An iodine mass-balance for Lake Constance, Germany: Insights into iodine speciation changes and fluxes

• Published in: Geochimica et cosmochimica acta 2010-06, Vol.74 (11), p.3090-3111
• Main Authors: Gilfedder, B.S., Petri, M., Wessels, M., Biester, H.
• Format: Article
• Language: English
• Subjects: Fluxes; Iodates; Iodides; Iodine; Lakes; Rivers; Sediments; Speciation
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2010.03.008

Lake Constance is one of Europe’s largest oligotrophic lakes and provides a water source for more than 4.5 million people in Germany and Switzerland. We present here a 12 month study on iodine concentrations, speciation and fluxes to and from the lake to gain a quantitative understanding of the limnic iodine cycle. Monthly water samples were obtained from all major tributaries (14) and the outflow to construct a mass-balance model. Sediment traps were also deployed in the lake for two years at two different stations. Total soluble iodine (TSI) in aqueous samples were analysed by ICP-MS and speciation (iodide, iodate and soluble organically bound iodine, SOI) by ion chromatography–ICP-MS. Iodine concentrations in the Alpine tributaries (1–2 μg l −1) decreased over the summer months due to increasing proportions of snow and glacial melt water from the Alps, while iodine levels in the lowland rivers (∼2–10 μg l −1) increased over the summer. Deposition of TSI to the catchment (16,340 kg I yr −1) was similar to the TSI out-flux by rivers (16,000 kg I yr −1). By also including the particulate riverine iodine flux out of the catchment (∼12,350 kg I yr −1) it is shown that the catchment is a net source of iodine, with the highest particulate fluxes coming from the Alpine rivers. The total TSI flux to the lake was 16,770 kg I yr −1, the largest proportion coming from the Alpenrhein (43%), followed by the Schussen (8%) and Bregenzer Ach (7.7%). Overall the mass-balance for TSI in the lake was negative, with more iodine flowing out of the lake than in (−2050 kg I yr −1; 12% of TSI in-flux). To maintain mass-balance, 8.8 μg I m −2 d −1 from the Obersee and 23 μg I m −2 d −1 from the Untersee must be released from the sediments into the water column. Thus, in comparison with the total iodine flux to the sediments measured by the sediment traps (4762–8075 kg I yr −1), up to 39% of the deposited iodine may be mobilised back into the lake. SOI was the dominant iodine fraction entering the lake, with a total flux of 10,290 kg I yr −1 (64% of TSI input), followed by iodate (3120 kg I yr −1) and iodide (2760 kg I yr −1). Net formation of SOI from iodide and iodate was also noted within the lake, with an estimated production of 6560 kg I yr −1, suggesting a strong role for biology in iodine cycling. In conclusion, organically bound iodine was the dominant iodine species in aqueous and solid phases in Lake Constance, despite low DOC concentrations (