[1] Shelf seas are an important global carbon sink. In the seasonal thermocline, the subsurface chlorophyll maximum (SCM) supports almost half of summer shelf production. Using observations from the seasonally stratified Celtic Sea (June 2010), we identify wind-driven inertial oscillations as a mechanism for supplying the SCM with the nitrate needed for phytoplankton growth and carbon fixation. Analysis of wind, currents, and turbulent dissipation indicates that inertial oscillations are triggered by a change in the wind velocity. High magnitude, short-lived dissipation spikes occur when the shear and wind vectors align, increasing the daily nitrate flux to the SCM by a factor of at least 17. However, it is likely that the sampling resolution of turbulent dissipation does not always capture the maximum wind-driven peak in mixing. We estimate that wind-driven inertial oscillations supply the SCM with~33% to 71% of the nitrate required for new production in shelf seas during summer. Citation: Williams, C., J. Sharples, C. Mahaffey, and T. Rippeth (2013), Wind-driven nutrient pulses to the subsurface chlorophyll maximum in seasonally stratified shelf seas, Geophys. Res. Lett., 40,[5467][5468][5469][5470][5471][5472]
A key parameter in determining the exchange of CO 2 across the ocean-atmosphere interface is the sea surface partial pressure of carbon dioxide (pCO 2 ). Temperate seasonally stratified shelf seas represent a significant sink for atmospheric CO 2 . Here an analytical model is used to quantify the impact of vertical mixing across the seasonal thermocline on pCO 2 . The model includes the impacts of the resultant dissolved inorganic carbon, heat, salt, and alkalinity fluxes on the solubility of CO 2 and the effect of the inorganic carbon sink created by the primary production fuelled by the flux of limiting nutrient. The results indicate that diapycnal mixing drives a modest but continuous change in pCO 2 of order 1-10 matm d21 . In quantifying the individual impacts of the fluxes of the different parameters, we find that the impact of the fluxes of DIC and nitrate fluxes dominate. In consequence, both the direction and magnitude of the change in pCO 2 are strongly dependent on the C:N uptake ratio in primary production. While the smaller impacts of the heat and salt fluxes tend to compensate for each other at midshelf locations, the heat flux dominates close to the shelf break. The analysis highlights the importance of the accurate parameterization of the C:N uptake ratio, the surface-mixed layer depth, and the TKE dissipation rate within the seasonal thermocline in models to be used to predict the air-sea exchange of carbon dioxide in these regimes. The results implicate storms as key periods of pCO 2 perturbation.
Lay Abstract One fundamental aim of marine science is to be able to accurately quantify and predict how mixing in the ocean can affect primary production and the global carbon budget. Shelf seas are the boundary between the coastal regions and the deep ocean. They are important areas for fisheries, as well as for the absorption of carbon from the atmosphere by microscopic organisms called phytoplankton. Phytoplankton living in the well lit surface layer of temperate shelf seas (between latitudes 23.5° and 66.5°) during summer rely on the turbulent supply of nutrients to sustain their growth. By using an instrument that is able to measure fine‐scale ocean currents, we are able to quantify mixing rates in the ocean. The chemical analysis of nutrients in seawater combined with these physical measurements allowed us to quantify the turbulent supply of nutrients from the deep ocean to surface water where phytoplankton live and thus estimate the importance of various mixing mechanisms to biological processes. In the western Irish Sea, we found that, for primary production to be maintained during summer, relatively large‐scale mixing events must take place to supply the nutrients required by phytoplankton. The background mixing rate does not supply sufficient nutrients to phytoplankton, and thus storms, enhanced tidal mixing, or both are likely to be vital in sustaining primary production in this marine ecosystem.
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