Near-shore environments are a significant source of atmospheric methane but the size of this source is poorly constrained, particularly for fjords and fjards. This study investigated the methane emission rates and the drivers controlling the dynamics of dissolved methane in the Saguenay Fjord, a deep, stratified, and well‑oxygenated subarctic fjord system in eastern Canada. Dissolved methane concentrations ([CH₄]) in the water column were measured in October 2016 and June, October, and November 2017, with stable carbon isotope composition of methane (δ¹³C_CH4) analyzed during the November 2017 survey. Surface-water [CH₄] ranged from 16 to 184 nmol L⁻¹ and decreased with increasing salinity in a bi-segment linear manner, inferring a temporally constant marine endmember but a freshwater discharge-dependent river endmember. The multi-cruises dataset yields a mean [CH₄] saturation ratio of 12.7 (range: 4.5–48.7) and a mean emission rate of 53.4 μmol m⁻² d⁻¹ (range: 16.4–256.9 μmol m⁻² d⁻¹). [CH₄] was generally higher in surface water than in deep water. However, sill-induced mixing could homogenize [CH₄] near the mouth of the fjord and sedimentary input of biogenic methane (δ¹³C_CH4: −57.660‰) in the fjord's head region increased [CH₄] in the overlying bottom water up to 459 nmol L⁻¹. The longitudinal pattern of [CH₄] below the surface layer was primarily controlled by deep-water renewal events. Deep-water [CH₄] declined with rising apparent oxygen utilization, suggestive of aerobic microbial methane oxidation at rates estimated to be <0.1 nmol L⁻¹ d⁻¹. The δ¹³C_CH4 data yields a carbon isotopic fractionation factor of 1.08 in both the surface and deep waters that points to microbial oxidation dictating the carbon isotopic fractionation of methane in the fjord. Mass-balance budgeting reveals that river runoff accounts for 81% of the total methane input to the fjord (12.13 × 10⁶ mol year⁻¹) and that microbial oxidation of methane (4.45 × 10⁶ mol year⁻¹) is comparable to emission to the atmosphere (4.27 × 10⁶ mol year⁻¹). This study demonstrates the important roles of river runoff and deep-water renewal in controlling the dynamics of [CH₄], δ¹³C_CH4, and methane emission to air in fjords receiving large terrestrial freshwater discharges and experiencing frequent deep-water renewals. The areal methane emission rates for deep fjords obtained by this and earlier studies are one to two orders of magnitude higher than the mean flux estimate for global coastal oceans, placing fjords and fjards as a potentially significant contributor to coastal methane emission.

近岸环境是大气中甲烷的重要来源，但这种来源的大小受到限制，尤其是对于峡湾和弗里德峡湾而言。这项研究调查了萨格奈峡湾（加拿大东部深层，分层且充氧良好的亚弧峡湾系统）中的甲烷排放速率和控制溶解甲烷动态的驱动因素。溶解的甲烷浓度（[CH ₄ ]）在水柱在十月2016和六月，十月，和2017年11月进行测量，与甲烷的碳同位素组成（δ ¹³ C ^ _CH4）的2017年11月测量期间进行分析。表面水[CH ₄ ]的范围为16至184 nmol L ^-1并随着盐度的增加以双段线性方式降低，从而推断出时间上恒定的海洋末段而依赖于淡水排放的河流末段。多巡航数据集的平均[CH ₄ ]饱和比为12.7（范围：4.5-48.7），平均发射率为53.4μmolm ^-2 d ^-1（范围：16.4-256.9μmolm ^-2 d ^-1）。[CH ₄ ]通常在地表水中比在深水中高。然而，下纵梁诱导混合可以均匀化[CH ₄ ]峡湾和生物甲烷的沉积输入（δ口附近¹³ Ç _CH4：-57.660‰）在峡湾的头部区域中增加[CH ₄]在上层底部水中，直至459 nmol L ^-1。表面层以下的[CH ₄ ]的纵向模式主要受深水更新事件控制​​。深水[CH ₄ ]随着表观氧气利用率的增加而下降，这表明有氧微生物甲烷氧化的速率估计为<0.1 nmol L ^-1 d ^-1。的δ ¹³ Ç _CH4在表面和深水域它指向微生物氧化口授甲烷的碳同位素分馏在峡湾两个数据产生1.08的碳同位素分馏因子。质量平衡预算显示，河流径流占峡湾甲烷总输入量的81％（12.13×10⁶  mol年^-1）和甲烷的微生物氧化（4.45×10 ⁶  mol年^-1）相当于向大气的排放（4.27×10 ⁶  mol年^-1）。这项研究表明，河川径流和深水重建中的重要作用的控制[CH的动态₄ ]，δ ¹³ C ^ _CH4，以及在接收大量陆地淡水排放并经历频繁的深水更新的峡湾中向空气中的甲烷排放。通过此项研究和较早的研究得出的深峡湾的甲烷甲烷排放速率比全球沿海海洋平均通量估计值高一到两个数量级，这使得峡湾和峡湾成为沿海甲烷排放的潜在重要贡献者。