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Recent Progress in CO₂ Emission from the World’s Rivers

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A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Hydrology".

Deadline for manuscript submissions: closed (31 December 2023) | Viewed by 33520

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Special Issue Editors

Prof. Dr. Yijun Xu

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Guest Editor

Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA
Interests: hydrology; fluvial geomorphology; biogeochemical cycling; sediment transport
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Siyue Li

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Guest Editor

School of Environmental Ecology and Biological Engineering, Institute of Changjiang Water Environment and Ecological Security, Hubei Key Laboratory of Novel Reactor and Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430205, China
Interests: watershed ecology; biogeochemical cycle and global change; river runoff adaptability and material flux; water environment geochemistry

Special Issue Information

Dear Colleagues,

We are guest-editing a feature paper Special Issue of Water, titled “Recent Progress in CO₂ Emissions from the World’s Rivers”. Manuscripts will be selected through both invitation and open call, and the publisher Water will waive the 1000 CHF fee for all accepted papers in this feature paper Special Issue.

Terrestrial carbon exports from river systems to coastal and marine environments are a crucial component of the global carbon cycle. However, current global estimates of CO₂ flux from rivers to the atmosphere vary largely, i.e., nearly nine-folds from the lowest to the highest estimate, and the large discrepancy may stem from a number of factors. This feature paper Special Issue aims to bring together the latest research on CO₂ emissions from the world’s rivers, from headwaters to lowland and estuarine rivers. We encourage submissions of high-quality studies concerning dissolved carbon transportation and transformation in riverine systems across climatic, geological, and land-use gradients. We especially encourage papers that address CO₂ from the word’s major rivers and that stimulate critical thinking pertinent to constraining uncertainties in the global estimation of riverine CO₂ emissions.

Prof. Dr. Y. Jun Xu
Prof. Dr. Siyue Li
Guest Editors

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Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Water is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

riverine CO₂ emission
seasonal and diurnal fluctuation of riverine pCO₂
large river carbon transformation
land use effects on riverine CO₂
climate change effects on riverine CO₂
lateral vs. vertical transport of dissolved carbon
stream order effects on predicting pCO₂ and CO₂
uncertainties in CO₂ estimation at different spatial and temporal scales
advancement in measurement and modeling of riverine pCO₂ and CO₂ outgassing

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Published Papers (11 papers)

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Research

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15 pages, 9848 KiB

Open AccessArticle

Quality and Biodegradation Process of Dissolved Organic Carbon in Typical Fresh-Leaf Leachate in the Wuhan Urban Forest Park

by Xiaokang Tian and Siyue Li

Water 2024, 16(4), 558; https://doi.org/10.3390/w16040558 - 12 Feb 2024

Viewed by 1323

Abstract

The study investigated the leaching and biodegradation of dissolved organic carbon in leaf leachates from typical fresh leaves in the Wuhan Urban Forest Park, Central China. The fresh leaf-leached dissolved organic carbon quality and biodegradability, as well as their potential determinants, were examined for 12 major tree species, including deciduous trees and shrubs. A 28-day indoor incubation was conducted at two temperature conditions of 20 °C and 30 °C. Sampling was conducted within the planned time frame for experimental measurements, and a first-order kinetic model for dissolved organic carbon degradation was fitted. The utilization of the fir tree as the predominant deciduous species and cuckoo as the primary shrubs provided advantages in increasing the carbon sequestration capacity of urban forests. There was no significant difference in the degradation rate of the leaching solution at different temperatures, but the k value of the first-order kinetic model was different. At 20 °C, the dissolved organic carbon degradation rate was positively correlated with electrical conductivity and total dissolved nitrogen, while it was negatively correlated with the humification index and ratio of dissolved organic carbon to total dissolved nitrogen. At 30 °C, the degradation rate of dissolved organic carbon showed a positive correlation with total dissolved phosphorus and total dissolved nitrogen, while it was negatively correlated with the humification index, ratio of dissolved organic carbon to total dissolved nitrogen and ratio of dissolved organic carbon to total dissolved phosphorus. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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19 pages, 3307 KiB

