Date of Award

Spring 2026

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean, Earth & Atmospheric Sciences

Committee Director

Richard Hale

Committee Member

Joseph Tamborski

Committee Member

Margaret Mullholland

Abstract

The Lafayette River, a shallow, eutrophic tributary of the Chesapeake Bay, experiences seasonal harmful algal blooms (HABs). While management efforts focus on reducing external nutrient inputs, internal loading from benthic sediments remains poorly constrained. This study quantifies spatial and seasonal variability in nutrient and radium isotope fluxes across the sediment–water interface at three sites where HABs are known to initiate and represent contrasting sediment textures and hydrodynamic conditions. Sediment cores and porewater samples were collected in summer 2023 and winter 2024 to measure grain size, porosity, dissolved nutrients (NH₄⁺, NOₓ⁻, PO₄³⁻), and radionuclide activities (²²⁴Ra and ²²⁸Th). Radium isotope disequilibrium revealed active porewater exchange at all sites, with molecular diffusion accounting for only 2–23% of total ²²⁴Ra fluxes, indicating dominance of enhanced porewater exchange processes.

Fine-grained sediments at lower-energy Sites exhibited PO₄³⁻ fluxes elevated relative to literature values. This likely reflects enhanced nutrient retention and microbial remineralization, as well as PO₄³⁻ release under low-oxygen conditions via iron reduction. In contrast, coarse grained sediments at higher-energy sites supported elevated NH₄⁺ fluxes in both seasons possibly due to greater permeability and thus advective porewater exchange. Nutrient fluxes (NH₄⁺ = 656–7,483 µmol m⁻² d⁻¹; PO₄³⁻ = 239–11,716 µmol m⁻² d⁻¹; NOx=6.06-5276.4 µmol m⁻² d⁻¹) fall within ranges reported for other estuarine and coastal systems. These results demonstrate that benthic sediments represent a significant and seasonally variable nutrient source to the overlying water column. Intra-system nutrient fluxes should therefore be incorporated into nutrient budgets and management strategies aimed at mitigating eutrophication and harmful algal blooms in urbanized estuaries.

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ISBN

9798197810526

ORCID

0009-0006-1925-0749

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