Date Awarded

Summer 2021

Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Deborah A Bronk

Committee Member

Rachel Simpler

Committee Member

Juliette Smith

Committee Member

Willy Reay


Nitrogen (N) is one of the essential building blocks for all life and is available in the form of dissolved N in aquatic ecosystems. It is important to understand how this N can support primary and secondary production mediated by phytoplankton and bacteria, respectively, as it can affect both microbial loop biogeochemistry and the higher trophic levels of food webs. Nitrogen studies have traditionally focused on dissolved inorganic N (DIN) as a labile N source. Dissolved organic N (DON), while still often considered refractory, has been increasingly recognized as an important N source supporting primary and secondary production. However, the inclusion of DON into uptake studies is still limited. Expanding N research to encompass DON will be important as researchers continue to assess how nutrient cycles respond to a changing climate. The goal of this dissertation was to expand the understanding of how phytoplankton and bacteria use N by investigating uptake rates of a suite of DIN and DON substrates in two different ecosystems. Research for this dissertation was conducted in the York River, VA and the coastal Alaskan Arctic. In both systems, nutrient uptake rates were measured using 13C and 15N stable isotopes for N and carbon (C) substrates. In the York River, N uptake (>0.3 µm size class) was investigated in alternating months during a period of elevated precipitation. Ammonium (NH4+) uptake was found to be the greatest, but urea uptake was elevated relative to other substrates in late fall. Rates of NH4+ regeneration were lower than measured uptake rates, which indicates that autochthonous production was insufficient and allochthonous sources were needed to meet the N demand. Finally, this study also reported the rates of NH4+ release from urea, finding that urea provided minimal NH4+, averaging <1% of NH4+ needed to support measured NH4+ uptake rates. Further study in the York River used 16S rDNA sequencing to determine if wastewater effluent with different DIN and DON content affected the composition and diversity of the microbial communities in receiving waters. Overall, addition of minimally treated effluent with high DIN lowered microbial diversity, while exposure to more heavily treated effluents resulted in communities that were more similar to the control community without effluent addition. In the Alaskan Arctic, late season N uptake was investigated through 15N and 13C substrate incubation experiments in the Chukchi and Beaufort Seas over two summers for the >0.3 µm size class. During these experiments, urea uptake was often greater than nitrate, but NH4+ was taken up at the highest rate. Differing sea-ice conditions were also found to support different rates of NH4+ regeneration and C uptake. Collectively, the results of this dissertation demonstrate that while DIN is the form of N primarily used in coastal and marine ecosystems, DON can be an important nutrient source to aquatic microbial communities. Future studies should aim to incorporate DON substrates as both nutrient cycling and community composition will likely continue to shift as anthropogenic activity alters ecosystems.




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