Open AccessArticle

Economic and Industrial Development SignificantlyContribute to Acidity and Ionic Compositions of Rainwaterin China

by Xi Huang and Siyue Li

Water 2024, 16(2), 193; https://doi.org/10.3390/w16020193 - 5 Jan 2024

Cited by 1 | Viewed by 1333

Abstract

To achieve a holistic understanding of the intricate interactions among human activities, atmospheric chemistry, and acid rain in China, a rigorous analysis of rainwater chemistry was made using a dataset comprising 2656 data points from 24 sites. The main cation and anion in the chemical composition of precipitation were Ca²⁺ and SO₄²⁻ in China, with an average concentration of 169.9 μeq/L and 135.4 μeq/L, respectively. Acid rain generally occurs in southern cities such as Shenzhen, Guangzhou, Zhuhai, Xiamen, and Chongqing. There were evident regional disparities in acidity and ion concentrations in rainwater, with an increase in acidity and a decrease in ion concentrations from north to south across China. Utilizing positive matrix factorization, the study found that NH₄⁺, SO₄²⁻, and NO₃⁻ mainly originated from anthropogenic sources such as fossil fuel combustion, vehicle exhaust emissions, agricultural fertilization, and industrial emissions (as reflected by F3 and F4). Ca²⁺ mainly stems from crustal factors, including industrial dust and natural crust (as represented by F1 and F4). Na⁺ and Cl⁻ were traceable from marine sources (as reflected by F5), while Mg²⁺ originated from crust origin (as presented by F1). K⁺ was mainly derived from a mixed source of crust, marine, and biomass burning (as indicated by F2 and F3). The correlation analyses showed that SO₄²⁻ and NO₃⁻ showed significant correlations with GDP and population. F⁻ was associated with wastewater, which may be linked to the production of brick and tiles from clay with high fluoride contents. The pH was negatively related to industrial wastewater. Long-term analysis of precipitation chemistry in four cities suggested a clear decrease in the proportion of SO₄²⁻ but a considerable increase in the proportion of NO₃⁻ in anions in metropolitans of Shanghai and Chongqing due to the environmental measures that targeted reducing sulfur dioxide (SO₂) emissions and increase of vehicles. This showed that pollution control strategies had an impact on precipitation ion concentrations. These results can conclude that economic and industrial growth, which will increase energy consumption, utilization of coal combustion, and a subsequent rise in pollutant emissions, can contribute to the change in the chemical compositions of rainwater and the exacerbation of acid rain. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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15 pages, 3704 KiB

Open AccessArticle

The Influence of Seasonal Variability of Eutrophication Indicators on Carbon Dioxide and Methane Diffusive Emissions in the Largest Shallow Urban Lake in China

by Bingjie Ma, Yang Wang, Ping Jiang and Siyue Li

Water 2024, 16(1), 136; https://doi.org/10.3390/w16010136 - 29 Dec 2023

Cited by 1 | Viewed by 1779

Abstract

Eutrophication is prevalent in urban lakes; however, a knowledge gap exists regarding eutrophication influences on carbon dynamics in these ecosystems. In the present study, we investigated the carbon dioxide (CO₂) and methane (CH₄) concentration and diffusion fluxes in Lake Tangxun (the largest shallow Chinese urban lake) in the autumn and winter of 2022 and spring and summer of 2023. We found that Lake Tangxun served as a source of GHGs, with average emission rates of 5.52 ± 12.16 mmol CO₂ m⁻² d⁻¹ and 0.83 ± 2.81 mmol CH₄ m⁻² d⁻¹, respectively. The partial pressure of dissolved CO₂ (pCO₂) (averaging 1321.39 ± 1614.63 μatm) and dissolved CH₄ (dCH₄) (averaging 4.29 ± 13.71 μmol L⁻¹) exceeded saturation levels. Seasonal variability was observed in the pCO₂ and dCH₄ as well as CH₄ fluxes, while the CO₂ flux remained constant. The mean pCO₂ and dCH₄, as well as carbon emissions, were generally higher in summer and spring. pCO₂ and dCH₄ levels were significantly related to total nitrogen (TN), total phosphorus (TP), and ammonium-nitrogen (N-NH₄⁺), and N-NH₄⁺ was a main influencing factor of pCO₂ and dCH₄ in urban eutrophic lakes. The positive relationships of pCO₂, dCH₄ and trophic state index highlighted that eutrophication could elevate CO₂ and CH₄ emissions from the lake. This study highlights the fact that eutrophication can significantly increase carbon emissions in shallow urban lakes and that urban lakes are substantial contributors to the global carbon budget. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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26 pages, 9079 KiB

