Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian river systems – concepts, emerging trends, and research challenges
Authors: Park Ji-Hyung , Nayna Omme K. , Begum Most S. , Chea Eliyan , Hartmann Jens , Keil Richard G. , Kumar Sanjeev , Lu Xixi , Ran Lishan , Richey Jeffrey E. , Sarma Vedula V. S. S. , Tareq Shafi M. , Xuan Do Thi , Yu Ruihong 
Published: May 2018

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.