Seminar

Abstract:

Nitrogen limits productivity in most of the ocean. To understand how nitrogen flows in a system is crucial in many aspects. Due to multiple micro-organism’s competing metabolism, nitrogen cycles rapidly through different pathways. In this talk, I will prepare you two cases of nitrogen dynamics. First one is about 15N labeling technique to detect nitrogen flows in an incubation bottle filled with oligotrophic seawater. Recent advances in 15N measurements allows us to examine multiple pathways of simultaneously-occurring nitrogen processes in oligotrophic ocean. Experimental results showed that community succession may happen in a bottle within 24 hours leading to biased rate measures. Proper estimators of rate of multiple nitrogen processes will be presented. The second case is about the application of nature abundance of 15N in a plume system under the influence of complex hydrodynamics. An endmember mixing model coupled with isotope mass conservation, nitrogen flows and associated biological processes were discerned. A conceptual diagram, which is wide applicable to estuary and plume systems, was constructed according to field observations.

Abstract:

Oceanic mesoscale eddies with typical sizes of 30-200 km contain more than half of the kinetic energy of the ocean. With an average lifespan of several months, they are major contributors to the transport of heat, nutrients, plankton, dissolved oxygen and carbon in the ocean. Mesoscale eddies have been observed and studied over the past 50 years, nonetheless our understanding of the details of their structure remains incomplete due to lack of systematic high-resolution measurements. To bridge this gap, a survey of a mesoscale anticyclone was conducted in early 2014 in the South China Sea capturing its structure at submesoscale resolution. By modeling an anticyclone of comparable size and position at three horizontal resolutions the authors verify the resolution requirements for capturing the observed variability in dynamical quantities, and quantify the role of ageostrophic motions on the vertical transport associated with the anticyclone. Results indicate that different submesoscale processes contribute to the vertical transport depending on depth and distance from the eddy center, with frontogenesis playing a key role. Vertical transport by anticyclones cannot be reliably estimated by coarse-resolution or even mesoscale-resolving models, with important implications for global estimates of the eddy-driven vertical pumping of biophysical and chemical tracers.