A Lagrangian SF6 tracer study of an anticyclonic eddy in the North Atlantic:: patch evolution, vertical mixing and nutrient supply to the mixed layer

Biological and biogeochemical change in the surface mixed layer of an anticyclonic eddy at 60 degreesN in the North Atlantic were monitored within a Lagrangian time-series study using the tracer sulphur hexafluoride (SF6). Four ARGOS buoys initially released at the patch centre remained closely associated with the SF6 patch over a 10-day period, with the near-circular eddy streamlines contributing to the stability and coherence of the patch. Progressive deepening of the surface mixed layer was temporarily interrupted by a storm, which increased mixed-layer nitrate and accelerated the transfer of SF6 to the atmosphere. Diapycnal exchange of SF6 was relatively rapid due to the shallow pycnocline gradient, and a vertical eddy diffusivity (K-z) of 1.95 cm(2) s(-1) at the base of the mixed layer was estimated from vertical SF6 profiles at the patch centre. Application of K-z to the nutrient gradients inferred vertical nitrate and phosphate fluxes of 1.8 and 1.25 mmol m(-2) d(-1), respectively, for the pre-storm period, which accounted for 33% and 20% of the reported in vivo uptake rates. Integration of the vertical nitrate flux and decline in surface layer nitrate suggest a total loss of 0.54 mmol N m(-3) d(-1) during the 5-day pre-storm period, of which in vivo nitrate consumption only accounted for 49%. Vertical transport of ammonium regenerated in the pycnocline accounted for up to 25% of in vivo phytoplankton uptake. The results suggest that the contribution of vertical turbulence to the mixed-layer nutrient pool was less important than that recorded in other regions of the open ocean, inferring that advective processes are more significant in an eddy. This study also emphasises the potential of SF6 for oceanic Lagrangian time series studies, particularly in dynamic regions, and in constraining estimates of new production. (C) 2001 Elsevier Science Ltd. All rights reserved.