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 40-20 50% of the global fluxes. The primary object of this review was to provide a conceptual framework for assessing human impacts on Asian river C fluxes, along with a latest update on anthropogenic alterations of riverine C fluxes, focusing on the impacts of water pollution and river impoundments 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. Recent booms in dam construction across Asia have created a host of environmental problems; yet only a small number of studies have 25 explicitly investigated altered rates of greenhouse gas (GHG) emissions and organic C transport. There have been contrasting reports on impoundment effects: decreases in GHG emissions in the reservoirs exhibiting enhanced primary production vs. increased emissions from the flooded vegetation and soils in the early years following dam construction or from the impounded river 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 greatly vary longitudinally over time, as 30 a combined result of diel shifts in the balance between autotrophy and heterotrophy, seasonal fluctuations between the dry and monsoon periods, and a long-term change from a leaky post-construction phase to a gradual C sink. Rapid pace of urbanization across southern and eastern Asian regions has dramatically increased municipal water withdrawal, generating annually 120.2 km 3 of wastewater in 24 countries, which comprises 38.6% of the global municipal wastewater production disproportionately affect the receiving river water, particularly downstream of rapidly expanding metropolitan areas, including eutrophication, increases in the amount and lability of organic C, and pulse emissions of CO2 and other greenhouse gases (GHGs). As reviewed for three representative rivers (the Ganges, Mekong, and Yellow River), the lower reaches of these rivers and their polluted tributaries tend to exhibit higher levels of organic C and the partial pressure of CO2 (pCO2) than the eutrophic reservoirs and less impacted upstream reaches. More field measurements of pCO2, together with accurate 40 flux calculations based on river-specific model parameters, are urgently required to provide more accurate estimates of GHG emissions from the Asian rivers that are now underrepresented in the global C budgets. Researchers working on individual river systems need to be linked to collaborative research networks to facilitate global synthesis of local field data. These synthesis efforts, combined with conceptual and mathematical models, will contribute to a better understanding of how anthropogenic perturbations in rapidly urb...

Localized Pollution Impacts on Greenhouse Gas Dynamics in Three Anthropogenically Modified Asian River Systems

“…The Ganges, Mekong, and Yellow River, which originate in the Himalayan mountains and/or Tibetan Plateau, drain some of the largest watersheds in the world with a total area exceeding 2,500 × 10 3 km 2 (Milliman & Farnsworth, 2011). These river systems share common hydrologic and demographic features, including intense monsoon‐driven seasonality in discharge and high population densities in the river basin (Park et al., 2018). More detailed information on each of these river basins is provided in Supporting Information.…”

“…Consequently, Asian rivers are significant contributors to global riverine GHG emissions; for instance, N 2 O emissions from Asian rivers account for 33% (32.2 Gg N 2 O‐N yr −1 ) of the global riverine emission (Hu et al., 2016). Despite large uncertainties in estimating CO 2 and CH 4 emissions from Asian rivers due to scarcity of field measurements (Park et al., 2018), CO 2 emissions from SE Asian rivers were estimated to exceed 10 g C m −2 yr −1 , similar to rates observed in large tropical rivers, that is, the Amazon and Congo (Lauerwald et al., 2015). Large dams in Asia (1,906 dams with surface areas >0.1 km 2 ) were estimated to have a total storage capacity 1,625 km 3 (Lehner et al., 2011), representing an important anthropogenic perturbation to riverine biogeochemical fluxes.…”

“…The traditional concept of the river continuum (Vannote et al., 1980) has been criticized for its inability to account for discontinuities in riverine biogeochemical processes along the reaches affected by dams, wastewater, and other perturbations (Park et al., 2018). The emissions of GHGs from impounded waters vary with time; for instance, high emissions of GHGs within years of dam construction have been associated with the degradation of flooded terrestrial OM (Deemer et al., 2016; Teodoru et al., 2012).…”

“…Consequently, phytoplankton‐derived OM and nutrients accumulated in benthic sediments create anoxic conditions that can either facilitate CH 4 and N 2 O production (Deemer et al., 2016) or N 2 O consumption. Concomitantly, large amounts of wastewater (>500 × 10 6 m 3 ) return to rivers carrying nutrients and particulate and dissolved OM (POM and DOM, respectively) (Park et al., 2018; Ulliman et al., 2020). Discharge from wastewater treatment plants (WWTPs) can significantly contribute to GHG emissions from downstream rivers (Begum et al., 2019; Garnier et al., 2013; Jin et al., 2018; Wang et al., 2017; Yoon et al., 2017).…”

River network saturation concept: factors influencing the balance of biogeochemical supply and demand of river networks

“…Other potentially important factors include variation of river network structure, flow regimes, the effect of disturbance, loading distributions (in space and time), seasonality of process rates, internal aquatic sources, role of water column processes, and their interactions. All these factors can differ among watersheds and among biomes (Mineau et al 2015;Ruegg et al 2016;Helton et al 2017;Marcé et al 2018;Park et al 2018;Gardner and Doyle 2018).…”