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From the land to the sea: Impacts of submarine groundwater discharge on the coastal ocean of California and Alaska


Submarine groundwater discharge to oceans is an ever-increasing topic of study in the earth sciences due to the dynamic geochemical and biological effects it imposes on aquatic systems both by the chemical reactions it induces and the constituents it transports. Studies indicate groundwater discharge can represent a major source of nutrients, fecal indicator bacteria, caffeine, trace metals, and mercury to aquatic systems. In some systems, groundwater discharge can rival rivers and upwelling, as a source of nutrients. The implications of groundwater discharge include harmful algal blooms caused by nutrient loading and poor water quality caused by pollutants transported through groundwater discharge. Much research has been directed at quantifying the flux of groundwater and associated constituents through groundwater discharge using radium isotopes as geochemical tracers. Radium isotopes represent some of the most common geochemical proxies used to calculate groundwater discharge rates to lakes, oceans, and other water bodies. However well-established these tracers are in the scientific community, they are often used as a single proxy for groundwater discharge resulting in large errors due to natural variability and dependence on residence time, and these errors compound through the calculations. Large errors in the estimates of the volume flux of groundwater discharge lead to large errors in constituent fluxes through groundwater discharge, which diminishes the usefulness of such efforts. Additionally most SGD-focused studies stop at calculating the SGD and associated constituent fluxes, and pondering the impacts of the constituents on the marine ecosystem, with out directly measuring the impact on the system through other methods. The focus of my thesis is to integrate methods of calculating SGD fluxes based on multi-radium isotope measurements with mixing models, bioassay incubation experiments, and water isotopes to better understand the impacts of SGD on marine systems. First (Chapter 1) I will integrate a multi-radium isotope method to calculate SGD fluxes with water isotopes to better understand the hydrology of the systems to understand the governing processes and importance of SGD as a conduit of methane to the North Pacific and Artic Oceans. Then (Chapter 2) I will combine a single radium isotope SGD flux model with a bioassay incubation experiment to determine the ability of SGD to impact phytoplankton ecology in Monterey Bay, California. Last (Chapter 3) I will combine a multi-radium isotope and nutrient flux mixing model with a multi-radium isotope SGD flux model to determine the importance of SGD as a nutrient source compared to sub-thermocline water and river water in Monterey Bay, California.

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