Nitrogen plays a dual role in coastal oceans. It sustains primary production, the foundation of coastal marine food webs, but can also cause degradation in the forms of eutrophication, harmful algal blooms, and hypoxia when supplied in excessive quantity from urban and agricultural land uses. In this dissertation, I examined the biogeochemical, land use, and hydrologic factors that regulate the concentration of dissolved inorganic nitrogen (DIN) in two nitrogen suppliers to the nearshore coastal ocean in the Santa Barbara, California region: watershed streams and beach pore water. In addition, I examined the influence of nearshore seawater and beach pore water dissolved organic matter (DOM) composition on the uptake of DIN by bacterioplankton, the primary decomposers in coastal oceans. I found that that the relationships between stream nitrate concentration and stream runoff were consistent within three broad land uses: agricultural, urban, and undeveloped.

As stream runoff increased, three agricultural watersheds showed nitrate dilution, two urban watersheds showed invariance, and one undeveloped watershed showed nitrate enrichment. The undeveloped upper mountainous watershed regions were dominant contributors of water and nitrate to stream runoff and nitrate across the whole watershed during periods of high runoff, such that the variability in stream nitrate concentrations during stormflow (rainstorms) was ~ 12% of the nitrate variability during baseflow (dry periods). In beach pore water, fresh groundwater with potentially elevated nitrate is largely precluded from discharge through beach sands by the dominant coastal bluff geomorphology of the region. However, tides circulate seawater into and out of beach sands, introducing marine particulate organic matter (POM), such as phytoplankton, giant kelp (Macrocystis pyrifera), and detrital material that is then degraded by heterotrophic microbes in beach sands to DOM and DIN.

I integrated pore water radon residence time distributions with DIN temporal evolution curves to calculate volume-weighted mean (VWM) DIN concentrations, which represented the DIN concentration of pore water discharging to the coastal ocean. I found that the combination of temporal variability in the residence time distribution due to spring-neap tidal cycles with the temporal evolution of DIN concentrations due to POM and DOM loading can result in substantial variability in pore water VWM DIN concentrations that should be accounted for in calculating DIN flux to coastal oceans. When pore water mixes with nearshore seawater upon discharge from the beach sands, DIN that was produced within beaches becomes available for uptake by bacterioplankton.

I used excitation emission matrix (EEM) fluorescence spectroscopy, respiratory quotient determinations (i.e., ratio of CO2 production to O 2 consumption), and DIN concentration analysis in a 7-day time-series dark incubation to determine how variability in DOM composition in seawater and pore water influenced DIN uptake by bacterioplankton in nearshore seawater. I found that mixing of seawater and pore water promoted greater DIN uptake than occurred in pore water alone, which is likely the result of the presence of more labile seawater DOM and/or production of more labile DOM under seawater and pore water mixing. Together, the studies described herein enhance our understanding of nitrogen dynamics in coastal marine regions, and thus can enhance coastal zone management and development practices that seek to preserve the functioning of coastal marine ecosystems.