Changes in iodine speciation across coastal hydrographic fronts in southeastern United States continental shelf waters

Abstract

The hydrographic front that separates the turbid inner shelf water from the “clearer” midshelf water on the southeastern U.S. continental shelf is accompanied by dramatic changes in the speciation of iodine. The total iodine to salinity ratio, or specific iodine, in the inner shelf was slightly lower, usually about 10% lower, than that in the midshelf, which was in turn slightly lower than those observed in the open oceans. This suggests that the efficiency for the removal of dissolved iodine to the particulate phase may increase progressively from the open ocean to the midshelf water to the inner shelf water. On the other hand, the average concentration of iodate increased by a factor of about three from the inner shelf to the midshelf water while the concentration of iodide decreased by about 30%. Above a concentration of iodate of about 0.05 μM, the concentration of iodide was inversely related to that of iodate with a ratio of about −1 suggesting that the in situ reduction of iodate to iodide may play an important role in determining the distribution and speciation of iodine in these waters. The relationship between the concentration ratio of iodate to iodide and the concentration of nitrate plus nitrite also falls into a definable pattern. Waters with high concentration ratios of iodate to iodide (>0.5) were found mostly when the concentration of nitrate plus nitrite fell below 0.5 μM. Above this concentration of nitrate plus nitrite, the iodate to iodide ratio was almost invariably below 0.5.