BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-15-3049-2018Reviews and syntheses: Anthropogenic perturbations to carbon fluxes in Asian
river systems – concepts, emerging trends, and research challengesAnthropogenic perturbations to carbon fluxes in Asian
river systemsParkJi-Hyungjhp@ewha.ac.krNaynaOmme K.BegumMost S.https://orcid.org/0000-0001-5214-1153CheaEliyanHartmannJenshttps://orcid.org/0000-0003-1878-9321KeilRichard G.KumarSanjeevLuXixiRanLishanhttps://orcid.org/0000-0002-4386-1471RicheyJeffrey E.SarmaVedula V. S. S.TareqShafi M.XuanDo ThiYuRuihongDepartment of Environmental Science and Engineering, Ewha Womans
University, Seoul, 03760, Republic of KoreaDepartment of Environmental Science, Royal University of Phnom Penh,
Phnom Penh, CambodiaInstitute for Geology, Universität Hamburg, Hamburg, GermanySchool of Oceanography, University of Washington, Seattle, 98112, USAGeosciences Division, Physical Research Laboratory, Ahmedabad, 380009,
IndiaDepartment of Geography, National University of Singapore, SingaporeDepartment of Geography, University of Hong Kong, Pokfulam Road, Hong
KongNational Institute of Oceanography, Council of Scientific and
Industrial Research, Visakhapatnam, IndiaDepartment of Environmental Sciences, Jahangirnagar University, Dhaka,
1342, BangladeshDepartment of Soil Science, Cantho University, Cantho, VietnamCollege of Environment and Resources, University of Inner Mongolia,
Hohhot, ChinaJi-Hyung Park (jhp@ewha.ac.kr)17May20181593049306919December201716January201825April201826April2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://bg.copernicus.org/articles/15/3049/2018/bg-15-3049-2018.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/15/3049/2018/bg-15-3049-2018.pdf
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.
Introduction
Inland waters play a pivotal role in the global carbon (C) cycle by storing,
transporting, or transforming inorganic and organic C components along the
hydrologic continuum linking the land and oceans (Kempe, 1982, 1984; Cole et
al., 2007; Battin et al., 2009). Recent syntheses have provided greater
estimates for the riverine transport of dissolved organic C (DOC) and
particulate organic C (POC) and the exchange of CO2 between the
atmosphere and inland waters than previous studies (Raymond et al., 2013;
Regnier et al., 2013; Wehrli, 2013; Ward et al., 2017). Monitoring data are
sparse for many river systems in Asia and Africa, leaving many blind spots in
global syntheses of riverine C transport and emission. Although Asian rivers
have been estimated to account for up to 40–50 % of the global inorganic
and organic C fluxes from the land to the oceans (Degens et al., 1991; Ludwig
et al., 1996; Schlünz and Schneider, 2000; Dai et al., 2012), the lack of
high-quality data and poor spatial coverage have constrained our ability to
estimate the contributions of Asian river systems to the global riverine C
fluxes in general and CO2 outgassing in particular (Schlünz and
Schneider, 2000; Lauerwald et al., 2015; Li and Bush, 2015a, b). For
instance, obtaining pCO2 data measured in Southeast Asian rivers
was suggested as a top priority to reduce the large uncertainty in estimating
the global riverine CO2 outgassing (Lauerwald et al., 2015).
Major river systems of South, Southeast, and East Asia that belong
to top 30 global rivers based on discharge (Raymond and Spencer, 2015). The
base map and the inset world map were modified from the ArcGIS online Ocean
Basemap and Milliman and Farnsworth (2011), respectively. Rivers addressed in
the review and other large rivers are distinguished by different colors.
Three Asian regions comprise the majority of Asian countries included in the
regional categories “Asia” (indicated by yellow on the inset world
map) and “Oceania” (dark green) used by Milliman and Farnsworth (2011).
