Effects of land use and water quality on greenhouse gas emissions from an urban river system

Abstract

Abstract. Rivers act as a natural source of greenhouse gases (GHGs) that can be released from the metabolisms of aquatic organisms. Anthropogenic activities can largely alter the chemical composition and microbial communities of rivers, consequently affecting their GHG emissions. To investigate these impacts, we assessed the emissions of CO2, CH4, and N2O from Cuenca urban river system (Ecuador). High variation of the emissions was found among river tributaries that mainly depended on water quality and neighboring landscapes. By using Prati and Oregon Indexes, a clear pattern was observed between water quality and GHG emissions in which the more polluted the sites were, the higher were their emissions. When river water quality deteriorated from acceptable to very heavily polluted, their global warming potential (GWP) increased by ten times. Compared to the average estimated emissions from global streams, rivers with polluted water released almost double the estimated GWP while the proportion increased to ten times for very heavily polluted rivers. Conversely, the GWP of good-water-quality rivers was half of the estimated GWP. Furthermore, surrounding land-use types, i.e. urban, roads, and agriculture, significantly affected the river emissions. The GWP of the sites close to urban areas was four time higher than the GWP of the nature sites while this proportion for the sites close to roads or agricultural areas was triple and double, respectively. Lastly, by applying random forests, we identified dissolved oxygen, ammonium, and flow characteristics as the main important factors to the emissions. Conversely, low impact of organic matter and nitrate concentration suggested a higher role of nitrification than denitrification in producing N2O. These results highlighted the impacts of land-use types on the river emissions via water contamination by sewage discharges and surface runoff. Hence, to estimate of the emissions from global streams, both their quantity and water quality should be included.

Carbon dioxide, methane, and dissolved carbon dynamics in an urbanized river system

“…The average CO 2 flux in the River Kelvin was greater than in the Cuenca urbanized river basin in Ecuador (tropical climate, 3.28 ± 0.61 g C·m −2 ·d −1 from sites close to urban area (Ho et al, 2020), Table 5). In Cuenca River, DO may be the most important limitation on CO 2 concentration, as the lower DO concentration (<5.6 mg·L −1 ) may restrict the respiration rate.…”

“…However, CH 4 emission from the River Kelvin was much lower than from the Adyar River (Table 5) in India (Nirmal Rajkumar et al, 2008), which is highly-polluted because of extensive and unregulated domestic developments. The global warming potential of rivers increases sharply as river water quality degrades due to pollution (Ho et al, 2020). The Adyar River experienced severe hypoxia (DO: 31.4 ± 17.7%) and eutrophication in the watershed, making the river a zone of CH 4 production.…”

“…Urbanized and anthropogenically altered catchments are often significant sources of dissolved carbon (Ho et al, 2020; McGinnis et al, 2016; Ni et al, 2019; Wang et al, 2020; Yu et al, 2017). Estimated global urban cover may reach 1.7 × 10 6 km 2 by 2050 (Zhou et al, 2019), and will be home to 70% of the human population, up from 55% in 2018 (World Bank, 2020).…”

“…The annual export of dissolved CH 4 from the River Kelvin to the Clyde Estuary was estimated at 434 ± 100 kg CÁyr À1 . The average CO 2 flux in the River Kelvin was greater than in the Cuenca urbanized river basin in Ecuador (tropical climate, 3.28 Models of gas effluxes and outlet-export for CH 4 , CO 2 *, DIC, and DOC Daily and annual data of discharge, carbon losses via gas evasions and exports to outlet at the mouth of the River Kelvin The comparison of area-normalized gas evasions (A) and export of dissolved carbon (B) in worldwide urban rivers 0.61 g CÁm À2 Ád À1 from sites close to urban area(Ho et al, 2020),…”

“…Daily and annual data of discharge, carbon losses via gas evasions and exports to outlet at the mouth of the River Kelvin 0.61 g CÁm À2 Ád À1 from sites close to urban area(Ho et al, 2020), Table5). In Cuenca River, DO may be the most important limitation on CO 2 concentration, as the lower DO concentration (<5.6 mgÁL À1 ) may restrict the respiration rate.…”