Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Deborah Bronk


The logistical difficulties of research in extremely low temperatures and lack of access to the Arctic have meant that there is a historic dearth of knowledge of coastal Arctic biogeochemistry, especially during winter when sea ice is present. Recent observations, however, indicate that the Arctic is changing rapidly. Changes include increased temperatures, decreased extent and volume of sea ice, and increased freshwater inputs. How these changes influence biogeochemical cycles is an open question, especially in the highly productive coastal regions of the Chukchi Sea. Here I present nitrogen (N) uptake and regeneration rates for phytoplankton and bacteria measured in the shallow waters and landfast sea ice near Barrow, Alaska. Experiments were performed using tracer-level incubations of stable isotope (15N) ammonium, nitrate, urea, and amino acids during January, April, and August over two successive years (2010 --- 2012). Autotrophic versus heterotrophic N uptake was measured with traditional size fractionation. In brief, I found that outside of the spring phytoplankton bloom period, ammonium and amino acids were the preferred N substrates assimilated. Regeneration of N and nitrification were especially high during winter. A high-speed cell sorting flow cytometer was used to distinguish bacterial sub-populations and their uptake rates. Low nucleic acid populations were active in taking up N compounds, although not at quite the same rate as high nucleic acid cells. The difference was less pronounced during winter compared to summer. Additional experiments were designed to artificially warm the samples to demonstrate that ammonium uptake rates increased with temperature and substrate availability, whereas nitrification rates did not. Uptake and regeneration of ammonium and nitrate along with nitrification was also measured in landfast sea ice. This is the first report of N uptake from within the sea ice matrix in the Chukchi Sea. Given the paucity of information on N cycling in the Arctic Ocean, these data can inform modeling efforts to predict future changes in the system and also provide a baseline by which to compare future observations.



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