Surface CO<sub>2</sub> gradients challenge conventional CO<sub>2</sub> emission quantification in lentic water bodies under calm conditions
<p>Lakes are hotspots of inland carbon cycling and are important sources of greenhouse gases (GHGs) such as carbon dioxide (CO<span class="inline-formula"><sub>2</sub></span>). The significant role of CO<span class="inline-formula"><sub>2...
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Copernicus Publications
2025-04-01
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| Series: | Biogeosciences |
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| author | P. Aurich U. Spank M. Koschorreck |
| author_facet | P. Aurich U. Spank M. Koschorreck |
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| collection | DOAJ |
| description | <p>Lakes are hotspots of inland carbon cycling and are important sources of greenhouse gases (GHGs) such as carbon dioxide (CO<span class="inline-formula"><sub>2</sub></span>). The significant role of CO<span class="inline-formula"><sub>2</sub></span> in the global carbon cycle makes quantifying its emission from various ecosystems, including lakes and reservoirs, important for developing strategies to mitigate climate change. The thin boundary layer (TBL) method is a common approach to calculating CO<span class="inline-formula"><sub>2</sub></span> fluxes from CO<span class="inline-formula"><sub>2</sub></span> measurements in both water and air, as well as wind speed. However, one assumption for the TBL method is a homogeneous CO<span class="inline-formula"><sub>2</sub></span> concentration between the measurement depth and the water surface where gas exchange takes place. This assumption might not be true under calm conditions, when microstratification below the surface slows the vertical exchange of gases. We used a floating outdoor laboratory to monitor CO<span class="inline-formula"><sub>2</sub></span> concentrations at 5 and 25 cm depth, CO<span class="inline-formula"><sub>2</sub></span> concentration in the air, wind speed, and water temperature profiles for 1 week in the Bautzen Reservoir, Germany. While we found homogeneous CO<span class="inline-formula"><sub>2</sub></span> concentrations in the two depths at wind speeds above 3 m s<span class="inline-formula"><sup>−1</sup></span>, there was a vertical gradient observed during windless nights. The concentrations observed temporally ranged from undersaturation to supersaturation at 25 and 5 cm, respectively. Fluxes calculated from the measured concentrations therefore would change from negative to positive, depending on the measurement depth. Simultaneous eddy covariance (EC) measurements showed that even the measurements close to the surface underestimated the actual CO<span class="inline-formula"><sub>2</sub></span> concentration. Oxygen measurements support our hypothesis that plankton respiration at the water surface causes a periodic CO<span class="inline-formula"><sub>2</sub></span> concentration gradient from the surface to the underlying water. Until now, the depth of CO<span class="inline-formula"><sub>2</sub></span> measurements has not been questioned, as long as measurements were done in the upper mixed layer and close to the surface. Our results provide evidence that representative measurements of CO<span class="inline-formula"><sub>2</sub></span> in the water strongly depend on depth and time of measurements.</p> |
| format | Article |
| id | doaj-art-76c9480be69c4dfaa4fd7f2756995bbc |
| institution | DOAJ |
| issn | 1726-4170 1726-4189 |
| language | English |
| publishDate | 2025-04-01 |
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| spelling | doaj-art-76c9480be69c4dfaa4fd7f2756995bbc2025-08-20T03:06:53ZengCopernicus PublicationsBiogeosciences1726-41701726-41892025-04-01221697170910.5194/bg-22-1697-2025Surface CO<sub>2</sub> gradients challenge conventional CO<sub>2</sub> emission quantification in lentic water bodies under calm conditionsP. Aurich0U. Spank1M. Koschorreck2Department Lake Research, Helmholtz Centre for Environmental Research – UFZ, Brückstraße 3a, 39114 Magdeburg, GermanyChair of Meteorology, Institute of Hydrology and Meteorology, Faculty of Environmental Sciences, Technische Universität Dresden, Pienner Straße 23, 01737 Tharandt, GermanyDepartment Lake Research, Helmholtz Centre for Environmental Research – UFZ, Brückstraße 