Substantial Contribution of nocturnal oxic methane production in eutrophic freshwaters. Data

Sampling activities and field experiments were organized into the following three sections:<br>1) Diel sampling was conducted from 12:00 a.m. on August 31 to 12:00 a.m. on September 1 on a deck floating in the center of the bay. During the Diel cycle, collections were made every three hours for chemical analyses and air-water gas fluxes (CO2 and CH4. Sampling depths were 0.5 m, 3.0 m, 6.0 m, 12 m, 18 m, and 21 m below the water surface, respectively.A multiparameter water quality probe (YSI® EXO2, USA) was deployed in the field to continuously obtain water temperature, dissolved oxygen (DO), pH, and other water quality parameters along the vertical profiles. Physical and chemical parameters were continuously obtained along the vertical profile.<br>2) Quantitative evaluation of oxygenated CH4. The field experiment consisted of the following three additional pathways: a) Funnel-collected sediment-water and air-water boiling fluxes of CH 4 in the coastal region of the bay.4 Sediment-water diffusion of CH4 was estimated by sampling and analyzing the transport of sediment pore water within the bay.4 The mass of the water column was estimated by acoustic Doppler velocimetry (Flow Quest, US) on both sides of the floating deck in the Two sampling transects were sampled and flow measurements were made along vertical profiles, i.e., inside (InCx) and outside (OutCx Supplementary Figure S1<br>3)4 In-situ incubation experiments were conducted for 4 days at depths of 0.5 m, 6 m and 20 m below the water surface to track microbial metabolism-driven CH 4 kinetics. Water samples were collected in situ and sealed in 624 mL brown glass vials wrapped in black plastic bags to prevent photosynthesis during incubation. Four identical incubation bottles were deployed in the field at each depth to facilitate daily sampling over a 24-hour period.4 concentration changes and their stable isotope signatures I isotope analyzer (Picarro®, CA, USA.). Triplicate copies were performed for quality control.<br>

本次研究的采样活动与野外实验分为以下三个部分：<br>1) 昼夜采样（Diel sampling）：于8月31日00:00至9月1日00:00在海湾中心的浮动甲板上开展。在昼夜周期内，每3小时采集一次样品，用于化学分析与二氧化碳（CO₂）、甲烷（CH₄）的气-水界面气体通量测定。采样水深分别为水面下0.5 m、3.0 m、6.0 m、12 m、18 m及21 m。野外部署多参数水质探头（YSI® EXO2，美国），以连续获取垂向剖面的水温、溶解氧（dissolved oxygen, DO）、pH及其他水质参数，实现物理化学参数的原位连续监测。<br>2) 含氧甲烷的定量评估。本次野外实验包含以下3种额外实验路径：a) 利用漏斗采集海湾沿岸区域的沉积物-水与气-水界面甲烷冒泡通量⁴。通过采集并分析海湾内沉积物孔隙水的运移过程，估算甲烷的沉积物-水扩散通量⁴。本研究在浮动甲板两侧的两条采样断面上（分别为断面内侧InCx与断面外侧OutCx，详见补充图S1）部署声学多普勒测速仪（Flow Quest，美国）以估算水柱质量，并沿垂向剖面开展采样与流速测量工作。<br>3) 原位培养实验：在水面下0.5 m、6 m及20 m水深处开展为期4天的原位培养实验，以追踪微生物代谢驱动的甲烷动力学过程。原位采集水样并封装于624 mL棕色玻璃瓶中，外层以黑色塑料袋包裹以避免培养过程中发生光合作用。每个水深点位部署4个平行培养瓶，以支持24小时周期内的每日采样工作。利用同位素分析仪（Picarro®，美国加利福尼亚州）测定甲烷浓度变化及其稳定同位素特征。实验设置3次平行重复以保障质量控制。