EAS Fall 2017 Seminar Series presents Dr. Angie Knapp, Florida State University

Identifying the spatial distribution of the largest di-nitrogen (N2) fixation fluxes to the ocean remains a critical goal of chemical oceanography because the answer informs our understanding of the environmental sensitivities of N2 fixation as well as the capacity for the dominant marine nitrogen (N) source and sink processes to respond to each other and thus influence the global carbon cycle and climate.

I will present results from studies in both the western and eastern South Pacific, regions of particular interest because of relatively low nitrate to phosphate concentration ratios in surface waters, as well as large gradients in the flux of atmospheric dust to surface waters. 

My work quantifies rates of N2 fixation and its importance for supporting export production using d15N budgets, however, 15N2 incubation-based N2 fixation rates as well as molecular studies were done in parallel by other groups in both field studies. N2 fixation rates in the eastern tropical South Pacific were very low, and did not support the “P*” hypothesis based on modeling of nutrient distributions and fluxes, and indicate that iron likely limits N2 fixation in the region. N2 fixation rates in the western tropical South Pacific were much higher at stations proximal to significant atmospheric dust sources, and at some stations supported a majority of export production, a unique result. 

Additionally, d15N budget-based N2 fixation rates were typically lower than 15N2 incubation-based N2 fixation rates, although molecular characterization of diazotroph abundance and activity agreed more closely with d15N budget-based N2 fixation rates.