The release of C from anthropogenic sources in rapidly urbanizing watersheds
around the world increases the uncertainty of the current global riverine C
flux estimates (Regnier et al., 2013). Concurrent anthropogenic perturbations
to the river systems, including eutrophication, altered sediment regimes, and
increased water residence time in impounded rivers, can significantly change
the riverine processing of organic matter (OM) and greenhouse gas (GHG)
outgassing (Stanley et al., 2012; Regnier et al., 2013; Crawford et al.,
2016). Many streams and rivers across Asia are highly polluted by
agricultural runoff and domestic and industrial wastewater, with their water
quality often exhibiting large seasonal variations associated with regional
monsoon rainfall regimes (Park et al., 2010, 2011; Evans et al., 2012, Bhatt
et al., 2014). Although enhanced lability and mineralization of organic C
have been observed in streams and rivers draining urbanized watersheds (Hosen
et al., 2014; Kaushal et al., 2014), little is known about organic C export
and CO2 outgassing from streams and rivers draining rapidly urbanizing
watersheds in developing Asian countries (Bhatt et al., 2014). A few recent
studies conducted in the metropolitan areas of China and Korea have suggested
that GHG emissions from polluted waterways carrying urban runoff and
wastewater treatment plant (WWTF) effluents may be underappreciated as
sources of GHGs (Wang et al., 2017; Yoon et al., 2017). Despite recent booms
in the construction of large dams on many large rivers across the region,
little attention has been paid to impoundment effects on GHG emissions (Chen
et al., 2009; Hu and Cheng, 2013). Considering the role of dams in storing
huge amounts of sediment and organic C (Syvitski et al., 2005; Maavara et
al., 2017), a mechanistic understanding of GHG emissions from impounded
rivers can provide more insights into the anthropogenic perturbations to the
C fluxes of dammed river systems.
This review aims to provide a conceptual framework for assessing the human
impacts on riverine C fluxes and an update on major anthropogenic
perturbations affecting C fluxes in Asian river systems, focusing on the
impacts of water pollution and river impoundments on riverine CO2
dynamics in South (S), Southeast (SE), and East (E) Asian regions that
account for the largest fraction of river discharge and C exports from Asia
and Oceania (Fig. 1). An important goal was to integrate various concepts of
riverine biogeochemical processes into a conceptual framework for assessing
human impacts on the riverine C fluxes in human-modified river systems. Given
the pace and wide-ranging impacts of urbanization and river impoundments, the
traditional view of the river continuum developed for natural streams and
rivers (Vannote et al., 1980) needs to be revised to reflect altered regimes
of riverine metabolic processes and material fluxes. We compared reported
values of pCO2 that had been either measured or estimated for
major river basins in three Asian regions, including the Ganges, the Mekong,
and the Yellow River as representative systems of three regions, to assess
the current status. We also compared pCO2 values among different
components of the river basin (main stem, headwater, tributary, and
impoundment) to examine how water pollution and impoundments alter riverine
metabolic processes and CO2 emissions. Many of the reported values
have been estimated from pH and alkalinity data available from the literature
and water quality databases such as GLORICH (Global River Chemistry Database;
Hartmann et al., 2014). Considering potential overestimations of water
pCO2 associated with organic acid contributions and increased
sensitivity to alkalinity in acidic, organic-rich waters with low carbonate
buffering (Abril et al., 2015), we provided methodological details for the
calculated pCO2 values if the cited references considered these
pH and alkalinity effects. This review and the following synthesis efforts
are expected to provide scientifically robust conceptual frameworks and data
that are required for a better understanding of how human-induced
perturbations in rapidly urbanizing watersheds across Asia transform riverine
metabolic processes and C fluxes away from the “natural” states assumed in
the traditional river continuum model.
