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沃茨中发现了甲烷（CH 4）在好氧沃茨中的积累。在我们的Aquameth项目（GR 1540/21-1）中，我们已经审查和评估了这种现象在中贫营养湖Stechlin的潜在机制。通过建立CH 4在线监测系统，可以揭示湖泊富氧水体中蓝藻和隐藻等藻类的动态变化与CH 4之间的密切时空关系。虽然最近建立的甲基膦酸酯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CH 4的第一个证据。然而，可能的机制和它的贡献，在好氧沃茨中产生的总CH 4通量仍然在很大程度上是未知的。通过结合两个已建立的研究小组的专业知识-理想地相互补充，包括仪器应用，我们的目标是评估CH 4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CH 4循环中的作用。因此，将详细量化德国两个湖泊的全部CH 4收支，即通过详细的质量平衡方法结合原位培养实验评估CH 4的整体源和汇。我们的对比现场/湖泊（寡-中营养湖Stechlin和富营养化Willersinnweiher）代表了两个主要的温带湖泊类型（深/营养贫乏和浅/营养丰富），这两个研究机构进行了充分的研究和生态地球化学特征。在这两个湖泊中，甲烷产生、积累和释放到大气中的具体过程取决于有利于某些生物和相关过程的物理、化学和生物因素的复杂相互作用。因此，我们的主要目标是纠缠环境变量和CH 4的形成，积累和释放过程之间的复杂的相互作用，并提供一个季节性的CH 4预算的湖泊，以确定区域和全球CH 4循环的相关性好氧CH 4形成。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，例如营养限制，各种光合自养生物可以直接形成CH 4。(2)甲烷形成与氧化解耦，这是由于好氧沃茨中甲烷氧化活性的空间和/或时间解耦。(3)温跃层中的甲烷是生物、化学和物理过程之间复杂相互作用的产物。(4)上层好氧水层中甲烷浓度的增加促进了水与大气之间的气体交换。虽然CH 4在上层的积累在很大程度上被忽视，它可能是一个主要的缺失环节，在全球CH 4预算。为了解决这些假设，现场测量和实验室实验将通过两个团队之间的密切互动相结合。

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.