Award: OCE-1829947

Understanding nitrogen (N) cycling in the ocean, and identifying the nitrogen sources that support marine food webs, is essential for better understanding how nutrients cycle, how food webs function, and how ocean ecosystems operate. The structure of ocean food webs—essentially, who eats whom—affects how efficiently energy reaches higher-level consumers (like fish) and how carbon is transported to the deep sea. This project used an advanced technique called Compound Specific Isotope Analysis of Amino Acids (CSIA-AA) to study nitrogen cycling processes in the open ocean. Some ("source") amino acids experience small nitrogen isotope values enrichment as they move through the food web, while others ("trophic" amino acids) show relatively significant changes at each trophic step in the food chain. This method allows researchers to identify the original nitrogen sources supporting the food web and estimate the number of steps in the food chain within particles produced by marine communities.

The study focused on the Equatorial Tropical North Pacific (ETNP) Oxygen Deficient Zone (ODZ)—an area with extremely low oxygen levels. During a research cruise on the R/V Sally Ride, we collected sinking and suspended particles using McLane in situ pumps and sampled zooplankton with a MOCNESS net system. We discovered that in regions with a secondary chlorophyll maximum (SCM), both suspended and sinking particles had lower amino acids nitrogen isotope values, suggesting that different types of primary producers relying on recycled nitrogen as their nutrient sources and such primary producers also contributed to particle export. Furthermore, in the ODZ, the breakdown of small particles by microbes was slower than in well-oxygenated waters.

These findings highlight the need to further investigate the roles of zooplankton and chemoautotrophs in particle export within ODZs, which may have a significant impact on nutrient and carbon cycling.

Project Results were presented in a Ph.D. dissertation consisting of four manuscripts, with publications in Rapid Communications in Mass Spectrometry and Global Biogeochemical Cycles. Researchers in this project delivered 23 presentations at international conferences such as the American Geophysical Union (AGU) and the Association for the Sciences of Limnology and Oceanography (ASLO). Project data are publicly available through the BCO-DMO website (https://www.bco-dmo.org/project/778644).

The project significantly contributed to training and education, with a focus on underrepresented groups: A female doctoral student completed her dissertation and successfully defended in November 2024. Additionally, a master’s student from an underrepresented group completed his thesis in the same month. Over 40 undergraduates participated in research activities, including many from McNair Scholars and Louis Stokes Alliances for Minority Participation (LSAMP) programs. These students gained valuable experience in isotope analysis, data interpretation, and experimental design. The doctoral student also completed extended training visits to a collaborator’s lab to gain hands-on experience with isotope ratio mass spectrometry.

This project advanced knowledge of nitrogen cycling in low-oxygen ocean environments and emphasized the role of particle dynamics in global biogeochemical cycles. By training a diverse group of students and sharing results through publications and conferences, the project promoted innovation, inclusion, and collaboration in the field of marine science.

Last Modified: 12/18/2024
Modified by: Lin Zhang