Iodide and iodate concentrations in eastern Australian subtropical waters, with iodide by ion chromatography

Abstract

Iodide and iodate concentrations were measured in the subtropical waters along the continental shelf and slope waters of eastern Australia. A thermocline between 50–100 m separated Coral Sea waters from Subtropical Lower Water on all transects. There was a sharp change in the iodide and iodate concentrations at the thermocline. The concentration of iodide in the shelf waters between 0 and 50 m ranged from 0.07 to 0.18 μM and ranged from 0.01 to 0.03 μM below a depth of 100 m. Iodate concentrations showed the inverse trend to iodide, ranging from 0.32 to 0.39 μM in the top 50 m, increasing to 0.46 μM below a 100 m. The ratio of iodate to phosphate in the Coral Sea waters was about eight times greater than deeper Subtropical Lower Water. This suggested that either (1) processes other than biological assimilation dominate in the Coral Sea, or (2) biological assimilation is greater in the Coral Sea, and more iodate is reduced to iodide, but this form of iodine does not enter the uptake cycle again as readily as phosphate, or (3) disparate paths for the biological cycling of iodine operate in different water bodies. Iodate was reduced to iodide in Coral Sea surface waters in the presence of both low concentrations of phosphate and nitrate. However, the reduction was for the most part effected before nitrate departed appreciably from its Redfield relationship with phosphate to become nutrient limiting.