The geographical scope, global implications, and emerging regional
trends of Asian river systems
Global syntheses of riverine C fluxes have been based on monitoring data
available for a limited number of large rivers (e.g., Degens et al., 1991;
Ludwig et al., 1996). We referred to these previous syntheses and a more
recent synthesis of global river discharge (Milliman and Farnsworth, 2011) to
scope the geographical extent of Asian river systems. We followed the
continental categories used by Milliman and Farnsworth (2011), namely Asia
and Oceania demarcated in Fig. 1, but did not consider Arctic rivers in
Russia and rivers in Australia and New Zealand. This review focuses on S, SE,
and E Asian regions where river systems are commonly affected by increasing
human impacts and for which data are available to address major review
themes. Ten large rivers in three Asian regions (Fig. 1) belong to top
30 global rivers based on discharge (Raymond and Spencer, 2015), 32 rivers
included in a previous global synthesis of riverine C fluxes (Ludwig et al.,
1996), or 34 rivers with basin areas greater than 500 000 km2
(Milliman and Farnsworth, 2011). The river systems in these Asian regions
share some common hydrologic and demographic features (Table 1), including a
large seasonality in discharge, high population densities
(80–513 km-2) compared to the global mean (70 km-2), and a
wide range of per capita annual discharge
(23–8594 m3 yr-1 person-1 vs. the global mean:
4901 m3 yr-1 person-1). Large seasonal variations in
precipitation and runoff associated with Asian monsoon systems play a
critical role in hydrologically mediated riverine processes including those
affecting C fluxes (Park et al., 2010). Recent demographic changes including
an unprecedented rapid growth in population may cause increasingly severe
perturbations to water and material flows along the rivers that are not only
regulated by dams and but are also polluted by urban sewage and agricultural
runoff.
Geographic and demographic features of the major river systems
addressed in the review in comparison with Asian and global sums.
RiverReceiving seaRegionBasin areaAnnualPopulationPopulationAnnual discharge(103 km2)discharge(×106)densityper capita(km3 yr-1)(per km2)(m3 yr-1)GangesBay of BengalS Asia9804904114191193BrahmaputraBay of BengalS Asia6706301452164353IndusArabian SeaS Asia9805 (90)22022423 (410)KrishnaBay of BengalS Asia26012 (62)101390118 (611)GodavariBay of BengalS Asia31092 (120)121390761 (993)MekongSouth China SeaSE Asia80055064808594YellowYellow SeaE Asia75015 (43)120160125 (358)YangtzeEast China SeaE Asia18009004752641894Pearl RiverSouth China SeaE Asia490260951932749Han RiverYellow SeaE Asia2517135131326Asia total32 51811 0004835148227(32 518)(13 196)Global total105 00036 0007345704901(106 326)(38 170)
River basin area and discharge data were obtained from Milliman and
Farnsworth (2011), supplemented with Asian and global sums in parentheses
from Ludwig et al. (1996). Pre-diversion discharge data are provided in
parentheses for the rivers where discharge has substantially decreased in
recent years because of river diversion, reservoir construction, and
irrigation. The Asia total discharge provided by Milliman and Farnsworth (2011) was estimated for all rivers in Asia and Oceania, excluding Arctic
rivers in Russia. Population data were taken from various sources including
Schmidt et al. (2017) and the CIA World Factbook
(https://www.cia.gov/library/publications/the-world-factbook/index.html;
last accessed: 10 January 2018). Asia total population is the population
for all countries belonging to Asia and Oceania excluding Russia.
According to an earlier estimation (Degens et al., 1991), Asian rivers
account for up to 35, 50, and 39 % of the global discharge, total organic
C (TOC) export, and dissolved inorganic C (DIC) export. Schlünz and
Schneider (2000) provided a lower estimate for TOC export by Asian rivers
(175. 2 Tg C yr-1; 40 % of the global TOC flux). Ludwig et
al. (1996) provided separate estimates for the export of DOC (69.01 Tg
C yr-1) and POC (76.40 Tg C yr-1) by Asian rivers, which
represented 34 and 44 % of the corresponding global fluxes, respectively.
More recent syntheses of the published data have corroborated the
quantitative importance of Asian rivers in the global fluvial C fluxes (Dai
et al., 2012; Huang et al., 2012; Galy et al., 2015). As indicated by the
lower ratio of DOC to POC (0.9) compared to other regions with the ratio
exceeding 1, many Asian rivers draining erosion-prone mountainous terrain
deliver more POC than DOC, particularly during the monsoon period (Ittekkot
et al., 1988; Ludwig et al., 1996). Monsoonal increases in discharge and POC
can have either a positive effect on riverine pCO2 levels through
the enhanced soil flushing of DIC and/or in-stream organic C biodegradation or a
negative effect caused by dilution, as observed in such turbid Asian rivers
as the Pearl (Yao et al., 2007), Yangtze (Li et al., 2012), and Mekong (Li et
al., 2013b). Constraining differential monsoon effects on the fluxes of DOC,
POC, and DIC including CO2 represents a key challenge in evaluating the
contribution of Asian rivers to the global riverine C fluxes.
