Award: OCE-2219973

In the North Pacific Subtropical Gyre, the physical supply of nutrients to the mixed layer (ML) is relatively small, however, periodic injections of nitrate into the euphotic zone have been shown to stimulate eukaryotic phytoplankton blooms and enhance carbon export. To investigate the response of eukaryotic phytoplankton to inputs of new (e.g., nitrate) and recycled (e.g., ammonium) nitrogen (N) sources, we conducted bioassay experiments using seawater collected from within the ML and at the deep chlorophyll maximum (DCM) during spring and late-summer. Various N regimes were simulated using assorted N amendments (as either ammonium, urea, nitrate or filtered deep seawater). Physiological and metatranscriptomic responses of dominant phytoplankton groups were assayed. We found that smaller cells  (<3 μm in spring and <5 μm in late-summer) were responsible for a substantial fraction of primary productivity and generally utilized recycled N in the ML while larger cells had a more appreciable contribution to productivity at the DCM and relied heavily on new N. Gene expression of the ML and DCM communities differed, with the ML phytoplankton expressing genes related to N acquisition regardless of N availability while phytoplankton at the DCM increased expression of specific genes in response to N inputs. For example, in the ML, haptophytes and dinoflagellates expressed nitrate transporters and reductases regardless of nitrate availability while at the DCM, these groups generally only expressed these genes following nitrate addition. This study provides evidence for niche partitioning between the two layers: low-N supply, small cell dominance and constitutive gene expression in the ML contrasts pulsed N supply, large cell dominance and responsive gene expression at the DCM. These results highlight how different N supply mechanisms and N source availability shape phytoplankton communities and their contributions to global carbon export.

As a part of this project, one post-doctoral fellow and one PhD student were fully or partially supported, and one undergraduate student was trained. Educational activities that were part of this project include participation in numerous public outreach events, and education conferences and workshops. Several scientific papers related to this project are in the final stages of preparation.

Last Modified: 01/29/2026
Modified by: Adrian Marchetti