Bromine species fluxes from Lake Constance’s catchment, and a preliminary lake mass balance

• Published in: Geochimica et cosmochimica acta 2011-06, Vol.75 (12), p.3385-3401
• Main Authors: Gilfedder, B.S., Petri, M., Wessels, M., Biester, H.
• Format: Article
• Language: English
• Subjects: biogeochemistry; Bromine; Catchments; Dissolution; Fluxes; Freshwater; ion exchange chromatography; Lakes; oceans; organic matter; Rivers; rocks; seasonal variation; Sediments; soil; Soils; summer; watersheds; winter; zirconium
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2011.03.021

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. 190 t yr −1 of total dissolved bromine (TDBr) was delivered to the lake via 14 rivers and precipitation, with the rivers Alpenrhein (84 t TDBr yr −1) and the Schussen (50 t TDBr yr −1) providing the largest sources. The estimated particulate bromine flux contributed an extra 24–26 t Br yr −1. In comparison, only 40 t TDBr yr −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 (∼12 t yr −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/C org data. In the lake bromine was irreversibly lost to the sediments, with best flux estimates based on mass-balance and sediment trap data of +50–90 μg Br m −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.