Open AccessFeature PaperArticle

Dissolved Carbon Concentrations and Emission Fluxes in Rivers and Lakes of Central Asia (Sayan–Altai Mountain Region, Tyva)

by Arisiya A. Byzaakay, Larisa G. Kolesnichenko, Iury Ia. Kolesnichenko, Aldynay O. Khovalyg, Tatyana V. Raudina, Anatoly S. Prokushkin, Inna V. Lushchaeva, Zoia N. Kvasnikova, Sergey N. Vorobyev, Oleg S. Pokrovsky and Sergey Kirpotin

Water 2023, 15(19), 3411; https://doi.org/10.3390/w15193411 - 28 Sep 2023

Cited by 1 | Viewed by 2049

Abstract

The carbon (C) cycle in inland waters, including carbon concentrations in and carbon dioxide (CO₂) emissions from water surfaces, are at the forefront of biogeochemical studies, especially in regions strongly impacted by ongoing climate change. Towards a better understanding of C storage, transport and emission in Central Asian mountain regions, an area of knowledge that has been extremely poorly studied until now, here, we carried out systematic measurements of dissolved C and CO₂ emissions in rivers and lakes located along a macrotransect of various natural landscapes in the Sayan–Altai mountain region, from the high mountains of the Western Sayan in the northwest of Tyva to the arid (dry) steppes and semideserts in the intermountain basins in the southeast of Tyva on the border with Mongolia. New data on major hydrochemical parameters and CO₂ fluxes (fCO₂) gathered by floating chambers and dissolved organic and inorganic carbon (DOC and DIC, respectively) concentrations collected over the four main hydrological seasons allowed us to assess the current C biogeochemical status of these water bodies in order to judge possible future changes under climate warming. We further tested the impact of permafrost, river watershed size, lake area and climate parameters as well as ‘internal’ biogeochemical drivers (pH, mineralization, organic matter quality and bacterial population) on CO₂ concentration and emissions in lakes and rivers of this region and compared them with available data from other subarctic and mountain settings. We found strong environmental control of the CO₂ pattern in the studied water bodies, with thermokarst lakes being drastically different from other lakes. In freshwater lakes, pCO₂ negatively correlated with O₂, whereas the water temperature exerted a positive impact on pCO₂ in large rivers. Overall, the large complexity of counteracting external and internal drivers of CO₂ exchange between the water surfaces and the atmosphere (CO₂-rich underground DIC influx and lateral soil and subsurface water; CO₂ production in the water column due to dissolved and particulate OC biodegradation; CO₂ uptake by aquatic biota) precluded establishing simple causalities between a single environmental parameter and the fCO₂ of rivers and lakes. The season-averaged CO₂ emission flux from the rivers of Tyva measured in this study was comparable, with some uncertainty, to the C uptake fluxes from terrestrial ecosystems of the region, which were assessed in other works. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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17 pages, 2035 KiB

