Spatial Variability and Hotspots of Methane Concentrations in a Large Temperate River

Ingeborg Bussmann,Uta Koedel,Matthias Koschorreck,Norbert Kamjunke,Claudia Schütze

doi:10.3389/fenvs.2022.833936

Abstract

Rivers are significant sources of greenhouse gases (GHGs; e.g., CH4 and CO2); however, our understanding of the large-scale longitudinal patterns of GHG emissions from rivers remains incomplete, representing a major challenge in upscaling. Local hotspots and moderate heterogeneities may be overlooked by conventional sampling schemes. In August 2020 and for the first time, we performed continuous (once per minute) CH4 measurements of surface water during a 584-km-long river cruise along the German Elbe to explore heterogeneities in CH4 concentration at different spatial scales and identify CH4 hotspots along the river. The median concentration of dissolved CH4 in the Elbe was 112 nmol L−1, ranging from 40 to 1,456 nmol L−1 The highest CH4 concentrations were recorded at known potential hotspots, such as weirs and harbors. These hotspots were also notable in terms of atmospheric CH4 concentrations, indicating that measurements in the atmosphere above the water are useful for hotspot detection. The median atmospheric CH4 concentration was 2,033 ppb, ranging from 1,821 to 2,796 ppb. We observed only moderate changes and fluctuations in values along the river. Tributaries did not obviously affect CH4 concentrations in the main river. The median CH4 emission was 251 μmol m−2 d−1, resulting in a total of 28,640 mol d−1 from the entire German Elbe. Similar numbers were obtained using a conventional sampling approach, indicating that continuous measurements are not essential for a large-scale budget. However, we observed considerable lateral heterogeneity, with significantly higher concentrations near the shore only in reaches with groins. Sedimentation and organic matter mineralization in groin fields evidently increase CH4 concentrations in the river, leading to considerable lateral heterogeneity. Thus, river morphology and structures determine the variability of dissolved CH4 in large rivers, resulting in smooth concentrations at the beginning of the Elbe versus a strong variability in its lower parts. In conclusion, groin construction is an additional anthropogenic modification following dam building that can significantly increase GHG emissions from rivers.

Highlights

By monitoring the CH4 mixing ratio in the atmosphere in parallel, we explored the potential effects of fluctuating atmospheric CH4 concentrations on the calculated CH4 emissions, the possibility of detecting hotspots based on atmospheric measurements and to improve the estimation of the diffusive CH4 flux from the Elbe
There was a clear increase in CH4 concentration to approximately 200 nmol L−1 after km-430 (Figure 2A)
The Elbe River was consistently oversaturated with CH4, rendering it a steady source of the emission of this greenhouse gases (GHGs) to the atmosphere

Summary

Introduction

Awareness regarding the significant contributions of inland waters, such as lakes, reservoirs, or rivers, to the global CH4 budget (103 Tg CH4 year−1) has been increasing (Rosentreter et al, 2021); relatively few studies have investigated CH4 dynamics in flowing waters (Bastviken et al, 2011). Streams and rivers have garnered much attention as the sources of atmospheric CH4 (Stanley et al, 2016). Regardless of being well-aerated, lotic waters are typically oversaturated with CH4 compared to water in equilibrium with atmospheric CH4, resulting in significant diffusive emissions to the atmosphere (Stanley et al, 2016). The precise quantification of these CH4 emissions is challenging because of large spatiotemporal heterogeneities. The magnitude of these diffusive emissions depends on the water turbulence, physical gas transfer coefficient, and CH4 partial pressure difference between water and the atmosphere (Donelan et al, 2002). While outgassing to the atmosphere is the most important CH4 loss process, microbial CH4 consumption in the water is relatively slow (Matoušů et al, 2019). Possible phytoplanktonmediated CH4 production under oxic conditions has been discussed (Tang et al, 2016)

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Discussion

Conclusion