3a, 39114 Magdeburg, Germany<p>Lakes are hotspots of inland carbon cycling and are important sources of greenhouse gases (GHGs) such as carbon dioxide (CO<span class="inline-formula"><sub>2</sub></span>). The significant role of CO<span class="inline-formula"><sub>2</sub></span> in the global carbon cycle makes quantifying its emission from various ecosystems, including lakes and reservoirs, important for developing strategies to mitigate climate change. The thin boundary layer (TBL) method is a common approach to calculating CO<span class="inline-formula"><sub>2</sub></span> fluxes from CO<span class="inline-formula"><sub>2</sub></span> measurements in both water and air, as well as wind speed. However, one assumption for the TBL method is a homogeneous CO<span class="inline-formula"><sub>2</sub></span> concentration between the measurement depth and the water surface where gas exchange takes place. This assumption might not be true under calm conditions, when microstratification below the surface slows the vertical exchange of gases. We used a floating outdoor laboratory to monitor CO<span class="inline-formula"><sub>2</sub></span> concentrations at 5 and 25 cm depth, CO<span class="inline-formula"><sub>2</sub></span> concentration in the air, wind speed, and water temperature profiles for 1 week in the Bautzen Reservoir, Germany. While we found homogeneous CO<span class="inline-formula"><sub>2</sub></span> concentrations in the two depths at wind speeds above 3 m s<span class="inline-formula"><sup>−1</sup></span>, there was a vertical gradient observed during windless nights. The concentrations observed temporally ranged from undersaturation to supersaturation at 25 and 5 cm, respectively. Fluxes calculated from the measured concentrations therefore would change from negative to positive, depending on the measurement depth. Simultaneous eddy covariance (EC) measurements showed that even the measurements close to the surface underestimated the actual CO<span class="inline-formula"><sub>2</sub></span> concentration. Oxygen measurements support our hypothesis that plankton respiration at the water surface causes a periodic CO<span class="inline-formula"><sub>2</sub></span> concentration gradient from the surface to the underlying water. Until now, the depth of CO<span class="inline-formula"><sub>2</sub></span> measurements has not been questioned, as long as measurements were done in the upper mixed layer and close to the surface. Our results provide evidence that representative measurements of CO<span class="inline-formula"><sub>2</sub></span> in the water strongly depend on depth and time of measurements.</p>https://bg.copernicus.org/articles/22/1697/2025/bg-22-1697-2025.pdf |
| spellingShingle | P. Aurich U. Spank M. Koschorreck Surface CO<sub>2</sub> gradients challenge conventional CO<sub>2</sub> emission quantification in lentic water bodies under calm conditions Biogeosciences |
| title | Surface CO<sub>2</sub> gradients challenge conventional CO<sub>2</sub> emission quantification in lentic water bodies under calm conditions |
| title_full | Surface CO<sub>2</sub> gradients challenge conventional CO<sub>2</sub> emission quantification in lentic water bodies under calm conditions |
| title_fullStr | Surface CO<sub>2</sub> gradients challenge conventional CO<sub>2</sub> emission quantification in lentic water bodies under calm conditions |
| title_full_unstemmed | Surface CO<sub>2</sub> gradients challenge conventional CO<sub>2</sub> emission quantification in lentic water bodies under calm conditions |
| title_short | Surface CO<sub>2</sub> gradients challenge conventional CO<sub>2</sub> emission quantification in lentic water bodies under calm conditions |
| title_sort | surface co sub 2 sub gradients challenge conventional co sub 2 sub emission quantification in lentic water bodies under calm conditions |
| url | https://bg.copernicus.org/articles/22/1697/2025/bg-22-1697-2025.pdf |
| work_keys_str_mv | AT paurich surfacecosub2subgradientschallengeconventionalcosub2subemissionquantificationinlenticwaterbodiesundercalmconditions AT uspank surfacecosub2subgradientschallengeconventionalcosub2subemissionquantificationinlenticwaterbodiesundercalmconditions AT mkoschorreck surfacecosub2subgradientschallengeconventionalcosub2subemissionquantificationinlenticwaterbodiesundercalmconditions |