The distributions of nitrate reductase activity (NRA), iodate and iodide we
re determined in May 1996 in the southern East China Sea in a transect that
traversed across the upwelling center northeast of Taiwan where topographi
cally induced upwelling occurred as the Kuroshio interacted with the shelf
edge. Rationalized iodate (R-iodate or R-IO3- in nM) and iodide (R-iodide o
r R-I- in nM), the concentrations of iodate and iodide normalized to a sali
nity of 35, were strongly correlated to each other so that
[R-IO3-] = -1.06(+/-0.06)[R-I-] + 424(+/-15), r(2) = 0.87.
Since the slope was indistinguishable from -1 mol-IO3- mol(-1)-I-, this imp
lies that, if iodate is depleted in the surface oceans by biologically medi
ated uptake, it re-appears almost quantitatively as iodide so that little i
odate is retained in the particulate phase. In the upwelling zone, R-iodate
and R-iodide were also strongly correlated to NRA (in nM-N h(-1)) such tha
t
[R-IO3-] = -10.2(+/-1.0)NRA + 392(+/-12), r(2) = 0.84,
[R-I-] = 9.8(+/-1.1)NRA + 27(+/-5), r(2) = 0.76.
Since NRA is an indicator of nitrate uptake and the source water to the upw
elling zone: the Kuroshio Subsurface Water, is rich in iodate and almost de
void of iodide, these relationships are consistent with the conceptual mode
l that iodate reduction and nitrate uptake occur in tandem through the acti
vities of the enzyme nitrate reductase, Since NRA represents an instantaneo
us rate while [R-IO3-] and [R-I-] are concentrations, these strong correlat
ions also suggest that iodate reduction and hence nitrate uptake had operat
ed at approximately constant rates during the residence time of the water i
n the upwelling zone. Thus, the depletion of iodate and the gain in iodide
in the upwelled water relative to its source water were integrations of the
uptake and reduction of iodate over this residence time. The slope of the
relationship between R-iodate and NRA represented the product of this resid
ence rime and the biological discrimination of iodate reduction relative to
nitrate uptake by the enzyme. Outside of the upwelling area, R-iodate and
R-iodide did not correlate well with NRA, probably as a result of the maski
ng effect of mixing among water masses with undefined concentrations of pre
-existing iodate and iodide. (C) 2001 Elsevier Science Ltd. All rights rese
rved.