Revising the methane cycling in lakes: sources and sinks in two German lakes with specific consideration of methane accumulation in oxic waters

近年来，在世界范围内的淡水和海水中都发现了甲烷（CH4）在含氧水体中的积累。在我们的Aquameth项目（GR1540/21-1）中，我们回顾并评估了中贫营养湖泊Stechlin中这种现象的潜在机制。通过建立在线CH4测量系统，揭示了湖泊含氧水层中蓝藻、隐藻等藻类与CH4动态的密切时空关系。虽然最近建立的甲基膦酸盐代谢可能存在于湖中，但我们发现了藻类在光合作用过程中直接产生CH4的第一个证据。然而，可能的机制及其在含氧水体中产生的CH4对总CH4通量的贡献在很大程度上仍然未知。通过结合两个已建立的研究小组的专业知识-理想地互补包括仪器应用，我们的目标是评估CH4形成和氧化过程的精确化学和生物学，以更好地了解湖泊在区域和全球CH4循环中的作用。因此，将详细量化德国两个湖泊的完整CH4收支，即通过详细的质量平衡方法结合原位孵育实验评估CH4总量源和汇。我们对比的野外站点/湖泊（寡中营养化的Stechlin湖和富营养化的Willersinnweiher湖）代表了两个研究所研究的两种主要温带湖泊类型（深/贫营养化和浅/富营养化）和生物地球化学特征。在这两个湖泊中，CH4产生、积累及其向大气释放的特定过程取决于有利于某些生物体和相关过程的物理、化学和生物因素的复杂相互作用。因此，我们的主要目标是将环境变量与CH4形成、积累和释放过程之间复杂的相互作用纠缠在一起，并提供湖泊的季节性CH4收支，以确定氧CH4形成与区域和全球CH4循环的相关性。我们假设：(1)甲烷的产生与光合作用直接相关，在特定的环境条件下，如养分限制，各种光自养生物可以直接形成CH4。(2)由于氧化水体中甲烷营养活性的空间和/或时间解耦，甲烷的形成与氧化解耦。(3)温跃层中的甲烷是生物、化学和物理过程复杂相互作用的产物。(4)上层氧化水层CH4浓度的增加促进了水与大气之间的气体交换。尽管上层水层的CH4积累在很大程度上被忽视了，但它可能是全球CH4收支中缺失的一个主要环节。为了解决这些假设，两个团队之间的密切互动将结合现场测量和实验室实验。

Recently, methane (CH4) accumulation in oxic waters has been found in freshwater and marine waters worldwide. In our Aquameth project (GR1540/21-1) we have reviewed and evaluated potential mechanisms for this phenomenon in meso-oligotrophic Lake Stechlin. By establishing an online CH4 measurement system, we could show a close spatio-temporal relationship between the dynamics of alga (e.g. cyanobacteria and cryptophytes) and CH4 in the oxic water layers of the lake. Whilst the recently established methyl phosphonate metabolism might be present in the lake we found first evidence that algae directly produce CH4 during photosynthesis. However, the possible mechanism and its contribution to CH4 produced in oxic waters to total CH4 fluxes remains largely unknown. By combining the expertise of two established research groups - that ideally complement each other including instrumental applications, we aim to evaluate the precise chemistry and biology of CH4 formation and oxidation processes to better understand the role of lakes in regional and global CH4 cycling. Therefore, the complete CH4 budget of two lakes in Germany will be quantified in detail, i.e. CH4 bulk sources and sinks will be evaluated by a detailed mass balance approach combined with in situ incubation experiments. Our contrasting field sites/lakes (oligo-mesotrophic Lake Stechlin and eutrophic Willersinnweiher) represent 2 main temperate lake types (deep /nutrient-poor and shallow /nutrient-rich) well studied and biogeochemically characterized by the two research institutes. In both lakes, the presence of specific processes of CH4 production, accumulation and its release to the atmosphere depend on a complicated interplay of physical, chemical and biological factors favoring certain organisms and related processes. Thus our main objectives are to entangle the complicated interplay between environmental variables and processes of CH4 formation, accumulation and release, and to provide a seasonal CH4 budget of the lake to determine the relevance of oxic CH4 formation for regional and global CH4 cycling. We hypothesize that (1) Methane production is directly linked to photosynthesis and CH4 can be directly formed by various photoautotrophic organisms at specific environmental conditions, e.g. nutrient limitation. (2) Methane formation is decoupled from oxidation due to spatial and/or temporal decoupling of methanotrophic activity in oxic waters. (3) Methane at the thermocline is the product of a complicated interplay between biological, chemical and physical processes. (4) Increased CH4 concentrations in the upper oxic water layer facilitate the gas exchange between water and atmosphere. Although CH4 accumulation in the upper water layers has been largely neglected, it may represent a major missing link in the global CH4 budget. To address these hypotheses, field measurements and lab experiments will be combined by close interactions between both teams.