Search

Abstract: Bromine was historically termed a cyclic salt in terrestrial freshwater environments due to its perceived conservative cycling between the oceans and the continents. This basic assumption has been challenged recently, with evidence that bromine is involved in dynamic chemical cycles in soils and freshwaters. We present here a study on dissolved bromine species (bromide, organically bound bromine, DOBr) concentrations and fluxes as well as sediment trap bromine levels and fluxes in Lake Constance, a large lake in southern Germany. Water samples were obtained from all major and some minor inflows and outflows over one year, where-after dissolved bromine species were measured by a combination of ICP-MS and ion chromatography coupled to an ICP-MS (IC-ICP-MS). Sediment traps were deployed at two locations for two years with Br, Ti and Zr levels being measured by μ-XRF. 190tyr−1 of total dissolved bromine (TDBr) was delivered to the lake via 14 rivers and precipitation, with the rivers Alpenrhein (84tTDBryr−1) and the Schussen (50tTDBryr−1) providing the largest sources. The estimated particulate bromine flux contributed an extra 24–26tBryr−1. In comparison, only 40tTDBryr−1 was deposited to the lake’s catchment by precipitation, and thus ∼80% of the riverine TDBr flux came from soils and rocks. Bromide was the dominant species accounting for, on average, 78% of TDBr concentrations and 93% of TDBr flux to the lake. Despite some high concentrations in the smaller lowland rivers, DOBr was only a minor component of the total riverine bromine flux (∼12tyr−1, 7%), most of which came from the rivers Schussen, Bregenzer Ach and Argen. In contrast, most of the bromine in the sediment traps was bound to organic matter, and showed a clear seasonal pattern in concentrations, with a maximum in winter and minimum in summer. The summer minimum is thought to be due to dilution of a high Br autochthonous component by low bromine mineral and organic material from the catchment, which is supported by Ti, Zr and Br/Corg data. In the lake bromine was irreversibly lost to the sediments, with best flux estimates based on mass-balance and sediment trap data of +50–90μgBrm−2 d−1. Overall, it appears that bromine is not simply a cyclic salt in the case of Lake Constance, with a clear geological component and dynamic lacustrine biogeochemistry. [Copyright &y& Elsevier]

Abstract: 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 12month 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 (1–2μgl−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 (∼2–10μgl−1) increased over the summer. Deposition of TSI to the catchment (16,340kg I yr−1) was similar to the TSI out-flux by rivers (16,000kg I yr−1). By also including the particulate riverine iodine flux out of the catchment (∼12,350kg 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 16,770kg 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 (−2050kg I yr−1; 12% of TSI in-flux). To maintain mass-balance, 8.8μg I m−2 d−1 from the Obersee and 23μg 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 (4762–8075kg 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 10,290kg I yr−1 (64% of TSI input), followed by iodate (3120kg I yr−1) and iodide (2760kg I yr−1). Net formation of SOI from iodide and iodate was also noted within the lake, with an estimated production of 6560kg 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 (<2mgl−1), and thus is expected to play an important role in iodine cycling in most freshwater environments. [Copyright &y& Elsevier]