Date of Award
Doctor of Philosophy (PhD)
Ocean & Earth Sciences
Margaret R. Mulholland
Nitrogen (N) is central to marine primary production; its availability often limits the capacity and rates of primary productivity in most of the world’s oceans. Contrastingly, estuaries frequently receive anthropogenic N loading, oftentimes resulting in eutrophication, harmful algal blooms (HABs), and substantially degraded water quality. Nutrient variability in both estuaries and oceanic regimes results from meteorological forcing and physical processes, including wind-induced, tidal, and mesoscale mixing and upwelling. In this dissertation, a comprehensive investigation of N variability and cycling and its links to physical-biogeochemical processes was conducted using time-series monitoring approaches, flux estimations, satellite imaging, biogeochemical measurements, and molecular analyses. The study focused on three contrasting regions along the east coast of the United States.
The study regions involved the lower Chesapeake Bay where the dinoflagellate, Margalefidinium polykrikoides, develops harmful blooms almost every year, to the coastal waters of Atlantic and Gulf Stream off Cape Hatteras, and the New England shelf break (NES) frontal zone dominated by diatoms and cyanobacteria. Cyanate, a recently identified N cycle intermediate, was present in nanomolar concentrations in the lower Chesapeake Bay, and its variability was regulated by algal production, degradation, and sediment resuspension. Water and nutrient fluxes were strongly driven by tidal cycles, and wind speed and direction. In the coastal ocean off Cape Hatteras, total N uptake rates by phytoplankton were significantly higher in the Mid-Atlantic Bight than the South Atlantic Bight (SAB), likely due to co-limitation by phosphorus in the SAB. Among six N resources tested, nitrate and urea comprised >60% of the total N uptake. The NES front acted as an ecological boundary with more nitrifiers in the diatom hotspots of the Slope Sea than on the shelf, suggesting higher levels of N recycling than on the shelf. Several slope stations had unexpectedly high phytoplankton biomass due to the supply of deep-water nutrients via isopycnal lifting induced by Gulf Stream intrusion, which may have stimulated both ammonium regeneration and nitrification. Consequently, N cycling is strongly affected by a combination of physical and biological processes that create distinct oceanographic zones in western Atlantic Ocean.
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"Nitrogen Cycling in the Lower Chesapeake Bay and Mid- and South Atlantic Bight"
(2023). Doctor of Philosophy (PhD), Dissertation, Ocean & Earth Sciences, Old Dominion University, DOI: 10.25777/j7mb-9x76