Preprints
https://doi.org/10.5194/bg-2022-62
https://doi.org/10.5194/bg-2022-62
 
11 Mar 2022
11 Mar 2022
Status: this preprint is currently under review for the journal BG.

Temporal patterns and potential drivers of CO2 emission from dry sediments of a large river

Matthias Koschorreck1, Klaus Holger Knorr2, and Lelaina Teichert1,2 Matthias Koschorreck et al.
  • 1Department of Lake Research, Helmholtz Centre for Environmental Research – UFZ, Magdeburg, 39114, Germany
  • 2Institute of Landscape Ecology, Westfälische Wilhelms-University, Münster, Germany

Abstract. River sediments falling dry at low water level are sources of CO2 to the atmosphere. While the general relevance of CO2 emissions from dry sediments has been acknowledged and some regulatory mechanisms identified, knowledge on mechanisms and temporal dynamics is still sparse. Using a combination of high frequency measurements and detailed studies we thus aimed to identify processes responsible for CO2 emissions and to assess temporal dynamics of CO2 emissions from dry sediments at a large German river.

CO2 emissions were largely driven by microbial respiration in the sediment. Observed CO2 fluxes could be explained by patterns and responses of sediment respiration rates measured in laboratory incubations. We exclude groundwater as a significant source of CO2 because potential evaporation rates were too low to explain CO2 fluxes by groundwater evaporation. Furthermore, CO2 fluxes were not related to radon fluxes, which we used to trace groundwater derived degassing of CO2.

CO2 emissions were strongly regulated by temperature resulting in large diurnal fluctuations of CO2 emissions with emissions peaking during the day. The diurnal temperature – CO2 flux relation exhibited a hysteresis which highlights the effect of transport processes in the sediment and makes it difficult to identify temperature dependence from simple linear regressions. The temperature response of CO2 flux and sediment respiration rates in laboratory incubations was identical. Also deeper sediment layers apparently contributed to CO2 emissions because the CO2 flux was correlated with the thickness of the unsaturated zone, resulting in CO2 fluxes increasing with distance to the local groundwater level and with distance to the river. Rain events lowered CO2 emissions from dry river sediments probably by blocking CO2 transport from deeper sediment layers to the atmosphere. Terrestrial vegetation growing on exposed sediments largely increased respiratory sediment CO2 emissions. We show that the regulation of CO2 emissions from dry river sediments is complex. Diurnal measurements are mandatory and even CO2 uptake in the dark by phototrophic micro-organisms has to be considered when assessing the impact of dry sediments on CO2 emissions from rivers.

Matthias Koschorreck et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-62', Anonymous Referee #1, 08 Apr 2022
  • RC2: 'Comment on bg-2022-62', Kenneth Thorø Martinsen, 24 Jun 2022

Matthias Koschorreck et al.

Matthias Koschorreck et al.

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Short summary
At low water level parts of the bottom of rivers fall dry. These beaches or mudflats are emitting the greenhouse gas carbon dioxide (CO2) to the atmosphere. We found that those emissions are caused by microbial reactions in the sediment and that they are changing with time. Emissions were influenced by many factors like temperature, water level, rain, plants, and light.
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