Previous syntheses based on a small number of data sets collected in several
large Asian rivers during the 1970s and 1980s provide only limited
information when we assess the effects of “ongoing” environmental changes
on riverine C fluxes. Therefore, this review aims to provide an update on
Asian river C fluxes, focusing on river impoundment and pollution as two of
the most important environmental changes affecting river systems across three
target Asian regions. There have been few systematic assessments of the
effects of impoundments and water pollution on the C fluxes of Asian rivers
(Sarma et al., 2011; Ran et al., 2014; Li and Bush, 2015a, b). For example, a
cascade of dams constructed along the upper Mekong River since the 1990s have
been implicated to cause a wide range of downstream impacts including
decreases in water and sediment flow (Li and Bush, 2015a). Although it is
expected that declining sediment flux can significantly alter POC and
associated C fractions along downstream reaches, little is known about
impoundment effects on the fluxes of POC, DOC, DIC, and CO2 in the
upper and lower Mekong River. This lag between the real-time environmental
changes and scientific assessments based on outdated data is quite surprising
given the magnitude and pace of the environmental changes occurring across
Asia. Deforestation and associated peatland drainage in tropical areas
represent another important but rarely explored topic with regard to
CO2 outgassing from Asian rivers (Baum et al., 2007; Wit et al.,
2015). A recent study suggested that peatland drainage could enhance organic
matter degradation in the coastal peatlands and organic-rich soils of
Southeast Asian lowland areas and islands, increasing CO2
outgassing from the rivers draining the affected areas (Wit et al., 2015).
However, this issue cannot be addressed in detail here because only a few
studies have been conducted in Indonesia and Malaysia.
A schematic diagram describing river discontinuity in the
anthropogenic land-water-scape: an example of river continuum observed in a
minimally impacted river (a); three-dimensional connectivity vectors
along the upper, middle, and lower river reaches (b); longitudinal
variations in a hypothetical riverine biogeochemical process X(c). The connectivity
vectors and the plots depicting river continuum and serial discontinuity were
modified from Stanford and Ward (2001) and Poole (2010), complemented with
some additional considerations including pollution-induced pulsatile
discontinuities (red-colored pulses) and the vertical vector of air–water
gas exchange (blue and red, indicating the potential effects of dams and
water pollution, respectively).
Conceptual framework for understanding interactive effects of changing
land-water-scape and climate on riverine C fluxes
Human-induced land changes, as manifested in agricultural lands and urban
areas, drive changes in biogeochemical cycles and climates, with altered
terrestrial biogeochemical cycles often leading to pollution in downstream
aquatic systems (Grimm et al., 2008). As a consequence of global
urbanization, human influences are pervasive across the interacting
terrestrial and aquatic patches of riverine landscapes or riverscapes (Allan,
2004; McCluney et al., 2014). To emphasize the dominant human influences on
connectivity and the interactions among terrestrial and aquatic patches of
the riverine networks, we term these anthropogenically modified riverscapes
“anthropogenic land-water-scapes”. Compared to the previous use of the term
“land-water-scape” focusing on terrestrial–aquatic boundary conditions in
urbanized watersheds (Cadenasso et al., 2008), our use is more general and
inclusive, covering longitudinal linkages between less or more modified
upstream and downstream reaches.
Rivers dominated by the effluents of wastewater treatment plants (WWTPs)
provide an example of how human activities, through water withdrawal and
wastewater generation, modify flows of water and materials across this
anthropogenic land-water-scape. In the case of rivers draining arid areas,
WWTP effluents can not only increase river flow but also provide a source of
water feeding into habitats for various aquatic organisms along downstream
reaches (Luthy et al., 2015). Rapid, concurrent changes in land use and river
flow and chemistry make Asian rivers a perfect test bed for exploring how
human-induced perturbations alter hydro-biogeochemical cycles across the
components of these anthropogenic land-water-scapes.
