Nitrogen (N) biogeochemical cycling in the marine environment is subject to complex biological and environmental feedbacks with unknown climate sensitivities. Past studies have attempted to characterize the marine N inventory using biogeochemical tracers, such as nutrient stoichiometry, nitrogen isotopes and N2 gas, but these studies have been a snapshot, focusing on a single process in a specific region of the ocean at a time. Such studies were forced to ignore other important and related controlling mechanisms. To resolve the magnitude and type of environmental controls on the processes that influence the N inventory, processes need to be considered simultaneously. Researchers from the University of California at Los Angeles propose to model the spatial distributions and rates of the primary N budget fluxes. In order to derive a global marine N budget, researchers propose to use an inverse model framework that is consistent with all available tracer data. The global ocean circulation model will contain biogeochemical cycles of tracers and will be constrained to optimize the fit to physical tracers (T, S, 14-C-age, CFCs). To examine how environmental variables affect the magnitude of each flux, they will be regressed against the modeling results, providing insight into the effects of anticipated climate change on the N budget. Furthermore, analysis of the inverse solution's error structure will allow researchers to examine the validity of tracer-related assumptions. This work could set up a statistically rigorous framework that will enable researchers to assimilate rapidly developing data streams and will highlight areas in the oceans where data is lacking, allowing resources to be used strategically. The project will provide education and training for undergraduate and graduate students, as well as a post-doc, and will provide important information and insights into the biogeochemical cycling of N necessary to understand the impact of changing climate on ocean processes.
The biological productivity of marine ecosystems are limited by the availability of nitrogen (N) across vast swaths of the ocean. The total amount of N in the ocean is largely controlled by specialized micro-organisms, whose activity can therefore regulate overall productivity. However the rates at which N is added and removed from the ocean are difficult to observe, leading to fundamental questions about whether the inputs and outputs can become out of balance. These questions can be greatly illuminated by looking at the chemical products of microbial activities that add or remove N, commonly referred to as "tracers". This collaborative project constructed an inverse model of the global ocean N cycle to provide the most definitive tracer-based estimates of major N fluxes and their sensitivities to environmental parameters. An inverse model takes observed quantities (e.g. the concentration of N) and makes quantitative inferences about the rates of input or output of that quantity, using information about how the ocean circulates. In the first two years, we tackled the N loss terms in the global budget, and constructed the first global 3D inverse model to derive those fluxes from a wide range of tracer data, including nutrients, isotopes, and gas ratios. The project also produced some unanticipated side-benefits, including a mechanistic model of particle fluxes that predicts a more dynamic P cycle than has been assumed, and large spatial variation in the oxygen-to-phosphate ratios in the subsurface ocean. A post-doc who was trained through the project has become a professor. The results of the research supported 12 peer-reviewed publications and over 20 presentations at scientific conferences and academic institutions.