Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Deborah Bronk


The goal of this study was to determine the significance of dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON) to harmful phytoplankton. Two regions that experience frequent and persistent harmful algal blooms (HABs) were examined, the Eastern Gulf of Mexico and York River, Virginia. Nitrogen uptake by harmful algae in these regions was examined using a combination of stable isotopic (15N) tracer techniques and nutrient bioassays. In the Eastern Gulf of Mexico, kinetic parameters for uptake of N substrates by K. brevis were determined and indicated the greatest preference for ammonium (NH4+), although all substrates tested were taken up. Investigation of N uptake by K. brevis in the light and dark revealed periodicity of diet uptake rates with the maximum and minimum rates measured early in the light and dark periods, respectively. The highest rates of uptake were observed for NH4+. Ratios of NH4+ regeneration:uptake were ∼1, indicating the importance of regeneration processes to blooms. Three strains of K. brevis exhibited significant differences in N uptake rates. The observed nutritional flexibility of K. brevis likely helps it flourish under a range of conditions spanning bloom initiation in oligotrophic offshore waters to bloom maintenance inshore. In the York River, N uptake was dominated by NH4+ with the highest uptake rates at all stations, for all size fractions and for all seasons, ranging from 34 to 80% of total absolute uptake. Rates of N uptake by A. monilatum are the first reported for this species in the York River, and demonstrate uptake of a diverse suite N substrates. During the A. monilatum bloom NH4+ regeneration rates equaled those of uptake, indicating the importance of regeneration to blooms. Additionally, three anthropogenic N sources were used to assess their role in exacerbation of a HAB during a 7 day bioassay. Urban parking lot run-off (+ Urban), soil from a construction site (+ Soil) and paper mill run-off (+ Industrial) were added to a natural bloom assemblage. Results indicated the anthropogenic sources had unique N compositions; DIN comprised ∼9%, 91% and 20% of + Urban, + Soil, and + Industrial, respectively. All N sources stimulated the growth of phytoplankton with the + Urban and + Soil treatments eliciting the greatest response, a doubling in Chl a and/or cell concentrations along with nutrient drawdown of both DIN and DON within two days. The results of this dissertation emphasize the importance of a flexible metabolism to the success of the HAB species investigated here. All harmful phytoplankton studied were able to utilize the variety of DIN and DON sources supplied. Additionally, a universal preference for NH4+ was observed in all studies despite the distinct regions examined and unique characteristics of each species.



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