Biologically available nitrogen (“fixed N”) is the limiting nutrient in much of the modern ocean. Changes in the distribution and fluxes of N are of growing interest in research at the interface of climate, biogeochemistry, and ecosystem function. For example, temporal changes in the global ocean inventory of fixed N could generate changes in the strength of the ocean’s “biological pump” (i.e., the biologically-drive sequestration of atmospheric CO2 in the deep ocean) large enough to contribute significantly to glacial/interglacial variations in atmospheric CO2. Additionally, N is one of the two major nutrients required universally by phytoplankton (the microscopic plants living in the sunlit surface ocean that are responsible for ~50% of global photosynthesis), and the only one with two stable isotopes. The oceanic budget of N, more so than any other nutrient, is dominated by biologically-controlled inputs and outputs; indeed, N biogeochemistry, which is an integral component of coupled ocean-climate models, cannot be understood or predicted without consideration of ocean biology.
Broadly, my research is focused on the relationships between N fluxes and primary productivity in the ocean, knowledge essential to our understanding of modern and past ocean-climate interactions and to our ability to predict the ocean’s response to perturbation (e.g., climate change). My work is largely focused on the stable isotopes of dissolved and particulate N forms in natural waters (e.g., nitrate, nitrite, particulate organic N, dissolved organic N), which provide an integrative view of biogeochemical and physical processes that are highly variable in time and space.