Open AccessArticle

Large Uncertainties in CO₂ Water–Air Outgassing Estimation with Gas Exchange Coefficient K_T for a Large Lowland River

by Anamika Dristi and Y. Jun Xu

Water 2023, 15(14), 2621; https://doi.org/10.3390/w15142621 - 19 Jul 2023

Cited by 2 | Viewed by 1908

Abstract

Aquatic CO₂ emission is typically estimated (i.e., not measured) through a gas exchange balance. Several factors can affect the estimation, primarily flow velocity and wind speed, which can influence a key parameter, the gas exchange coefficient K_T in the balancing approach. However, our knowledge of the uncertainty of predictions using these factors is rather limited. In this study, we conducted a numeric assessment on the impact of river flow velocity and wind speed on K_T and the consequent CO₂ emission rate. As a case study, we utilized 3-year (2019–2021) measurements on the partial pressure of dissolved carbon dioxide (pCO₂) in one of the world’s largest alluvial rivers, the lower Mississippi River, to determine the difference in CO₂ emission rate estimated through three approaches: velocity-based K_T, wind-based K_T, and a constant K_T (i.e., K_T = 4.3 m/day) that has been used for large rivers. Over the 3-year study period, river flow velocity varied from 0.75 ms⁻¹ to 1.8 ms⁻¹, and wind speed above the water surface fluctuated from 0 ms⁻¹ to nearly 5 ms⁻¹. Correspondingly, we obtained a velocity-based K_T value of 7.80–22.11 m/day and a wind-speed-based K_T of 0.77–8.40 m/day. Because of the wide variation in K_T values, the estimation of CO₂ emission using different approaches resulted in a substantially large difference. The velocity-based K_T method yielded an average CO₂ emission rate (FCO₂) of 44.36 mmol m⁻² h⁻¹ for the lower Mississippi River over the 3-year study period, varying from 6.8 to 280 mmol m⁻² h⁻¹. In contrast, the wind-based K_T method rendered an average FCO₂ of 10.05 mmol m⁻² h⁻¹ with a small range of fluctuation (1.32–53.40 mmol m⁻² h⁻¹,), and the commonly used constant K_T method produced an average FCO₂ of 11.64 mmol m⁻² h⁻¹, also in a small range of fluctuation (2.42–56.87 mmol m⁻² h⁻¹). Based on the findings, we conclude that the effect of river channel geometry and flow velocity on CO₂ outgassing is still largely underestimated, and the current estimation of global river CO₂ emission may bear large uncertainty due to limited spatial coverage of flow conditions and the associated gas exchange variation. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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17 pages, 3463 KiB

Open AccessArticle

Interconnected River–Lake Project Decreased CO₂ and CH₄ Emission from Urban Rivers

by Chunlin Wang, Yuhan Xv, Siyue Li and Xing Li

Water 2023, 15(11), 1986; https://doi.org/10.3390/w15111986 - 24 May 2023

Cited by 4 | Viewed by 2270

Abstract

Urban riverine networks are hotspots of CO₂ and CH₄ emissions, due to river impoundment and pollution. The river–lake connection is considered to be an important way to improve the ecological environment of urban rivers; however, its impact on CO₂ and CH₄ emissions from urban rivers and regulatory mechanisms are still unclear. Rivers and lakes have been studied separately by lots of traditional studies. In this study, we investigated the concentration and emission of CO₂ and CH₄ from March 2021 to December 2021 in an interconnected river–lake system in Central China. We found that the urban river–lake system was a hotspot of CO₂ and CH₄ emissions. CO₂ and CH₄ emissions from urban rivers were much higher than those from the lakes, which are 2.7 times and 11.9 times that of lakes, respectively. The correlation analysis indicated that the spatial variation of CO₂ and CH₄ emissions was determined by nutrient content. The abundant nutrients promoted microbial growth and consumed dissolved oxygen (DO), thus resulting in high emissions of CO₂ and CH₄ in the isolated urban rivers (UR). The average CO₂ and CH₄ emissions of urban rivers are 991.56 and 14.82 mmol m⁻² d⁻¹, respectively. The river–lake connection decreased the nutrients of urban rivers connected to lakes (LUR). The moderate nutrients wreaked in situ respiration, exhibiting moderate CO₂ emission in the LUR. The average CO₂ emission of LUR is 543.49 mmol m⁻² d⁻¹. The river–lake connection increased the DO concentrations in the LUR, inhibited methanogenesis, and enhanced CH₄ oxidation, reducing CH₄ emission from LUR sharply. The average CH₄ emission of LUR is 1.26 mmol m⁻² d⁻¹. A correlation analysis showed that the seasonal variations of CO₂ and CH₄ emissions were controlled by DO and T. Hence, the highest emissions of CO₂ were observed in the spring and the lowest in the winter, and the CO₂ emissions in spring were 10.7 times that in winter. The highest emissions of CH₄ were observed in the summer and the lowest in the winter, and the CH₄ emissions in summer were 6.6 times those in winter. The connection of urban rivers and lakes changes the environmental factors, thereby varying the production and emission of greenhouse gases. This study advanced the knowledge of the greenhouse gas emission response to the river–lake connection, providing the theoretical basis for greenhouse gas emission reduction from urban rivers. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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16 pages, 1917 KiB