New conceptual templates can build on some existing concepts that have been
used to explain natural riverine processes. Above all, the river continuum
from headwaters to mouth has been one of the most widely used concepts to
represent longitudinal connectivity in river ecosystem structure and function
over the last 5 decades (Vannote et al., 1980; Webster, 2007). The
original river continuum concept envisaged gradual and continual changes in
OM composition and metabolic rates in correspondence to downstream variations
in environmental conditions and biotic communities along the river (Vannote
et al., 1980; Fig. 2). Recent biogeochemical studies based on the river
continuum concept include those of OM chemical diversity (Mosher et al.,
2015) and biodegradability (Catalán et al., 2016) and riverine CO2
outgassing (Hotchkiss et al., 2015). A prevailing idea underlying these
approaches has been the selective degradation of labile components of OM
during transit across the continuum, which has been successful in explaining
the critical role of water retention time for the downstream evolution of the
composition and biodegradability of DOM in river systems with a
relatively high proportion of natural lakes and/or low levels of
anthropogenic perturbations (Koehler et al., 2012; Weyhenmeyer et al., 2012;
Mosher et al., 2015; Catalán et al., 2016). According to the reactivity
continuum model, the composition of DOM becomes dominated gradually by highly
degraded compounds as a result of the prolonged exposure of DOM to biodegradation
and photodegradation, resulting in a downstream decline in DOM reactivity
(Koehler et al., 2012; Catalán et al., 2016).
Despite the wide use of the river continuum in studying various riverine
processes, it has been criticized for overlooking an increasingly recognized
reality that specific rivers are often divided into discrete segments that
are hierarchically nested in a river network (Townsend, 1996; Poole, 2002).
Discrete segments along a river network can occur as a result of “abrupt
transitions between adjacent segments with dissimilar physical structure”
within the hierarchically nested river network (Poole, 2002). These abrupt
transitions between discrete segments can occur temporarily, as illustrated
by seasonal variations in water connectivity (Casas-Ruiz et al., 2016). As
depicted in Fig. 2, examples of human-induced discontinuities include those
created by dams built on regulated rivers (Ward and Standford, 1983) and
pollution-induced perturbations to the production–respiration balance in the
eutrophic river (Kempe, 1984; Garnier and Billen, 2007). Because the river
continuum concept was originally proposed as a template for integrating
physical environments and biological processes of “natural, unperturbed
stream ecosystems” (Vannote et al., 1980), it has limitations in explaining
discontinuities in fluvial processes and biogeochemical fluxes, which might
be accentuated in many anthropogenically modified Asian river systems.
River impoundments and water withdrawal alter not only the rates of runoff
and sediment transport (Syvitski et al., 2005) but also aquatic primary
production and its effects on OM biodegradation (Stanley et al., 2012). In
response to disturbance events, unregulated rivers tend to reset physical and
ecological conditions toward the pre-disturbance state. However, river
impoundment induces long-lasting perturbations to those conditions along the
distance upstream or downstream of a dam, termed “discontinuity distance” (Ward
and Standford, 1983; Stanford and Ward, 2001). This discontinuity distance
was originally proposed as part of the “serial discontinuity concept”,
which states that stream regulation by multiple dams results in “an
alternating series of lentic and lotic reaches” (Ward and Stanford, 1983).
According to this concept, stream regulation by dams can induce disturbances
to the gradual processes envisaged in the river continuum concept, shifting a
given physical or biological parameter longitudinally (Ward and Stanford,
1983). For example, Vannote et al. (1980) envisaged that parameters such as
the ratio of production to respiration (P/R) and diel temperature
difference (ΔT) would exhibit a specific longitudinal pattern shown
in Fig. 2. Stream regulation can shift this longitudinal pattern along the
discontinuity distance. Although the serial discontinuity concept has been a
useful framework for assessing anthropogenic impacts on regulated lotic
systems, its presuppositions, including no disturbances other than impoundment
(Ward and Stanford, 1983), limit its application to investigating
environmental stresses other than impoundments, such as the high levels of organic
pollutants and nutrients observed in many Asian rivers receiving untreated
sewage and urban runoff.
