Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
The subterranean estuary (STE) is a subsurface ecosystem where fresh groundwater mixes with intruding, recirculated seawater at the coastline. Despite being present globally, STEs and their potential impacts as hotspots for biogeochemical processing, or as a source of nutrients to coastal systems, are not well understood. STEs have been recognized as highly reactive zones for biogeochemical reactions, which influence the fate and transport of nutrients, organic matter, and trace metals discharged to the coastal ocean. Biogeochemical processing of nitrogen (N) in STEs influences N in submarine groundwater discharge (SGD) including its availability for use by primary producers and cascading eutrophication. The overarching goal of this dissertation was to assess N cycling in STEs and to evaluate how these processes may impact exchanges of N with adjacent marine environments. In chapter two, seasonal, tidal, and spatial variation in geochemical gradients as well as N cycling rates were examined to determine their influence on nutrient fluxes in SGD in the Gloucester Point beach STE (GP-STE). Geochemical gradients varied significantly across seasons, but not over tidal and spatial scales, driving seasonal variance in STE nutrient concentrations and fluxes. The nitrate, ammonium, and dissolved oxygen gradients in the GP-STE suggested nitrification was a major biogeochemical process determining the fate of groundwater derived N. The GP-STE was identified as a source of N and phosphorus (P) to the overlying York River estuary in all seasons, but denitrification reduced N export. In chapter three, the microbial community of the sandy GP beach along with the composition and abundance of nitrifiers was examined with depth in four seasons. The microbial community varied significantly with depth, but not with season. Nitrifiers were present in the top one meter of the beach indicating the genetic potential for nitrification in the system, but their abundances decreased with depth. Multivariate analysis indicated that porewater nutrient concentrations, pH, and dissolved oxygen were major drivers of subsurface nitrifier abundance. In chapter four, STE nitrification rates were measured using conservative mixing models, an in situ tracer experiment, and ex situ sediment slurry incubations. All three methods indicated nitrification is occurring in the GP-STE; however, the in situ tracer experiment revealed variation in nitrification rates over space and time that was not captured by the mixing model calculations or slurry incubations. These data suggest that, due to heterogeneity in the subsurface environment, in situ experiments may be the best approach for estimating STE process rates. Chapter five used a global meta-dataset to examine the groundwater nutrient pool, determine STE behavior with regards to nutrients, and assess the impact STE processing has on groundwater fluxes to the global ocean. The composition and concentrations of the groundwater nutrients were influenced by sample salinity, latitude, land use, and site type. DON represented >30% of the groundwater N pool. STEs, at the global scale, produced DIN and DIP, but reduced DON concentrations in groundwater nutrients, resulting in higher DIN and DIP fluxes, but lower DON fluxes in fresh SGD to the global ocean. Total SGD fluxes of DIN and DON were estimated to be higher than riverine fluxes the ocean, but DIP fluxes from total SGD were half the riverine input. Overall, this dissertation reveals the importance of STE biogeochemical processes on exchanges of nutrients along the land-ocean continuum from groundwater to the coastal ocean.
© The Author
Wilson, Stephanie J., "The Fate And Transport Of Nitrogen In Subterranean Estuaries" (2022). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1673281650.
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