New insight into the drivers, magnitude and sources of fluvial CO2 efflux in temperate and arctic catchments

Freshwater systems are generally found to be sources of CO2 to the atmosphere and evasion of CO2 from fluvial systems is now recognised to be a significant component of the global carbon cycle. However detailed understanding of fluvial carbon dynamics and controls on the system is lacking and global coverage of published data is sparse, but thorough understanding across a broad range of locations is crucial if global carbon budgets are to be refined. This research addresses this lack of understanding by investigating the magnitude, controls and sources of CO2 efflux across five catchments with different catchment characteristics, global locations and climate-change sensitivities. In doing so new understanding is used to explore a novel method for large-scale upscaling of CO2 efflux, time series reconstruction of the source and magnitude of CO2 efflux and incorporation of an Arctic region into the global fluvial carbon budget. The magnitude of and controls on CO2 efflux are not well understood, although it has been suggested that increased flow velocity and turbulence can enhance CO2 efflux rates. This research uses direct and contemporaneous measurements of CO2 efflux (range: -3.53 to 107 μmol CO2 m−2 s−1), flow hydraulics (e.g. mean velocity range: 0.03 to 1.39 m s-1; shear Reynolds number range: 350 to 174000), and water chemistry (e.g. pCO2 range: 388 to 4660 ppm), at sites in three UK catchments to assess whether flow intensity (a term which is used to describe one or more measures of flow strength and turbulence) is a primary control on CO2 efflux. These field sites have been chosen as they have contrasting size and land use: Drumtee Water (DW), 5.7 km2 and rural, the River Kelvin (RK), 335 km2 and urban, and the River Etive (RE), remote and snow-melt influenced. At the more soil-dominated sites DW and RK, a strong positive logarithmic relationship exists between CO2 efflux and measures of flow intensity (e.g. shear Reynolds number, overall R2 = 0.69), but this relationship is strengthened by including pCO2 (overall R2 = 0.72). Flow intensity may have a key influence on CO2 influx, although data are limited. A method using visual classification of flow intensity shows promise for supporting large-scale upscaling of fluvial CO2 efflux, if classification of water surface state can be standardised. Movement of dissolved inorganic carbon (DIC) through the hydrological cycle is an important component of global carbon budgets, and how they may respond to changing climatic conditions. However uncertainty remains about the hydrological and biogeochemical controls on DIC transmission through a catchment. Using contemporaneous measurements of DIC concentration ([DIC]) and stable carbon isotope composition of the DIC pool (δ13CDIC), fluvial DIC at more soil dominated sites, DW and RK, is found to vary considerably in response to changes in catchment hydrology. At low flow groundwater dominates, and has similar composition in both systems ([DIC]: 1.5 mmol L-1 DW, 2.0 mmol L-1 RK; δ13CDIC: -9 ‰ DW and RK) indicating a common hydrogeological inheritance in DIC, that is comparable to that of other temperate and tropical locations. Differences in composition at high flow ([DIC]: 0.1 mmol L-1 DW, 1.0 mmol L-1 RK; δ13CDIC: -23 ‰ DW, -14 ‰ RK) reflect catchment land use, and a lower contribution of soil water to the DIC pool in the more urban catchment (RK). Measured diel cycles in DIC pool composition at DW indicate biological processes modify the pool, and time series reconstructions of pool composition and CO2 efflux at DW reveal seasonal- and flow-related patterns in this biological activity. Time series reconstructions also reveal that at DW terrestrial-aquatic-atmospheric carbon cycling is rapid during event flows, with large amounts of CO2, of soil-origin, effluxed to the atmosphere in relatively short periods of time. Conversely, at low flows, CO2 efflux is of smaller magnitude and primarily fuelled by groundwater, and terrestrial-aquatic-atmospheric carbon cycling is slower. The reconstructions allow for inter-year comparisons which are useful in assessing for behaviours in CO2 source and feedback that might be typical under climate change-induced changes in hydrology (e.g. wetter winters, drier summers, more frequent large flow events). Global ice melt and permafrost thaw are increasing due to climate change, effects of melting ice and thawing