Dissolved methane in the water column of the Saguenay Fjord

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 (δ13CCH4) 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 (δ13CCH4: −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 δ13CCH4 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], δ13CCH4, 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.