Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Deborah A. Bronk


Traditional measurements of phytoplankton N uptake have been confounded by bacterial retention on filters used in 15N uptake studies, and such methodological obstacles have limited our understanding of phytoplankton-bacterial interactions regarding N cycling. In this research, uptake of various inorganic and organic N substrates by phytoplankton and bacteria was measured in several marine ecosystems using two distinct approaches: size fractionation into phytoplankton and bacterial size classes, and flow cytometric (FCM) sorting of autotrophic cells. Comprehensive assessments of N uptake dynamics were conducted in Chesapeake Bay, the Mid-Atlantic Bight, and Raunefjord, Norway, with supplementary data collected from the York River, Virginia and the Gulf of Mexico. In Chesapeake Bay, the composition of the dissolved N pool shifted from being dominated by dissolved inorganic N (DIN) in the upper bay to mostly dissolved organic N (DON) in the lower bay. Accordingly, phytoplankton nitrate uptake was highest near the head, whereas uptake of urea and dissolved free amino acids generally increased southward. Nonetheless, ammonium was the dominant form of N used by phytoplankton and bacteria throughout the bay. In the Mid-Atlantic Bight, the surface layer was devoid of DIN but ambient urea concentrations were relatively high and this organic substrate supported a large majority of total measured N uptake. The dissolved N pool in the bottom water consisted of about two-thirds DIN, with ammonium contributing most to total uptake. Bacteria were especially active in the bottom water and contributed over half of the total DIN uptake, and there was evidence of bacterial urea uptake in the surface water. In Raunefjord, a mesocosm approach was used to examine N uptake by a bloom of colonial Phaeocystis as well as the competition between phytoplankton and bacteria for limited N resources. Despite amending with nitrate, ammonium was the primary N form supporting the bloom. In the unfertilized mesocosm, bacteria were responsible for about half the urea uptake, most of the DFAA uptake, and at least a third of DIN uptake. Overall, total dissolved N concentrations and total N uptake decreased from estuarine to oceanic waters, although uptake rates were highly variable within each ecosystem. The reduced N forms, ammonium and urea, were most important to phytoplankton N nutrition, and contrary to traditional belief, urea at times played an important role in bacterial N uptake. With respect to methodological approaches, traditional filtration resulted in significant overestimation of phytoplankton N uptake due to the inclusion of, and 15N enrichment in, bacterial biomass retained on filters. This research represents the first comprehensive assessment of phytoplankton-specific N uptake across various ecosystems. It highlights not only the need for careful qualification of uptake rates measured using traditional approaches, but also the potential application of FCM sorting to more detailed examination of N uptake by phytoplankton in general, but also by specific taxa in various marine ecosystems.



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