Diluting a salty soup: Impact of long-lasting salt pollution on a deep Alpine lake (Traunsee, Austria) and the downside of recent recovery from salinization

Saline emissions can elevate water densities and have the potential to alter stratification and mixing dynamics in lakes. In the light of rising anthropogenic salinization of temperate freshwater lakes, it is highly relevant to understand how saline emissions, as well as the rehabilitation from salt pollution might affect lake circulation patterns. In this study, we present the impact of industrial pollution with chloride salts on the deep Alpine lake Traunsee in Austria from 1930 to 2005 and the observed limnological changes during the recovery from salinization in the following 12years. We assembled and analysed a unique dataset of monthly sampling profiles covering chloride, conductivity, temperature and dissolved oxygen over a 87-year period. We quantified the impact of saline emissions on the lake's total chloride content, water densities, stability indices and deepwater oxygen concentrations. Time-series of water density profiles and stability indices revealed a significant impact of salinization on seasonal stratification and mixing. Higher dilution and shallower release of saline wastes and the short water retention in Traunsee helped to prevent a long-lasting density stratification. Nevertheless, two periods of salt-induced meromixis occurred in the lake. The first was caused directly by the disposal of saline wastes into the deepwater in the 1930s and 1940s, whereas the second was induced by the recovery from salinization in recent years. The naturally faster wash-out of salts from shallower water layers reinforced the density gradient and impeded vertical water circulation after salt pollution ceased. The rehabilitation from salinization had a stronger effect on stratification intensity and deepwater oxygen concentration than the continuous salt pollution over the last century.