Open AccessFeature PaperArticle

Variability of Carbon Export in the Lower Mississippi River during an Extreme Cold and Warm Year

by Lee Potter and Y. Jun Xu

Water 2022, 14(19), 3044; https://doi.org/10.3390/w14193044 - 27 Sep 2022

Cited by 3 | Viewed by 3209

Abstract

The Mississippi River (MR) discharges on average 474 km³ of water annually into the Northern Gulf of Mexico (NGOM) with a large quantity of carbon, playing a vital role in the ecosystem’s food chain and water quality. In this study, we analyzed exports of dissolved inorganic (DIC) and organic carbon (DOC) from January 2021 to December 2021, during which the contiguous United States experienced one of the coldest winters as well as the hottest summer on record. Bi-weekly in situ river measurements and water sampling were conducted in the lower MR at Baton Rouge in Louisiana, USA, approximately 368 km from the river’s mouth. We found that the MR transported 12.61 Tg C of DIC and 4.54 Tg C of DOC into the NGOM during the study period. Much of the DOC mass export occurred during the winter (~38%), while much of the DIC mass export took place in the spring months (~35%). The seasonality of DOC and DIC exports was affected by their concentrations, water temperature, and discharge. DIC concentrations were significantly higher in the fall (32.0 mg L⁻¹) than those during the winter (20.4 mg L⁻¹), while DOC concentrations were highest during the winter months (11.3 mg L⁻¹) and varied seasonally, however, not significantly. Partial pressure of dissolved carbon dioxide (pCO₂) in the MR averaged 1703 ± 646 µatm peaking in the summer at 2594 µatm and reaching a low in the winter at 836 µatm. Outgassing of CO₂ (FCO₂) peaked in the spring averaging 3.43 g C m² d⁻¹ and was lowest in the winter at 1.62 g C m² y⁻¹. Our findings validate our initial hypotheses that seasonal variability and weather extremes strongly affect terrestrial-aquatic carbon transfer, and that climate change will likely intensify carbon export from the Mississippi River Basin. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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30 pages, 4520 KiB

Open AccessArticle

The Seasonal Dynamics of Organic and Inorganic Carbon along the Tropical Usumacinta River Basin (Mexico)

by Ismael Soria-Reinoso, Javier Alcocer, Salvador Sánchez-Carrillo, Felipe García-Oliva, Daniel Cuevas-Lara, Daniela Cortés-Guzmán and Luis A. Oseguera

