Dissolved iodine in the Changjiang River Estuary, China

The distribution and behavior of total dissolved iodine (TDI) and its species – iodate, iodide, and dissolved organic iodine (DOI) in the Changjiang River Estuary (CJE) surface and subsurface waters were studied along the salinity gradient. The results show that TDI concentration in the freshwater endmember of the CJE was 0.037 μM and exists as iodide. Although the transformation of dissolved iodine forms is active, TDI shows a conservative behavior, ranging from 0.037 to 0.42 μM in the estuary. Iodate shows removed behavior (ranging from 0 to 0.277 μM), iodide shows added behavior (ranging from 0.037 to 0.131 μM), while DOI shows both added (0 < salinity < 20) (ranging from 0 to 0.099 μM) and removed (20 < salinity < 33.5) behaviors (ranging from 0.099 to 0.022 μM). The iodine atoms in DOI are mainly supplied by iodide when salinity is <10 but by iodate when salinity is >10. The I/C ratio in DOI from different sources is more than 10 times different. The annual flux of iodine in the Changjiang River is 4.0 × 109 g, accounting for about 4% of the global river iodine flux.