Is the Large Aral Sea turning meromictic? Tracing the ongoing changes in the physical and biogeochemical regime of the world’s most dynamic lake ecosystem (Large Aral and Meromixis)

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Anthropogenic water use and climate change may have drastic negative consequences on lake ecosystems. The most striking example of the fragility of inland waters, “the ecological disaster of the century”, is the Aral Sea. The consequences of the Aral Sea desiccation have not been fully estimated to date. A new finding of the project team, central to the proposed study, was the recent discovery of meromictic (permanent) stratification starting to develop in the largest remaining basin of the Sea, the Large Aral. Our observations in 2015-2018 suggest that the lake is currently undergoing a transitional period of “meromictisation”, accelerated by the water regulation in the inflowing rivers and by climate-driven changes in the regional water budget. Such a transition suggests acute changes in the biogeochemical regime, some of them, such as the rapid increase of methane concentrations in the lake, have already been registered by the project team. In order to quantify the mechanisms of meromictisation and assess its consequences at regional scales, we propose to apply a set field research methods—monitoring of stratification and the oxygen regime, direct estimations of microbial activity, methane concentrations and emissions together with turbulent mass transfer across the water column, stable isotope studies of the water balance—complemented by modeling and remote sensing observations. The present situation in the Large Aral allows us to trace the unprecedented lake ecosystem changes caused by the major threats to inland waters worldwide: increasing water use and climate-driven alteration of the water budget. As such, a timely and detailed investigation of the transformations in the Large Aral ecosystem would provide a “worst-case” scenario for other large terminal lakes of arid regions, threatened by the same anthropogenic drivers. The expected outcome of this project is a quantification of the present mixing regime in the Large Aral as well as an assessment of the future tendency in the seasonal to decadal mixing and its effect on the biogeochemistry, in particular, methane production, and biodiversity.