permafrost on the global carbon cycle, and carbon cycling dynamics of the melt/thaw waters are not well understood. Data from the River Etive has few similarities to that of DW and RK and indicates that snow- and ice- dominated systems may behave very differently to more soil-dominated systems in terms of magnitude and controls on efflux and sources and mixing of the DIC pool. This is confirmed by data collected from the melt/thaw waters of two cryospheric systems in Greenland: a Greenland Ice Sheet (GrIS) drainage river (Akuliarusiarsuup Kuua River, or AR) and the local permafrost-landscape surface-drainage systems (PLST). CO2 efflux appears independent of flow controls in both systems, and instead seems to be pCO2 limited (average pCO2: 115 ppm AR, 596 ppm PLST), with spatial variation in AR (efflux decreases downstream) and temporal variation in PLST (efflux decreases with melt season progression). The frequent occurrence of CO2 influx (measured in 64% and 14% of cases in AR and PLST respectively), which has rarely been reported from other rivers globally, reveals that Arctic fluvial systems can periodically act as net sinks of CO2 and this should be incorporated into global carbon budgets. The occurrence of CO2 influx, and dominance of air-water CO2 exchange in these low pCO2 systems, is reflected in the DIC pool composition which is 13C-enriched and approaches isotopic equilibrium with the atmosphere (~0 ‰), and indicates that soil and ground water contributes little to the DIC pool under frozen ground conditions. Radiocarbon analysis gives further insight into the source of carbon in these systems, revealing that the GrIS is releasing old DOC (~5200 to 6600 yrs BP) upon melting, which is considered to be highly biolabile and may prime bacterial activity and feedback to climate change, and meltwaters are returning old carbon (800 to 960 yrs BP) to the atmosphere via CO2 efflux. Thus it appears that climate change (via melting ice sheets) may be a driver of the age of atmospheric carbon composition. The effluxed CO2 being less old than the DOC indicates the source of CO2 efflux is a mixed pool of respired/UV-oxidised old DOC and modern atmospheric CO2 from drawdown. In contrast to GrIS meltwaters, and the permafrost of other global locations (e.g. the Siberian Yedoma deposits), the permafrost landscape of the Kangerlussuaq region of Greenland is cycling modern carbon and appears not to be degrading, as old carbon is not found in, or degassed from, the fluvial systems. In summary this research contributes to a greater understanding of fluvial carbon dynamics and the processes controlling the return of CO2 to the atmosphere via efflux, across an array of catchment types, sizes, land uses and global locations, and makes contributions of novel data to a number of areas of fluvial carbon cycling research where there are scarcities. Marked differences in the fluvial carbon cycling dynamics of cryospheric and snow-melt dominated systems compared to soil-dominated terrestrial systems are uncovered, novel upscaling attempts made using new findings of the research, and a number of exciting new research directions and opportunities that could enhance the findings of this work are identified. Overall, this research takes steps towards a greater understanding of fluvial carbon cycling dynamics on a global scale and improved projections of the likely response of fluvial systems to climate change, ultimately aiding the community to be more prepared for what our shifting climate will bring. This work has been funded by the UK Natural Environment Research Council (NERC), and undertaken and supported at the University of Glasgow within the College of Science and Engineering (CoSE), as part of activities within the Carbon Landscape and Drainage (CLAD) research group, led by Prof. Susan Waldron, in the School of Geographical and Earth Sciences (GES). Additional funding for the field campaign to Greenland was provided by the Sir Alwyn Williams Postgraduate Scholarship in Geographical and Earth Sciences, the CoSE Graduate School Mobility Funding, SAGES (Scottish Alliance for Geoscience, Environment and Society) Theme 2 Research Enabling Funding for field research support and the Dudley Stamp Memorial Award (Royal Geographical Society with Institute of British Geographers). The Scottish Universities Environmental Research Centre helped support the stable isotope analysis and radiocarbon measurements were funded by NERC Radiocarbon Analysis award Number 1790.0414.