Diel, seasonal, and inter-annual variation in carbon dioxide effluxes from lakes and reservoirs

Abstract

Accounting for temporal changes in carbon dioxide (CO2) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO2 based on the eddy covariance method from 13 lakes and reservoirs in the Northern Hemisphere, and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO2 flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO2. Annual CO2 emissions had an average of 25% (range 3-58%) interannual variation. Similar to studies on streams, nighttime emissions regularly exceeded daytime emissions. Biophysical regulations of CO2 flux variability were delineated through mutual information analysis. Sample analysis of CO2 fluxes indicate the importance of continuous measurements. Better characterization of short- and long-term variability is necessary to understand and improve detection of temporal changes of CO2 fluxes in response to natural and anthropogenic drivers. Our results indicate that existing global lake carbon budgets relying primarily on daytime measurements yield underestimates of net emissions.

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

“…Overall, EC measurements captured 23% and 19% of total CO 2 and CH 4 fluxes, respectively, over 2 years from FCR (Table S1 in Supporting Information S1), which is similar to previously reported deployments of EC systems at lakes and reservoirs (e.g., Golub et al, 2023;Reed et al, 2018;Waldo et al, 2021). The percentage of available data was relatively consistent across half-hourly periods (from 00:00 to 23:30), ranging from 14% to 34% of data availability for CO 2 for 22:00 and 12:30 half-hourly periods, respectively, and 11%-32% for CH 4 (22:00 and 12:30 half-hourly periods, respectively; Figure S3 in Supporting Information S1).…”

“…When scaling fluxes to the full year, FCR was a much smaller annual CH 4 source (1.02-1.29 g m −2 yr −1 ), yet a larger CO 2 source (633-731 g m −2 yr −1 ; Figure 5, Figure S12 in Supporting Information S1), than other reservoirs reported in the literature to date (A. K. Baldocchi et al, 2020;Deemer et al, 2016;Golub et al, 2023). While the total magnitude of CO 2 emissions from FCR was greater than most studies, Golub et al (2023) similarly found that data from 12 lakes and reservoirs over multiple years emitted substantial amounts of CO 2 in their synthesis of EC measured CO 2 fluxes in freshwaters (13.6-224 g C m −2 yr −1 ), except for one reservoir during one year which had a CO 2 flux of −53.6 g C m −2 yr −1 . As compared to other reservoirs with GHG flux data, FCR is old (>100 years old) which may lead to lower GHG emissions, particularly for CH 4 fluxes, likely as a result of reduced supply of organic matter substrate in the sediments as the reservoir ages (Barros et al, 2011;McClure et al, 2020;Prairie et al, 2018).…”

“…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.…”

“…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.…”

Characterizing Performance of Freshwater Wetland Methane Models Across Time Scales at FLUXNET‐CH4 Sites Using Wavelet Analyses

“…Wavelet analysis is especially useful for model evaluation since, compared to Fourier transform, it can identify not only the time scales that influence a signal but also inform when those time scales are significant. Previous studies have identified disagreement between models and observations for carbon dioxide (CO 2 ) fluxes across different ecosystems (Golub et al., 2023; Li et al., 2023; Richardson et al., 2012; Schwalm et al., 2010; Stoy et al., 2013) using wavelet analysis, and have found that (a) model errors in CO 2 flux peak at the diurnal scale and (b) the model error patterns are associated with model structure and environmental drivers. However, so far there is no assessment for FCH 4 .…”