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Iodine is a vital micronutrient for all mammals, including humans. Despite iodine's listing by the WHO as the world's most easily preventable cause of brain damage, there is very little known about iodine cycling in terrestrial freshwaters, particularly regarding temporal changes in speciation. This study presents iodine speciation (measured by IC-ICP-MS) data from one year of monthly sampling of a dimictic humic lake in the Black Forest (Mummelsee). Total soluble iodine (TSI) levels in the Mummelsee's water column averaged 1.9 ± 0.3 μg L-1. Soluble organically bound iodine (SOI) accounted for the majority of TSI in the lake (85 ± 7%) and inflow (76 ± 6%). SOI showed few changes throughout the year, despite stratification, anoxia in the hypolimnion and ice cover. Iodide was lost from the epilimnion during the summer, autumn and winter, which may be related to (micro)biological uptake and sedimentation of dead organic matter. Efficient biological uptake was also suggested by a sediment core, where we found high total iodine concentrations (av. 11.8 ± 1.7 mg kg-1) and a significant correlation with organic carbon content (p <0.001). In the hypolimnion a strong iodide flux was observed from the sediments into the water column during lake stratification and low DO levels. Based on I/Corg ratios in the sediment core, the iodide flux into the lake probably occurs by biological decomposition of organic material in the upper 20 cm of the sediments. Iodate levels in the epilimnion increased consistently over the year, whereas it was reduced below detection limits in the hypolimnion during anoxia. The winter turnover lead to reintroduction of oxygen into the hypolimnion and rapid formation of iodate and SOI. During the turnover event, near complete loss of iodide from the water column was also observed. SOI will play the largest role in iodine cycling in the Mummelsee; however, further studies are needed on the biological transformation of iodine species in freshwaters for a more complete understanding of iodine cycling

Iodine speciation and cycling in fresh waters: a case study from a humic rich headwater lake (Mummelsee)