Speciation of dissolved iodine: integrating nitrate uptake over time in the oceans

Abstract

The distributions of nitrate reductase activity (NRA), iodate and iodide were determined in May 1996 in the southern East China Sea in a transect that traversed across the upwelling center northeast of Taiwan where topographically induced upwelling occurred as the Kuroshio interacted with the shelf edge. Rationalized iodate (R-iodate or R-IO 3 − in nM) and iodide (R-iodide or R-I − in nM), the concentrations of iodate and iodide normalized to a salinity of 35, were strongly correlated to each other so that [R-IO 3 − ]=−1.06(±0.06)[R-I − ]+424(±15), r 2=0.87. Since the slope was indistinguishable from −1 mol-IO 3 − mol −1-I −, this implies that, if iodate is depleted in the surface oceans by biologically mediated uptake, it re-appears almost quantitatively as iodide so that little iodate 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 that [R-IO 3 − ]=−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 upwelling zone, the Kuroshio Subsurface Water, is rich in iodate and almost devoid of iodide, these relationships are consistent with the conceptual model that iodate reduction and nitrate uptake occur in tandem through the activities of the enzyme nitrate reductase. Since NRA represents an instantaneous rate while [R-IO 3 −] and [R-I −] are concentrations, these strong correlations also suggest that iodate reduction and hence nitrate uptake had operated at approximately constant rates during the residence time of the water in 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 residence time 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 masking effect of mixing among water masses with undefined concentrations of pre-existing iodate and iodide.