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. Results showed that TDI concentration in the freshwater endmember of CJE was 0.037 μM and existed as iodide. Although the transformation of dissolved iodine forms was active, TDI showed a conservative behavior, ranging from 0.037 μM to 0.42 μM in the estuary. Iodate showed removal behavior (ranging within 0-0.277 μM), iodide showed additive behavior (ranging within 0.037-0.131 μM), whereas DOI showed additive (0 < salinity < 20) (ranging within 0-0.099 μM) and removal (20 < salinity < 33.5) behavior (ranging within 0.099-0.022 μM). The iodine atoms in DOI were supplied primarily by iodide when salinity was <10 but by iodate when salinity was >10. The iodine-carbon ratios in DOI from different sources were more than 10 times different. The annual flux of iodine in the Changjiang River was 4.0 × 10⁹g, accounting for about 4% of the global river iodine flux.