HMSC Research Seminar: Shelf Nitrate Response to Upwelling on the Newport Hydrographic Line

Speaker: Andrew Scherer, PhD Student, College of Earth, Ocean and Atmospheric Science, Oregon State University

Topic: The Northern California Current System supports a productive marine ecosystem through wind-driven coastal upwelling of nitrate. Model studies show the amount of nitrate upwelled into the euphotic zone depends on the interaction of the cross-shelf flow and nitrate profiles, which vary on upwelling event and seasonal timescales; the long-term nitrate measurements necessary to observe such effects have only recently become available. We first develop an algorithm for the quality control of a decade of daily in situ nitrate profiles provided by the NSF Ocean Observatories Initiative Endurance Array and, alongside validation via ship-based bottle samples, provide a curated nitrate dataset suitable for future investigations. This nitrate dataset is then used with decades of physical oceanographic data collected on the Newport Hydrographic Line off Oregon, USA to investigate the response of continental shelf nitrate concentration to wind. The composite mean response of the cross-shelf nitrate flux, averaged over dozens of upwelling-favorable wind events, reveals a strong evolution of the near-bottom flux. A seasonal cycle is also evident in the near-bottom cross-shelf nitrate flux response during upwelling events, associated with a similar seasonal cycle in the monthly mean nitrate concentration. We estimate the cross-shelf nitrate flux and nitrate tendency terms in the continental shelf nitrate budget. The residual in this budget is often comparable in size to the estimated terms. The timing of changes in the residual when composited over upwelling events suggests the along-shelf nitrate flux divergence or biological nitrate flux are also important. The depth-averaged inner-shelf nitrate concentration is well correlated with cross-shelf nitrate flux and along-shelf wind stress. An analytic 2-D upwelling model reproduces 43% of the variance in inner-shelf nitrate based only on along-shelf winds.