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Sarah Fawcett - About Me

Sarah Fawcett
In Cargèse, Corsica, at the surface ocean lower atmosphere study (SOLAS) summer institute.


I got my bachelor’s degree from Harvard University in 2006, majoring in Earth and Planetary Science. I was first introduced to marine applications of isotope geochemistry during the two summers I spent on the Great Barrier Reef in Australia, investigating geochemical proxies for sea surface temperature and El Niño recorded in 10,000 year-old corals. As a Ph.D. student my greater interest is in ocean productivity and the nitrogen cycle. Specifically, I use nitrogen isotopes as a tool to gain insight into the sources of nitrogen supporting phytoplankton growth, with a view to understanding whether phytoplankton diversity is important for ocean processes such as the sequestration of carbon in the ocean interior. 

Research interests 

Sources of nitrogen (N) supporting phytoplankton growth; N isotopes in the subtropical ocean; rates of N assimilation by different phytoplankton taxa; contribution of various phytoplankton taxa to export, and implications for the biological pump.  

Research summary

Photosynthesis by marine phytoplankton is thought to account for half of the oxygen in our atmosphere. Biologically available nitrogen (“fixed N”) is essential for photosynthesis and phytoplankton growth, yet much of the global ocean appears to be N limited. Thus, N availability, as well as the form of N that is available, limits marine productivity and drives phytoplankton community composition. Nitrogen is predominantly supplied to the surface ocean either as nitrate from depth or as ammonium recycled within the surface ocean. Phytoplankton growth on deep nitrate ( NO3) is termed “new production” and ammonium-supported growth is known as “regenerated production.” On an annual timescale, new production must be balanced by the export of sinking organic matter out of shallow waters, resulting in the sequestration in the ocean interior of atmospheric CO2 fixed as biomass. This storage mechanism is known as the ocean’s “biological pump”, and the extent to which different phytoplankton drive the biological pump in various regions of the global ocean is not clearly understood. By identifying the sources of N supporting various phytoplankton taxa, we gain insight into their importance for transporting atmospheric CO2 into the ocean interior. 

I use various geochemical techniques (e.g., flow cytometry coupled with natural abundance N isotope analysis, isotope tracer incubation experiments) to investigate the sources of N supporting phytoplankton growth, and to elucidate the contribution of different phytoplankton to organic matter export. I am also interested in quantifying N uptake rates by different phytoplankton groups, with a view to better understanding N transformations between different organisms, as well as larger scale N cycle dynamics. (visit my Research page)


Fawcett, S.E. , Lomas, M.W., Casey, J.R., Ward, B.B., Sigman, D.M., Assimilation of upwelled nitrate by small eukaryotes in the Sargasso Sea, Nature Geoscience 4, 717-722 (2011) Abstract | Dept. News Article

Fawcett, S.E., Ward, B.B. Phytoplankton succession and nitrogen utilization during the development of an upwelling bloom, Marine Ecology Progress Series 428, 13-31 (2011) Abstract

Department: Geosciences

Bess Ward
(with Daniel Sigman)

Position: Graduate Student

Area: Biological Oceanography

Office: M45 Guyot Hall

Phone: 609-258-6294