Seasonal and Spatial Dynamics of Gas Ebullition in a Temperate Water‐Storage Reservoir

Abstract

Gas ebullition of river impoundments plays an increasingly significant role, particularly in transporting methane CH4 from their sediments to the atmosphere, and contributing to the global carbon budget and global warming. Quantifying stochastic and episodic nature of gas ebullition is complicated especially when conventionally conducted by using coverage‐limited gas traps. Current knowledge of seasonality in a reservoir's gas ebullition is lacking in the literature. For this reason, advanced acoustic surveying was intensively applied to determine spatiotemporal distributions of gas ebullition in a European water‐storage reservoir for two years. Additionally, the sampling was accompanied with gas collecting for analyzing gas composition. The gas released from the reservoir was primarily composed of CH4 (on average 52%, up to 94%). The longitudinal distribution of gas ebullition was mainly determined by a proximity to the river inflow as a source of organic matter. A magnitude of ebullitive fluxes within the reservoir varied up to 1,300 mL m−2 d−1 (30 mmol CH4 m−2 d−1). The most significant period of ebullition has turned out to be in fall, on average reaching a sevenfold ebullitive flux (70 mL m−2 d−1, 1.6 mmol CH4 m−2 d−1) higher than in the rest of the season. A substantial contribution to the fall peak was induced by an expansion of gas ebullition into greater depths, covering two thirds of the reservoir in late fall. The study demonstrates that the ebullitive fluxes of the temperate water storage reservoir were correlated to season, depth, and inflow proximity.

The Magnitude and Drivers of Methane Ebullition and Diffusion Vary on a Longitudinal Gradient in a Small Freshwater Reservoir

“…We selected candidate driver variables by identifying previously published predictors of CH 4 ebullition and diffusion at the ecosystem scale that could feasibly be sampled at FCR. The variables included depth (Deshmukh et al, 2014;Tušer et al, 2017), inflow discharge (Maeck et al, 2014), SWI temperature (Aben et al, 2017), phytoplankton biomass (West et al, 2016), change in atmospheric pressure (Tokida et al, 2007;Zhu et al, 2016), wind speed (Joyce & Jewell, 2003), and near-sediment and surface water turbulence (Joyce & Jewell, 2003;Yang et al, 2013). Dissolved oxygen at the SWI at each transect was measured but not included in the analysis because of sensor failure during the monitoring period.…”

“…For example, rapid decreases in barometric pressure and hydrostatic pressure that occur within short time periods (<24 hr) have been shown to substantially increase ebullition rates (Casper et al, 2000). Using alternate methods such as echo sounders (Ostrovsky, 2003;Tušer et al, 2017) and automated bubble traps (Delwiche & Hemond, 2017;Varadharajan et al, 2010) for measuring ebullition may provide insight on the drivers of ebullition rates at shorter time scales.…”

“…Similarly, studies have also demonstrated that the highest diffusion rates occur in shallow upstream sites and decrease toward the deeper sites in larger reservoirs Yang et al, 2013). This longitudinal heterogeneity has been well documented in large reservoirs (surface area > 0.5 km 2 ; DelSontro et al, 2010;Demarty et al, 2011;Marcelino et al, 2015;Soumis et al, 2004;Tušer et al, 2017), but it remains unknown how variable CH 4 emissions are on a longitudinal gradient within small reservoirs (surface area < 0.5 km 2 ), which may exhibit less predictable reservoir zonation because of their size.…”

“…Within reservoirs, CH 4 ebullition and diffusion rates can vary substantially along a longitudinal gradient from the upstream areas near the major inflows to the downstream areas near the dam (Beaulieu et al, , ; Huang et al, ; Sobek et al, ; Tušer et al, ). Ebullition is generally highest in shallow areas upstream, with much lower (but still detectable) rates in downstream areas (Beaulieu et al, , ).…”

“…Studies of CH 4 emissions in naturally formed lakes and reservoirs have suggested that ebullition and diffusion rates are controlled by both physical and biological variables (West et al, 2016), with the relative importance of these drivers likely varying spatially DelSontro et al, 2016;Hofmann, 2013;Natchimuthu et al, 2016;Tušer et al, 2017;Yang et al, 2013). For example, CH 4 ebullition rates have been shown to be related to shear stress at the sediment-water interface (SWI) caused by bottom currents (Joyce & Jewell, 2003;Yang et al, 2013), elevated wind speeds (Joyce & Jewell, 2003), changes in barometric pressure (Casper et al, 2000;Mattson & Likens, 1990;Peltola et al, 2018;Tokida et al, 2005;Yu et al, 2014), and varying depth (Tušer et al, 2017), all factors that may vary along a longitudinal gradient in a reservoir. Similarly, increases in temperature at the SWI during the summer, particularly in shallower sites, increase ebullition rates (Aben et al, 2017;DelSontro et al, 2016).…”