RAPID: Is Biomass Mobilization at Ice-covered Lake Fryxell, Antarctica reaching a Critical Threshold?

南极麦克默多干谷常年被冰覆盖的湖泊含有丰富的微生物垫子，这种垫子材料的出口可以肥沃周围的极地沙漠生态系统。这些沙漠土壤是地球上有机质最贫乏的土壤之一，但却拥有微生物群落。当气泡在湖垫中形成时，微生物垫物质从湖泊浅层、气体过饱和区域输出，将它们抬到冰盖上；常年冰盖保持气体过饱和。这些垫子冻结在一起，并通过冰蚀作用输出到周围的土壤中。弗莱克塞尔湖历史上最大的季节性降幅和最薄的冰盖是在2022-2023年的南方夏季记录的。在这个稀薄的冰年，水柱溶解氧比以前的观测数据有所增加，湖底被气泡破坏的垫子的面积是1980-1981年和2006-2007年观测到的两倍多。这项工作将限制在史无前例的冰层变薄时期麦克默多干谷弗莱克塞尔湖内外的垫子流动，以了解未来区域气候变化和预测的湖泊冰盖季节性丧失将如何影响麦克默多干谷的营养物质运输。假设特殊年份的垫子出口将对土壤群落的养分输出产生最显著的影响；因此，垫子升空的可变性可能在麦克默多干谷生态系统对气候变化的反应中发挥作用。南极洲麦克默多干燥河谷湖泊的常年冰盖调节了湖泊与周围土壤之间气体、有机和无机物质的转移。这些湖泊中生物量的输出是由于常年冰盖下浅水区大气气体的过饱和所致。气泡在垫子中成核，产生浮力，将它们抬到冰层底部，在那里它们被冻结，并通过冰蚀作用输出到周围的土壤中。这些垫子代表着麦克默多干燥山谷土壤的重要生物量和养分来源，这些土壤是地球上最贫瘠的有机土壤之一。然而，在麦克默多干河谷的有机碳循环模型中，这种生物量仍然没有被考虑在内。来自麦克默多干谷长期生态研究项目的冰盖数据显示，在过去的40年里，冰层厚度经历了周期性的变化，在2022-2023年的南半球夏季，达到了弗莱克塞尔湖有记录以来最大的季节性降幅和最薄的冰盖。初步工作表明，弗莱克塞尔湖的冰席升空面积是1980-1981年和2006-2007年观测到的面积的两倍多，恰逢冰盖史无前例地变薄，水柱溶解的氧气增加。这项研究将限制在史无前例的冰川变薄时期麦克默多干燥山谷弗莱克塞尔湖内外的生物质动员。研究人员假设，较薄的冰盖通过以下方式促进更多的生物量动员：1)在夏季光合作用期间，刺激现有气体过饱和水域产生额外的气泡，创造微生物垫升空；2)促进更深、更厚的微生物垫升空；3)通过表征生物量的化学特征和模拟最可能的沉积环境，可以将这些生物量追踪到土壤中。这项工作将使用微生物垫样本、湖泊溶解氧和光合作用有效辐射数据以及记录2023年1月升空垫深度分布的水下无人机镜头，以及麦克默多干谷长期生态研究项目收集的长期冰盖厚度、光合作用有效辐射和湖泊水位变化数据来验证假设1-3。暴露在弗莱克塞尔湖表面的升空垫的扩散将使用混合单粒子拉格朗日积分弹道(HYSPLIT)模型进行建模。假设像现在这样的特殊起飞年会对土壤群落产生最重大的影响，因为土壤呼吸速率随着碳的增加而增加。然而，未来10-40年的持续变暖可能会导致冰盖的季节性丧失和起飞垫出口的停止。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Perennially ice-covered lakes in the McMurdo Dry Valleys of Antarctica contain abundant microbial mats, and the export of this mat material can fertilize the surrounding polar desert ecosystems. These desert soils are one of the most organic-poor on earth yet host a community of microorganisms. Microbial mat material is exported from the shallow, gas-supersaturated regions of the lakes when gas bubbles form in the mats, lifting them to the ice cover; the perennial ice cover maintains gas supersaturation. These mats freeze in and are exported to the surrounding soils through ice ablation. The largest seasonal decrease and thinnest ice cover in the history of Lake Fryxell was recorded during the 2022-2023 Austral summer. In this thin ice year, the water column dissolved oxygen increased over prior observations, and the lake bottom surface area with bubble-disrupted mat was more than double that observed in 1980-1981 and 2006-2007. This work will constrain mat mobilization within and out of Lake Fryxell in the McMurdo Dry Valleys during a period of unprecedented ice thinning to understand how future changing regional climate and predicted seasonal loss of lake ice cover will affect nutrient transport in the McMurdo Dry Valleys. Exceptional years of mat export are hypothesized to have the most significant impact on nutrient export to soil communities; variability in mat liftoff may thus play a role in the McMurdo Dry Valleys ecosystem response to changing climate. The perennial ice cover of lakes in the McMurdo Dry Valleys of Antarctica modulates the transfer of gasses, organic and inorganic material, between the lakes and surrounding soils. The export of biomass in these lakes is driven by the supersaturation of atmospheric gasses in the shallow regions under perennial ice cover. Gas bubbles nucleate in the mats, producing buoyancy that lifts them to the bottom of the ice, where they freeze in and are exported to the surrounding soils through ice ablation. These mats represent a significant source of biomass and nutrients to the McMurdo Dry Valleys soils, which are among the most organic-poor on earth. Nevertheless, this biomass remains unaccounted for in organic carbon cycling models for the McMurdo Dry Valleys. Ice cover data from the McMurdo Dry Valleys Long Term Ecological Research Project shows that the ice thickness has undergone cyclical variation over the last 40 years, reaching the largest seasonal decrease and thinnest ice-cover in the recorded history of Lake Fryxell during the 2022-2023 austral summer. Preliminary work shows that the surface area with mat liftoff at Lake Fryxell is more than double that observed in 1980-1981 and 2006-2007, coinciding with this unprecedented thinning of the ice-cover and an increase in the water column dissolved O2. This research will constrain biomass mobilization within and out of Lake Fryxell in the McMurdo Dry Valleys during a period of unprecedented ice thinning. The researchers hypothesize that a thinner ice cover promotes more biomass mobilization by 1) stimulating additional production of gas bubbles from the existing gas-supersaturated waters during summertime photosynthesis to create microbial mat liftoff and 2) promoting mat liftoff in deeper, thicker microbial mats, and 3) that this biomass can be traced into the soils by characterizing its chemistry and modeling the most likely depositional settings. This work will use microbial mat samples, lake dissolved oxygen and photosynthetically active radiation data and underwater drone footage documenting the depth distribution of liftoff mats in January 2023, and long-term ice cover thickness, photosynthetically active radiation, and lake level change data collected by the McMurdo Dry Valleys Long Term Ecological Research Project to test hypotheses 1-3. The dispersal of the liftoff mat exposed at Lake Fryxell surface will be modeled using a Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Exceptional liftoff years like the present are hypothesized to have the most significant impact on the soil communities as the rates of soil respiration increase with the addition of carbon. However, continued warming in the next 10 - 40 years may result in seasonal loss of the ice cover and cessation of liftoff mat export.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.