Water 2022, 14(17), 2703; https://doi.org/10.3390/w14172703 - 30 Aug 2022

Cited by 10 | Viewed by 4245

Abstract

Rivers are important sites for carbon (C) transport and critical components of the global C cycle that is currently not well constrained. However, little is known about C species’ longitudinal and temporal changes in large tropical rivers. The Usumacinta River is Mexico’s main lotic system and the tenth largest in North America. Being a tropical river, it has a strong climatic seasonality. This study aims to evaluate how organic (DOC and POC) and inorganic (DIC and PIC) carbon change spatially and seasonally along the Usumacinta River (medium and lower basin) in rainy (RS-2017) and dry (DS-2018) seasons and to estimate C fluxes into the southern Gulf of Mexico. Concentrations of DOC, POC, DIC, and PIC ranged from 0.88 to 7.11 mg L⁻¹, 0.21 to 3.78 mg L⁻¹, 15.59 to 48.27 mg L⁻¹, and 0.05 to 1.51 mg L⁻¹, respectively. DOC was the dominant organic species (DOC/POC > 1). It was ~doubled in RS and showed a longitudinal increase, probably through exchange with wetlands and floodplains. Particulate carbon showed a positive relationship with the total suspended solids, suggesting that in RS, it derived from surface erosion and runoff in the watershed. DIC is reported for the first time as the highest concentration measured in tropical rivers in America. It was higher in the dry season without a longitudinal trend. The C mass inflow–outflow balance in the RS suggested net retention (DOC and POC sink) in floodplains. In contrast, in the DS, the balance suggested that floodplains supply (C source) autochthonous DOC and POC. The lower Usumacinta River basin is a sink for DIC in both seasons. Finally, the estimated annual C export for the Usumacinta-Grijalva River was 2.88 (2.65 to 3.14) Tg yr⁻¹, of which DIC was the largest transported fraction (85%), followed by DOC (10%), POC (4%), and PIC (<1%). This investigation is the first to present the C loads in a Mexican river. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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11 pages, 3328 KiB

Open AccessArticle

Total Organic Carbon Concentration and Export in a Human-Dominated Urban River: A Case Study in the Shenzhen River and Bay Basin

by Yilong Huang, Liping Zhang and Lishan Ran

Water 2022, 14(13), 2102; https://doi.org/10.3390/w14132102 - 30 Jun 2022

Cited by 9 | Viewed by 3050

Abstract

In order to understand the organic carbon dynamics in urban rivers, the present study monitored the total organic carbon (TOC) concentration and export in the Shenzhen River and Bay basin. The results show that the average TOC concentrations ranged from 7.04 to 17.50 mg/L in the study area, which exhibited pronounced spatial and temporal variations due to urbanization level, rainfall–runoff, and effluent of wastewater treatment plants (WWTPs). The TOC concentrations of rainwater were averaged at 4.03 mg/L during 2011–2012, which was higher than that of some urban river basins in developed countries. As an average rainfall year, the total TOC export in 2012 was 11.2 × 10⁶ kg/yr in the study basin, of which 37.5% was contributed by the effluent of WWTPs, 14.1% by wet deposition, and 48.4% by the surface non-point sources and endogenous pollution. The areal yield of TOC in the Shenzhen River and Bay basin was 23.73 × 10³ kg/(km².yr) in 2012, which was 2.86 times the Pearl River’s average value and 6.43 times the global average value. According to the predicted values of linear regression, the TOC concentration showed a gradual downward trend (R = 0.87, p < 0.001, n = 14) during the period 2006–2019, which also induced a decreasing TOC export (R = 0.23, p > 0.05, n = 14). Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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18 pages, 3249 KiB

Open AccessArticle

Driving Factors of Total Organic Carbon in Danjiangkou Reservoir Using Generalized Additive Model

by Yeqing Jiang, Kang He, Yuying Li, Mingqing Qin, Zhenzhen Cui, Yun Zhang, Yinlei Yao, Xiaonuo Chen, Minjie Deng, Andrew Gray and Bailian Li

