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会对湖泊生态系统产生严重的负面影响。内陆水域脆弱的最显著例子--“本世纪的生态灾难”--是咸海。到目前为止，咸海干化的后果还没有得到充分估计。作为拟议研究的核心，项目组的一项新发现是，最近发现在大咸海最大的剩余海盆--大咸海--开始形成亚米米期(永久性)分层。我们在2015-2018年的观察表明，由于流入河流的水量调节和气候驱动的区域水收支变化，该湖目前正在经历一个“分水岭化”的过渡期。这样的转变表明生物地球化学系统发生了剧烈变化，其中一些变化，例如湖泊中甲烷浓度的迅速增加，已经被项目组记录下来。为了量化分生石化的机制并在区域尺度上评估其后果，我们建议应用一套实地研究方法--监测分层和氧气制度，直接估计微生物活动、甲烷浓度和排放以及水柱上的湍流传质，水平衡的稳定同位素研究--并辅之以模拟和遥感观测。大咸海目前的情况使我们能够追踪到世界各地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：用水量增加和气候驱动的水预算变化。因此，及时和详细地调查大型咸海生态系统的变化将为干旱地区的其他大型终端湖泊提供“最坏的”情况，这些湖泊受到同样的人为驱动因素的威胁。该项目的预期成果是对大型咸水区目前的混合状况进行量化，并对季节混合到十年混合的未来趋势及其对生物地球化学的影响，特别是甲烷产量和生物多样性进行评估。

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.