Scales and drivers of temporal pCO₂ dynamics in an Alpine stream

Abstract

The role of inland waters for the global carbon cycle is now recognized and evidence increasingly suggests that stream ecosystems disproportionately contribute to the carbon cycle. Understanding the dynamics and drivers of stream water partial pressure of CO 2 (pCO 2 ) and CO 2 evasion fluxes from streams to the atmosphere is imperative for assessing the role of climate change on the carbon cycle in stream ecosystems. Monitoring pCO 2 over 3 years, we here report on the seasonal, diurnal, and event-driven dynamics of pCO 2 in the hyporheic zone and stream water of an Alpine stream and assess possible drivers of these dynamics. Our findings suggest that both catchment-derived CO 2 delivered by shallow groundwater into the stream and in-stream respiration continuously build up pCO 2 in the hyporheic zone. Depending on stream water temperature and assumedly on primary production (inferred from photosynthetically active radiation), hyporheic CO 2 contributes to stream water pCO 2 and ultimately to CO 2 outgassing to the atmosphere. Diurnal patterns of stream water pCO 2 increasingly built up during extended base flow and streambed-scouring storms caused the collapse of these diurnal patterns. Post storm recovery of the diurnal pCO 2 patterns was generally rapid. Our findings suggest that decreasing gas exchange velocity related to receding discharge drives recovery dynamics. We found that average CO 2 outgassing fluxes during night exceeded those during day by up to 1.8 times. Our study highlights temperature and hydrology-key components of climate change-as major drivers of pCO 2 dynamics in Alpine streams. They also underscore the necessity to consider day-night differences in CO 2 outgassing fluxes to properly establish carbon budgets and regional estimates of CO 2 outgassing to the atmosphere.

Biogeochemical contrast between different latitudes and the effect of human activity on spatio-temporal carbon cycle change in Asian river systems

“…The simulated result showed annual-averaged evasion flux during night exceeded that during day by up to 1.17 times at most in Africa (Fig.6c). This value was smaller than the previous measurement in temperate region (Peter et al, 2014), which showed averaged evasion flux during night exceeded that during day by up to 1.8 times. However, in local scale, the simulated evasion flux during night exceeded that during day 10 by up to 1.43, 3.92, and 1.76 times temporarily in summer in Ob, Yangtze, and Mekong Rivers (Figs 4d-f).…”

“…storage decreases in nighttime (Figs 4d-f), in particular, in Asia and Africa (CO2 evasion increase and sediment storage decrease are 6.35 and 1.32 TgC/yr in Asia, and 5.67 and 0.73 TgC/yr in Africa, respectively). This result also suggests clearly that measurement pCO2 during daytime only, in most of previous observation data, might seriously underestimate annual averages of CO2 outgassing fluxes from inland water to the atmosphere (Peter et al, 2014), and that it is necessary to observed pCO2 during nighttime furthermore not only in local scale but also in regional scale. 5…”

“…This result suggests that temperature-induced physico-chemical processes can't be ignored in aquatic metabolism whereas hydrologic cycle affects primarily the dominance 30 of organic or inorganic carbons in the river as some studies pointed out (Hope et al, 1994;Leach et al, 2016). This trend is similar to Peter et al (2014) in temperate region of Austria, which suggests that (i) enhanced outgassing would reduce CO2 accumulation in the stream water and hence depress pCO2 as stream water temperature increases during daytime, and that (ii) Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-447 Manuscript under review for journal Biogeosciences Discussion started: 7 November 2017 c Author(s) 2017. CC BY 4.0 License.…”

“…vegetation, and geology, and the boundary conditions of various chemical variables. Improvement of seasonal carbon flux also depends on a compilation of the seasonal or diurnal fluxes in the previous materials as much as possible (Peter et al, 2014;Leach et al, 2016), a new construction of observation network in spatio-temporal high resolution, and the further verification of the simulated fluxes with them in addition to annual-averaged data.…”

“…While pCO2 and pCH4 are necessary to evaluate CO2 and CH4 flux to the atmosphere (evasion), DOC, DIC, and POC are also important to evaluate CO2 flux to the ocean and sediment storage. As for a temporal scale, the diurnal variation of carbon flux remains poorly evaluated except some researches (Peter et al, 2014), which pointed out that CO2 outgassing fluxes during nighttime exceeds those during daytime. From this viewpoint, it is necessary to further develop a process-oriented model to obtain a better grasp of the biogeochemical cycle in the biosphere (terrestrial and aquatic 25 ecosystems) as 'unifying currency' through a holistic approach (Prairie and Cole, 2009).…”