Abstract
Carbon evasion from rivers is an important component of the global carbon cycle. The intensification of anthropogenic pressures on hydrosystems requires studies of human-impacted rivers to identify and quantify the main drivers of carbon evasion. In 2016 and 2017, four field campaigns were conducted in the Seine River network characterized by an intensively cropped and highly populated basin. We measured partial pressures of carbon dioxide (pCO2) in streams or rivers draining land under different uses at different seasons. We also computed pCO2 from an existing data set (pH, water temperature and total alkalinity) going back until 1970. Here we report factors controlling pCO2 that operate at different time and space scales. In our study, the Seine River was shown to be supersaturated in CO2 with respect to the atmospheric equilibrium, as well as a source of CO2. Our results suggest an increase in pCO2 from winter to summer in small streams draining forests (from 1670 to 2480 ppm), croplands (from 1010 to 1550 ppm), and at the outlet of the basin (from 2490 to 3630 ppm). The main driver of pCO2 was shown to be dissolved organic carbon (DOC) concentrations (R2 = 0.56, n = 119, p < 0.05) that are modulated by hydro-climatic conditions and groundwater discharges. DOC sources were linked to land use and soil, mainly leaching into small upstream streams, but also to organic pollution, mainly found downstream in larger rivers. Our long-term analysis of the main stream suggests that pCO2 closely mirrors the pattern of urban water pollution over time. These results suggest that factors controlling pCO2 operate differently upstream and downstream depending on the physical characteristics of the river basin and on the intensity and location of the main anthropogenic pressures. The influence of these controlling factors may also differ over time, according to the seasons, and mirror long term changes in these anthropogenic pressures.
Highlights
Streams and rivers are estimated to contribute significantly to carbon budgets, with two recent studies estimating carbon dioxide (CO2) emissions in the order of 0.65−+00..1270 PgC yr−1 and 1.80 ± 0.25 PgC yr−1 1,2
The streams and rivers sampled during our field campaigns were neutral or basic and carbonate-buffered (Fig. 2a,c), excluding the overestimation of calculated pressure of CO2 (pCO2) already shown to be linked to the low buffering capacity of the carbonate system[12]
The high total alkalinity measured in all the streams and rivers (Table 1) indicated that waters were carbonate-buffered due to the lithology of the basin, which is dominated by carbonate rocks (Fig. 1d)[35]
Summary
Streams and rivers are estimated to contribute significantly to carbon budgets, with two recent studies estimating carbon dioxide (CO2) emissions in the order of 0.65−+00..1270 PgC yr−1 and 1.80 ± 0.25 PgC yr−1 1,2. CO2 supersaturation in waters with respect to the CO2 atmospheric equilibrium can result from the bacterial mineralization of biodegradable organic material exported from soils and autochthonous production as well as inorganic carbon imports from soils (weathering of the bedrock, acidification of buffered waters, etc.)[6]. Around 284 ppm were measured in the Loire (croplands)[12], from 1604 to 6546 ppm in the podzolized Arcachon catchment’s streams, with higher values when discharge is low[13], and 2292 ppm in the carbonate-rock-dominated Meuse watershed, which is mostly covered by forests, grasslands and croplands[12]. A recent study of the Meuse River[14] revealed marked variations in pCO2 (34 to 10,033 ppm) the higher values being associated with watersheds dominated by agriculture and lower values with forested watersheds. CO2 undersaturation with respect to the atmospheric equilibrium has been demonstrated in the upstream part of the Danube River basin related to photosynthetic uptake in summer[15]
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