Water 2022, 14(6), 891; https://doi.org/10.3390/w14060891 - 12 Mar 2022

Cited by 7 | Viewed by 4112

Abstract

Dynamic changes in total organic carbon (TOC) concentration in lakes and reservoirs affect the functions of aquatic ecosystems and are a key component of water quality management, especially in drinking water sources. The Danjiangkou Reservoir is the water source area of the Middle Route Project of the South-to-North Water Diversion in China. Its water quality is of critical importance to the safety of water diversion. TOC concentration and other environmental factors at 19 sampling sites in the Danjiangkou Reservoir were investigated quarterly during 2020–2021 to explore the differences at the spatio-temporal scales. A generalized additive model (GAM) was used to analyze the environmental factors correlated with the observed spatio-temporal variations of TOC concentration. The results showed that the comprehensive trophic level index (TLI) of the Danjiangkou Reservoir was under the state of intermediate nutrition, and the water quality was overall good. In terms of temporal patterns, TOC concentration was higher in both spring and summer and lower in other seasons. Spatially, TOC concentrations were found in descending order from the site of outlet, Han reservoir, entrance of reservoir, and Dan reservoir. The single-factor GAM model showed that TOC correlated with different environmental factors across spatio-temporal scales. Water temperature (WT), permanganate index (COD_Mn), and ammonia nitrogen (NH₄⁺-N) were significantly correlated with TOC in autumn, but only total nitrogen (TN) and transparency (SD) were significant in winter. Spatially, WT, chemical oxygen demand (COD), NH₄⁺-N, TN, and conductivity (Cond) correlated with TOC in the Dan reservoir, but WT, COD, NH₄⁺-N, total phosphorus (TP), and chlorophyll a (Chl.a) were significant in the Han reservoir. The multi-factor GAM model indicated that the environmental factors correlated with TOC concentration were mainly WT, TN, Cond, COD_Mn, and TP, among which WT and Cond showed a significant linear relationship with TOC concentration (edf = 1, p < 0.05), while TN, COD_Mn, and TP had a significant nonlinear relationship with TOC concentration (edf > 1, p < 0.05). Comprehensive trophic level index (TLI) and TOC concentration revealed a highly significant correlation (R² = 0.414, p < 0.001). Therefore, the GAM model could well explain the environmental factors associated with the spatio-temporal dynamics of TOC concentration, providing a reference for the evaluation of water quality and research on the carbon cycle in similar inland reservoirs. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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20 pages, 804 KiB

Open AccessFeature PaperReview

India’s Contribution to Greenhouse Gas Emission from Freshwater Ecosystems: A Comprehensive Review

by Bipradeep Mondal, Kuldeep Bauddh, Amit Kumar and Nirmali Bordoloi

Water 2022, 14(19), 2965; https://doi.org/10.3390/w14192965 - 21 Sep 2022

Cited by 8 | Viewed by 5791

Abstract

In the modern era, due to urbanization, industrialization, and anthropogenic activities in the catchment, greenhouse gas (GHG; CO₂, CH₄, and N₂O) emissions from freshwater ecosystems received scientific attention because of global warming and future climate impacts. A developing country such as India contributes a huge share (4% of global) of GHGs from its freshwater ecosystems (e.g., rivers, lakes, reservoirs) to the atmosphere. This is the first comprehensive review dealing with the GHG emissions from Indian freshwater bodies. Literature reveals that the majority of GHG from India is emitted from its inland water, with 19% of CH₄ flux and 56% of CO₂ flux. A large part of India’s gross domestic product (GDP) is manipulated by its rivers. As a matter of fact, 117.8 Tg CO₂ year⁻¹ of CO₂ is released from its major riverine waters. The potential of GHG emissions from hydropower reservoirs varies between 11–52.9% (mainly CH₄ and CO₂) because of spatio-temporal variability in the GHG emissions. A significant contribution was also reported from urban lakes, wetlands, and other inland waters. Being a subtropical country, India is one of the global GHG hotspots, having the highest ratio (GHG: GDP) of 1301.79. However, a large portion of India’s freshwater has not been considered yet, and there is a need to account for precise regional carbon budgets. Therefore, in this review, GHG emissions from India’s freshwater bodies, drivers behind GHG emissions (e.g., pH, mean depth, dissolved oxygen, and nutrients), and long-term climatic risks are thoroughly reviewed. Besides research gaps, future directions and mitigation measures are being suggested to provide useful insight into the carbon dynamics (sink/source) and control of GHG emissions. Full article

(This article belongs to the Special Issue Recent Progress in CO₂ Emission from the World’s Rivers)

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