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)

人为用水和气候变化可能对湖泊生态系统产生严重的负面影响。内陆水域的脆弱性，被称为“世纪生态灾难”，最突出的例子是咸海。迄今为止，咸海干涸的后果还没有得到充分估计。项目小组的一项新发现是这项研究的核心，即最近发现的分生（永久）分层开始在海中最大的剩余盆地大咸海中发展。我们在2015-2018年的观察表明，该湖目前正在经历一个“小湖化”的过渡期，由于流入河流的水调节和气候驱动的区域水收支变化而加速。这种转变表明生物地球化学制度发生了剧烈变化，其中一些变化，如湖中甲烷浓度的迅速增加，已经被项目小组记录下来。为了量化甲基化的机制并评估其在区域尺度上的后果，我们建议应用一套实地研究方法——监测分层和氧状态，直接估计微生物活动，甲烷浓度和排放以及水柱上的湍流质量传递，稳定同位素研究水平衡，辅以建模和遥感观测。大咸海的现状使我们能够追踪由全球内陆水域面临的主要威胁所造成的前所未有的湖泊生态系统变化：不断增加的用水和气候驱动的水收支变化。因此，对大咸海生态系统的变化进行及时而详细的调查，将为干旱地区其他受到同样人为驱动因素威胁的大型终端湖泊提供“最坏”的情况。该项目的预期结果是对大咸海目前的混合情况进行量化，并评估季节性到年代际混合的未来趋势及其对生物地球化学，特别是甲烷生产和生物多样性的影响。

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.