin. Our simulations suggest that benthic Fe fluxes enhance surface primary production, supporting a positive feedback on benthic Fe release by decreasing oxygen in bottom waters. However, a reduction of phytoplankton Fe limitation by enhanced benthic fluxes near the coast may be partially compensated by increased nitrogen limitation further offshore, limiting the efficacy of this positive feedback. Chapter 3. While numerous studies have investigated the influence of hypoxia and anoxia on nutrient cycling and microbial activity in the sediment and water column of the SBC, studies that characterize the spatial distribution of dissolved oxygen in the channel, and its temporal variability, remain limited. I explored the spatial extent and temporal variability of dissolved oxygen in the SBC, with the goal of increasing our understanding of its dynamics and the consequences for the ocean biogeochemistry and phytoplankton productivity. To this end, I integrated historical observations of dissolved oxygen from California Cooperative Oceanic Fisheries Investigation (CalCOFI) and other programs with two highly spatially resolved oxygen surveys from recent cruises (AT42-19/2019 and AT50-11/2023) that deployed the Autonomous Underwater Vehicle (AUV) Sentry and the Remotely Operated Vehicle (ROV) Jason, equipped with oxygen sensors. This new data compilation provides the first spatially resolved characterization of dissolved oxygen across the SBC and in the bottom boundary layer. Chapter 4 Global deoxygenation is becoming an emerging issue in ocean policy that intersects with other ocean issues such as ocean acidification, climate impacts on marine species and marine conservation. However, deoxygenation has not appeared in international ocean policy even though the loss of oxygen transcends national and international jurisdiction. The Biodiversity Beyond National Jurisdiction Treaty (BBNJ) or “High Seas Treaty” provides an opportunity to implement deoxygenation for the High Seas. To do so we developed a framework for deoxygenation using scale and levels based on scientific understanding and consensus on the loss of oxygen that intersect with ocean governance. Our framework illustrates how ocean governance can advance ocean policy for deoxygenation that intersects with the initiative in BBNJ.