title: Exploring urbanisation impacts on the spatiotemporal dynamics of riverine dissolved carbon
Creators: name: Gu, Chao
Publisher: name: University of Glasgow
Year: 2022

description: Rivers, as a significant component of inland freshwater systems, have been recognised to be carbon (C) sources to the atmosphere and oceans. Rivers are primarily oversaturated in methane (CH4) and carbon dioxide (CO2) with respect to atmospheric equilibrium and thus function as a source of CH4 and CO2 to the atmosphere. Laterally, in the form of dissolved inorganic C (DIC) and dissolved organic C (DOC), a substantial amount of C is exported to oceans annually, constituting a significant part of riverine C budgets. However, riverine C dynamics are often complex and knowledge gaps in what role urbanized rivers play in global C cycling are poorly constrained. This research addresses this knowledge gap by investigating 1), dynamics, controls, and sources of dissolved C, 2), the response of dissolved C to hydrological events, and 3), anthropogenic land-use impacts on dissolved C within an urbanized river, the River Kelvin (RK), in Scotland. At the mouth of RK, CH4 and CO2 were consistently oversaturated with respect to atmospheric equilibria over a 2-year sampling period, leading to continual degassing to the atmosphere. Terrestrially derived C comprised most of the riverine CH4 and CO2* load while dissolved inorganic carbon (DIC) from groundwater was the main form of riverine DIC. The dynamics of CH4, CO2*, and DIC in the river were primarily hydrology-controlled, i.e., [CH4] and [CO2*] both increased with elevated discharge, total [DIC] decreased with elevated discharge while the proportion of biologically-derived DIC increased with increasing discharge. The concentration of DOC showed a weak relationship with river hydrology in summer and autumn and was likely influenced by the combined sewer overflows, alongside a buffering of catchment connectivity due to hard-infrastructure. More than 99% of C degassing was in the form of CO2 while for lateral C export, annual DIC export approximately double that of DOC. Per unit area, the River Kelvin was a smaller C source to the atmosphere than natural rivers / streams but showed elevated lateral fluxes of DIC and DOC under comparable conditions. Compared to base-flow conditions, hydrological events can temporarily elevate the vertical and lateral exports of dissolved C. The contribution of terrestrial C sources to the riverine C pool was enhanced significantly by the increased discharge. Between different events, the response of particular dissolved C species could be variable and could both increase and decrease under elevated discharge conditions demonstrating dissolved C was under the impact of fine scale drivers in addition to broader hydrological controls. Dissolved C displayed variable hysteresis patterns in different events and more diversiform patterns than natural fluvial systems due the influence of urbanization. For example, unlike the fast response to rainfall observed in many natural systems, DOC displayed a lag under increased discharge when rainfall was relatively light and exhibited a stronger connectivity between OC-rich soil in the catchment and the river channel during storm periods, in which DOC was flushed into the river with higher efficacy. Hydrological events in urbanized rivers can enlarge the estimation uncertainty in the global C budget and emphasize the necessity of high-resolution monitoring on riverine C emission and export for further constraining uncertainties in global C cycling. It is highlighted of the dominant contribution of terrestrial C input in urbanized rivers and the importance of rainfall to dissolved C dynamics and export. Critically, this research illustrates potentially significant effects of urbanization on hydrological response / sensitivity of different C species, alongside the potential reduced connectivity between catchment and river channel due to hard infrastructure buffering. Through year-long monitoring of 13 sites, this research shows that at the whole catchment scale, the river is almost exclusively a source of CH4 and CO2 to the atmosphere and dissolved C concentrations show significant spatial, as well as temporal variations, reflecting the dominant proximal sources and controls. For example, the dynamics of CH4 and CO2* were significantly influenced by the proximity of coal mine infrastructure in some upper-reach regions but were more influenced by adjacent landfills in the midstream section of the rivers main channel. The concentrations of both CO2* and DOC were positively correlated with OC-rich soil associated with agriculture, indicating a potentially significant anthropogenic control on the riverine export of both CO2 and DOC. However, DIC input via groundwater at the catchment scale primarily controlled the dynamics of riverine DIC. Further, the positive correlation between δ13C-DIC and δ13C-CO2 indicated the DIC pool was at times, also significantly influenced by soil respiratory CO2. Both DIC and DOC showed a weak but significant correlation with the proportion of urban / suburban land use, suggesting an overall enhanced export of dissolved C resulting from urban expansion in this system. The findings from the whole catchment survey highlight a series of potentially key effects anthropogenic activities and land-use practices can have on riverine C dynamics. Anthropogenic perturbations can significantly increase all kinds of dissolved C export from rivers and are likely to enhance the influence of human activities over natural forcing as fluvial systems are further modified. Given the increasing rate of urban expansion, further efforts to assess dissolved C export from urbanized rivers are critical in further constraining the global C cycle. This work illustrates the potential impact future urbanization may have on regional fluvial C dynamics and their (dis)similarities to natural systems and highlights the necessity for a comprehensive elucidation of urban river C cycle dynamics.