Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian river systems – concepts, emerging trends, and research challenges

Abstract

Abstract. Human activities are drastically altering water and material flows in river systems across Asia. These anthropogenic perturbations have rarely been linked to the carbon (C) fluxes of Asian rivers that may account for up to 40–50 % of the global fluxes. This review aims to provide a conceptual framework for assessing the human impacts on Asian river C fluxes, along with an update on anthropogenic alterations of riverine C fluxes. Drawing on case studies conducted in three selected rivers (the Ganges, Mekong, and Yellow River) and other major Asian rivers, the review focuses on the impacts of river impoundment and pollution on CO2 outgassing from the rivers draining South, Southeast, and East Asian regions that account for the largest fraction of river discharge and C exports from Asia and Oceania. A critical examination of major conceptual models of riverine processes against observed trends suggests that to better understand altered metabolisms and C fluxes in “anthropogenic land-water-scapes”, or riverine landscapes modified by human activities, the traditional view of the river continuum should be complemented with concepts addressing spatial and temporal discontinuities created by human activities, such as river impoundment and pollution. Recent booms in dam construction on many large Asian rivers pose a host of environmental problems, including increased retention of sediment and associated C. A small number of studies that measured greenhouse gas (GHG) emissions in dammed Asian rivers have reported contrasting impoundment effects: decreased GHG emissions from eutrophic reservoirs with enhanced primary production vs. increased emissions from the flooded vegetation and soils in the early years following dam construction or from the impounded reaches and downstream estuaries during the monsoon period. These contrasting results suggest that the rates of metabolic processes in the impounded and downstream reaches can vary greatly longitudinally over time as a combined result of diel shifts in the balance between autotrophy and heterotrophy, seasonal fluctuations between dry and monsoon periods, and a long-term change from a leaky post-construction phase to a gradual C sink. The rapid pace of urbanization across southern and eastern Asian regions has dramatically increased municipal water withdrawal, generating annually 120 km3 of wastewater in 24 countries, which comprises 39 % of the global municipal wastewater production. Although municipal wastewater constitutes only 1 % of the renewable surface water, it can disproportionately affect the receiving river water, particularly downstream of rapidly expanding metropolitan areas, resulting in eutrophication, increases in the amount and lability of organic C, and pulse emissions of CO2 and other GHGs. In rivers draining highly populated metropolitan areas, lower reaches and tributaries, which are often plagued by frequent algal blooms and pulsatile CO2 emissions from urban tributaries delivering high loads of wastewater, tended to exhibit higher levels of organic C and the partial pressure of CO2 (pCO2) than less impacted upstream reaches and eutrophic impounded reaches. More field measurements of pCO2, together with accurate flux calculations based on river-specific model parameters, are required to provide more accurate estimates of GHG emissions from the Asian rivers that are now underrepresented in the global C budgets. The new conceptual framework incorporating discontinuities created by impoundment and pollution into the river continuum needs to be tested with more field measurements of riverine metabolisms and CO2 dynamics across variously affected reaches to better constrain altered fluxes of organic C and CO2 resulting from changes in the balance between autotrophy and heterotrophy in increasingly human-modified river systems across Asia and other continents.

Longitudinal discontinuities in riverine greenhouse gas dynamics generated by dams and urban wastewater

“…A growing body of research on greenhouse gas (GHG) emissions from inland waters has recently generated various global 35 syntheses of CO 2 and CH 4 data (Cole et al, 2007;Bastviken et al, 2011;Raymond et al, 2013;Lauerwald et al, 2015;Stanley et al, 2016;Marx et al, 2017) and conceptual frameworks incorporating anthropogenic perturbations as a critical driver of riverine biogeochemical processes in human-impacted river systems (Kaushal et al, 2012;Regnier et al, 2013;Park et al, 2018). However, these efforts have been hampered by data scarcity and inequality and inadequate consideration of multiple GHGs co-regulated by a wide range of concurrent environmental changes including anthropogenic perturbations.…”

“…Many studies have examined impoundment effects on GHG emissions from various types and sizes of dams including hydroelectric dams, often reporting contrasting results such as large pulse emissions of CO 2 and CH 4 from the flooded vegetation and sediments following dam construction and an enhanced primary production and CO 2 sink in 75 eutrophic reservoirs (Abril et al, 2005;Chen et al, 2009;Barros et al, 2011;Hu and Cheng, 2013;Maeck et al, 2013;Crawford et al, 2016;Maavara et al, 2017;Shi et al, 2017). These contrasting impoundment effects can be explained by the shifting balance between autotrophy and heterotrophy at diel to decadal timescales (Park et al, 2018). However, little is known about complex interplays among multiple factors and mechanisms concurrently affecting the production and consumption of three GHGs.…”

“…Longitudinal surveys conducted along urbanized rivers have observed large pulsatile increases in CO 2 and other GHGs in rivers receiving the effluents from wastewater treatment plants (WWTPs) (Garnier et al, 2013;Burgos et al, 2015;Alshboul et al, 2016;Yoon et al, 2017). WWTP effluents and other anthropogenic 85 sources can not only exert direct effects on riverine GHG dynamics via discharged loads of GHGs, DOC, and nutrients but also modify riverine metabolic regimes (Garnier and Billen, 2007;Park et al, 2018). The complexity of involved metabolic processes, together with the multiplicity of environmental conditions and anthropogenic perturbations, poses a great challenge for a systematic understanding of GHG dynamics in human-impacted river systems.…”

“…Although headwater streams feeding into the upper reach drain largely forested mountainous terrain, the recent expansion of croplands on steep slopes has resulted in high levels of nutrients and suspended sediment in downstream waters receiving agricultural runoff, particularly during heavy monsoon rainfalls (Park et al, 2010). A series of dams on the middle reach of the North Han River may create "serial discontinuity" in riverine metabolic processes and GHG emissions 95 (Ward and Stanford, 1983;Park et al, 2018), while urban tributaries draining the densely populated Seoul metropolitan area with a population >25 million discharge pulsatile loads of nutrients, DOC, and GHGs to the lower Han River (Yoon et al, Biogeosciences Discuss., https://doi.org/10.5194/bg-2018-278 Manuscript under review for journal Biogeosciences Discussion started: 13 June 2018 c Author(s) 2018. CC BY 4.0 License.…”

“…Another 100 important goal was to track longitudinal variations in major sources of DOM and GHGs by combining measurements of C isotopes in DOC, CO 2 , and CH 4 with a basin-scale monitoring of three GHGs. Unlike other studies focused on the quantitative importance of inland waters as sources of GHGs, this study aims to examine basin-scale patterns of riverine GHG dynamics against the conceptual frameworks that can explain longitudinal variations in riverine dissolved organic matter (DOM) and GHGs (refer to Park et al, 2018 and references therein), so only concentration data are presented without providing estimated 105 emissions of three GHGs. We expected that the comparison of reach-specific spatial patterns of three GHGs and C isotopic composition in DOM, CO 2 , and CH 4 in the river system modified by river impoundment and pollution would provide invaluable field data essential for examining emerging concepts of anthropogenically created discontinuities in riverine metabolic processes and GHG emissions.…”