Award: OCE-1554474

Nitrogen is an essential element for life. It’s availability at the surface ocean influences the fertility of ecosystems. The naturally occurring isotope ratios of nitrogen (15N/14N) are measured at the surface ocean as they provide insights into the sources and cycling of N. To infer processes from isotope ratios requires a prior knowledge of how different biological processes influence the isotope ratios. To this end, we conducted laboratory experiments to determine how marine phytoplankton fractionated 15N and 14N isotopes of ammonium, an important compound in the surface ocean. We observed that phytoplankton cease fractionating ammonium isotopes at the low concentrations germane to the surface ocean. We also infer that fractionation during assimilation occurs during transport, due to the deprotonation of ammonium. We can now interpret isotope ratios of ammonium in the environment. Pioneering measurements at the surface of the Southern Ocean reveal relatively low isotope ratios, corroborating our finding that assimilation does not fractionate the N isotope ratios. I developed a service-learning course for undergraduate seniors, wherein students were involved in research to determine the causes of water quality impairment in local coves, in collaboration with a group of citizen scientists (Clean Up Sounds and Harbor). The course is highly popular, introducing students to laboratory and field methods, as well as data analysis, interpretation and presentation. Last Modified: 06/29/2022 Submitted by: Julie Granger