Nitrate supply and phytoplankton uptake kinetics in the euphotic layer of a Gulf Stream warm-core ring

Chcmiluminescent nitrate analysis was used in conjuction with 15N-labeled B = − B 0 e − 2 π a 0 x k to assess the rates of B = − B 0 e − 2 π a 0 x k uptake by phytoplankton in warm-core ring 82B. The relatively high precision of this method compared to conventional B = − B 0 e − 2 π a 0 x k analyses permits reliable estimates of B = − B 0 e − 2 π a 0 x k uptake in oligotrophic waters. Aggregation of uptake data from six profiles from 2 days of observation permitted the calculation of B = − B 0 e − 2 π a 0 x k turnover times ranging from about 4 h near the surface to 150 h at the top of the nitracline. Turnover times in the euphotic zone and the observed half saturation constant of 93 nmol kg −1 for B = − B 0 e − 2 π a 0 x k uptake imply nitrogen limitation for these populations. Extrapolation from the linear portion of the kinetic curve revealed that a B = − B 0 e − 2 π a 0 x k threshold concentration of about 16 nmol kg −1 was required for the initiation of uptake. These highly precise uptake measurements were used in a one-dimensional model to estimate the vertical flux of B = − B 0 e − 2 π a 0 x k . Maximum near-surface and deep-euphotic-zone eddy diffusivity values ( K z)were 3 × 10 −3 and 5 × 10 −4 m 2 s −1, respectively, prior toamajor storm. Following the storm K z values were substantially greater.