Spatial and temporal variability of <i>p</i>CO<sub>2</sub> and CO<sub>2</sub> emissions from the Dong River in south China

Abstract

Abstract. CO2 efflux at the water–air interface is an essential component of the riverine carbon cycle. However, the lack of spatially resolved CO2 emission measurements prohibits reliable estimation of the global riverine CO2 emissions. By deploying floating chambers, seasonal changes in river water CO2 partial pressure (pCO2) and CO2 emissions from the Dong River in south China were investigated. Spatial and temporal patterns of pCO2 were mainly affected by terrestrial carbon inputs (i.e., organic and inorganic carbon) and in-stream metabolism, both of which varied due to different land cover, catchment topography, and seasonality of precipitation and temperature. Temperature-normalized gas transfer velocity (k600) in small rivers was 8.29 ± 11.29 and 4.90 ± 3.82 m d−1 for the wet season and dry season, respectively, which was nearly 70 % higher than that of large rivers (3.90 ± 5.55 m d−1 during the wet season and 2.25 ± 1.61 m d−1 during the dry season). A significant correlation was observed between k600 and flow velocity but not wind speed regardless of river size. Most of the surveyed rivers were a net CO2 source while exhibiting substantial seasonal variations. The mean CO2 flux was 300.1 and 264.2 mmol m−2 d−1 during the wet season for large and small rivers, respectively, 2-fold larger than that during the dry season. However, no significant difference in CO2 flux was observed between small and large rivers. The absence of commonly observed higher CO2 fluxes in small rivers could be associated with the depletion effect caused by abundant and consistent precipitation in this subtropical monsoon catchment.

Basin‐Scale CO2 Emissions From the East River in South China: Importance of Small Rivers, Human Impacts and Monsoons

“…Based on the riverine CO 2 partial pressure ( p CO 2 ) and gas transfer velocity ( k ) data we collected in the ERB during five rounds of field campaigns from December 2018 to October 2019, including two (December and October) during the dry season (October–March) and three (April, July, and August) during the wet season (April–September), the total annual riverine CO 2 emissions was computed. Our earlier study in the ERB (B. Liu et al., 2021) showed that the spatial and temporal variability of CO 2 emissions mainly exhibited in three ways: (a) variability in CO 2 emissions among three sub‐basins in the ERB (i.e., the Upper, Middle, and Lower ERB) was driven by changes in their land use covers; (b) spatial variability in CO 2 emissions between large downstream rivers (fourth‐ to seventh‐order streams) and small headwater rivers (first‐ to third‐order streams) was regulated by their differences in hydrological conditions and controls of riverine p CO 2 and (c) temporal variability in CO 2 emissions between the wet and dry seasons was affected by seasonal changes in climate and hydrological conditions. Therefore, in each sub‐basin, we calculated the areal CO 2 emission fluxes from each Strahler stream order according to: $$${F}_{\mathrm{C}\mathrm{O}2}=k\times {k}_{0}\times \left({p\mathrm{C}\mathrm{O}}_{2}^{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}-{p\mathrm{C}\mathrm{O}}_{2}^{\mathrm{a}\mathrm{i}\mathrm{r}}\right)$$$ where, k is the gas transfer velocity (m d −1 ), k 0 is the solubility constants for CO 2 corrected for temperature and pressure (mol L −1 atm −1 ) from Weiss (1974), $${p\mathrm{C}\mathrm{O}}_{2}^{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$$ is the mean riverine p CO 2 for a given stream order within a given sub‐basin, and $${p\mathrm{C}\mathrm{O}}_{2}^{\mathrm{a}\mathrm{i}\mathrm{r}}$$ is average atmospheric p CO 2.…”

“…Due to the monsoon climate, precipitation is high and with substantial seasonal variability. The multi‐annual average precipitation is about 1,800 mm, of which 80% falls in the wet season from April to September (B. Liu et al., 2021). The multi‐annual average water discharge at the Boluo Hydrological Gauge, the lowermost gauge of the East River mainstem channel, is 2.37 × 10 10 m 3 yr −1, and about 80%–90% of the discharge is transported during the wet season (Y. D. Chen et al., 2011; S. Zhang et al., 2008).…”

“…Our earlier study in the ERB (B. Liu et al, 2021) showed that the spatial and temporal variability of CO 2 emissions mainly exhibited in three ways: (a) variability in CO 2 emissions among three sub-basins in the ERB (i.e., the Upper, Middle, and Lower ERB) was driven by changes in their land use covers; (b) spatial variability in CO 2 emissions between large downstream rivers (fourth-to seventh-order streams) and small headwater rivers (first-to third-order streams) was regulated by their differences in hydrological conditions and controls of riverine pCO 2 and (c) temporal variability in CO 2 emissions between the wet and dry seasons was affected by seasonal changes in climate and hydrological conditions. Therefore, in each sub-basin, we calculated the areal CO 2 emission fluxes from each Strahler stream order according to:…”

“…In the ERB, groundwater CO 2 input could be substantially diluted due to extensive precipitation in this watershed (B. Liu et al, 2021), implying that in-stream metabolism could be the key mechanic controlling the pCO 2 dynamics in small rivers. However, due to the short water residence time induced by shorter river lengths and steep river channels, organic matter inputs from surrounding landscapes could not have enough time to decompose (Hotchkiss et al, 2015;S.…”

“…The East River in South China is located in the understudied East Asia. Our recent work (B. Liu et al., 2021) reveal that the spatial and temporal characteristics of riverine CO 2 concentrations in the East River Basin (ERB) are affected by the monsoon climate and human impacts in the region, and various underlying processes in small and large rivers result in their difference in CO 2 emissions. However, it is still unclear how those factors affect the estimate of CO 2 emission fluxes and the regional C budget analysis.…”