Atmospheric CO₂ Exchange of a Small Mountain Lake: Limitations of Eddy Covariance and Boundary Layer Modeling Methods in Complex Terrain

Abstract

The world's streams, rivers, and lakes contribute substantially to the global carbon (C) cycle through the emission of carbon dioxide (CO 2 ) across the air-water interface. Current estimates quantified that 1-3.8 Pg C per year are released by inland waters to the atmosphere (

Eddy Covariance Data Reveal That a Small Freshwater Reservoir Emits a Substantial Amount of Carbon Dioxide and Methane

“…Finally, comparisons with diffusive grab samples suggest fluxes measured with the EC system were consistently higher than those estimated with diffusive grab samples, especially for CO 2 (Figure 2, Figure S11 in Supporting Information ), which is consistent with previous studies (Scholz et al., 2021, and references therein). Conversely, CH 4 fluxes calculated using the discrete diffusive methods were more comparable to those measured by the EC system (Figure 3, Figure S11 in Supporting Information ).…”

“…In addition, based on studies conducted in similar terrestrial ecosystems, we might expect negative CO 2 fluxes in the summer followed by substantial CO 2 emissions in the fall and winter; however, these patterns were not observed in FCR, suggesting the majority of fluxes measured in this study likely originated in the reservoir. When considered and interpreted cautiously, the data collected by the EC system provides a far more comprehensive time series than what is possible from discrete measurements (Anderson et al., 1999; Eugster, 2003; Huotari et al., 2011; Jonsson et al., 2008; Scholz et al., 2021), which is critical for increasing our understanding of GHG fluxes from small reservoirs on multiple temporal scales.…”

“…While strict filtering processes were enacted to limit non-local fluxes (i.e., filtering fluxes when the along-wind distance providing 90% of the cumulative contribution was outside the reservoir), we are unable to completely rule out potential non-local processes (e.g., land-lake interactions) which occur outside the footprint and are entrained or advected into the EC footprint area (Esters et al, 2021;Vesala et al, 2006Vesala et al, , 2012Figure S2 in Supporting Information S1). These processes may be particularly important in small freshwaters located in mountainous regions (Scholz et al, 2021). For example, Scholz et al (2021) hypothesized that reduced nighttime CO 2 emissions measured using the diffusive-flux method were due to low wind speeds and CO 2 sinking from the land to the lake surface at night in a mountainous Swiss lake.…”

“…Thus, EC systems can greatly increase the temporal resolution and spatial extent of measured fluxes in lakes and reservoirs, with the caveat that important considerations and data filtering are needed for EC systems in small waterbodies (Scholz et al., 2021). Specifically, a waterbody’s small surface area increases the likelihood of surrounding terrestrial vegetation impacting EC measurements of aquatic fluxes and decreases the area available for a well‐mixed, turbulent footprint (Esters et al., 2021; Scholz et al., 2021; Vesala et al., 2012).…”

“…Altogether, there is a clear need to measure annual-scale CH 4 and CO 2 fluxes from small freshwater ecosystems, especially small reservoirs. While several studies have measured annual CO 2 fluxes from freshwaters (e.g., A. K. Baldocchi et al, 2020;Golub et al, 2023;Huotari et al, 2011;Liu et al, 2016;Reed et al, 2018;Scholz et al, 2021;Shao et al, 2015), to the best of our knowledge, only one freshwater study has measured both CH 4 and CO 2 fluxes on an annual timescale (Jammet et al, 2017), while Taoka et al (2020) and Waldo et al (2021) measured only CH 4 fluxes at the annual scale. Specifically, Waldo et al (2021) used EC to measure annual CH 4 fluxes from a large (2.4 km 2 ), highly eutrophic temperate reservoir, measuring emissions up to 71.4 g CH 4 m −2 yr −1 , which is in the top quarter of those reported from lakes and reservoirs to date.…”