Collaborative Research: Quantifying the Relationship between Nitrogen Uptake and Net Community Production using a Synthesis of JGOFS Data and 1-D Modeling

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Sambrotto The measurement of Net Community Production (NCP) is a robust estimate of the amount of organic material produced in surface waters of the ocean. NCP can serve as a starting point for models of carbon export from surface waters and the ocean's capacity to absorb carbon dioxide from the atmosphere. However, most models of ocean production are based on a limiting nutrient such as phosphorus or nitrogen and the reliability of this class of models therefore will depend on how accurately the consumption of limiting nutrients can be converted to the absorption of carbon dioxide by the ocean. The PIs will calibrate the relationship between NCP and new (nitrate) production in order to assist in the goal of predicting the role of the ocean as a carbon sink. The question of how to best convert between surface ocean elemental cycles will be examined with the a null hypothesis being that the stoichiometric oxidative ratios found in ocean thermocline waters (the Redfield ratios) are a quantitative way to translate between nitrogen and NCP. A comparative approach will be used to examine the extent of coupling between the surface nitrogen and carbon cycles. In instances where carbon and nitrogen appear to be uncoupled in the Redfield sense, possible methodological causes will be carefully evaluated. Following this analysis, the role of early bloom vs. nutrient limited conditions; dominant taxa; dissolved organic matter (DOM) levels; N2 fixation; iron limitation and other factors will be considered as agents modifying the relationship between nitrogen and carbon fluxes. The degree of coupling in representative ocean regions will be evaluated in a one-dimensional numerical model with realistic upper layer physics. These results will allow biogeochemical modelers to judge when the use of Redfield ratios for conversion is appropriate and identify what (if any) circumstances merit an alternate conversion approach. This will contribute to more accurate predictions of carbon flux that are sensitive to the relevant biological processes.