A schematic diagram illustrating human-induced alterations of the
riverine C fluxes in the Yellow River as a model river system (modified from
Ran et al., 2014). The annual flux rate (Tg C yr-1) for each of the
described soil and fluvial processes was estimated for the period 1950–2010.
Slope and fluvial processes depicted in the figure include the following. SE: soil
erosion, HR: hillslope redistribution, SC: slope control of erosion, DT: dam
trapping, SD: sediment diversion, CD: channel deposition, CE: CO2
emission, and ST: seaward transport. Refer to Ran et al. (2014) for more
details on their flux and uncertainty estimations.
Kaushal and Belt (2012) proposed an urban watershed continuum framework that
recognizes a continuum linking engineered and natural hydrologic flow paths
across the urbanized watershed. From the perspective of spatial disconnection
within the hierarchically nested river network (Poole, 2002), this urban
watershed continuum is actually “a continuum with discontinuities” (sensu
Poole, 2002), in which the natural land–water hydrologic connectivity common
in low-order streams is replaced by urban structures such as sewers and storm
water drains. The lateral transfer of water and associated materials via
networks of engineered urban structures not only creates departures from the
natural patterns or “discontinuities” in the hydrologic paths across the
terrestrial–aquatic interface, but also exerts extraordinary impacts on the
downstream transport and transformations of OM and nutrients (Paul and Meyer,
2001; Allan, 2004; Garnier and Billen, 2007; Lookingbill et al., 2009;
Kaushal and Belt, 2012). As Hynes (1975) emphasized the importance of the
terrestrial–aquatic
connectivity in headwater systems by saying that “the valley rules the
stream”, human-induced changes in the watershed
would have large cascading effects on the structure and function of stream
ecosystems. In human-modified river systems, the “valley” is often
separated from the stream or replaced by engineered structures that release
pulses of water and materials, creating abrupt transitions across the
land–water interface and stream segments (Fig. 2). Urban structures across
the land–water interface can also result in pulsatile flows of water and
materials as a combined consequence of increased runoff from the impervious
urban surface and discharges from WWTPs, storm water drainages, and combined
sewer overflows (Paul and Meyer, 2001; Garnier and Billen, 2007; Kaushal and
Belt, 2012). Although wastewater can bring pulses of OM and nutrients to the
receiving urban water systems, its impact on riverine metabolism and
CO2 outgassing has rarely been investigated in Asian river systems
except for a few exploratory studies (Guo et al., 2014; Yoon et al., 2017).
Human-modified river networks often lack dynamic movements and flow
adjustment and are therefore limited in their ability to buffer against
disturbances such as floods and water stress (Palmer et al., 2008). Given the
large seasonality inherent in the monsoon climate, anthropogenic
land-water-scapes forming on monsoonal Asian river systems might be
particularly vulnerable to climatic variability and extremes, as exemplified
by the strengthened flashy storm responses of sediment and C export from
erosion-prone mountainous watersheds during extremely wet monsoon periods
(Park et al., 2010; Jung et al., 2012). Climate models have suggested that
perturbations in the global water cycle accompanying climatic warming can
increase river discharge in many parts of the world (Milly et al., 2005).
Although increases in river discharge have been detected for some large
basins over the last century (Labat et al., 2004), globally no consistent
pattern has been established. For example, cumulative discharge from many
midlatitude rivers, including rivers draining arid regions of Asia, have
decreased substantially as a result of concurrent changes in precipitation
and anthropogenic perturbations such as damming, irrigation, and inter-basin
water transfers (Milliman et al., 2008). While discharge from most Asian
rivers except Siberian rivers and the Brahmaputra has declined, the most
striking decrease exceeding -50 % was observed for the Indus and Yellow
River (Milliman et al., 2008). In many dammed Asian rivers, observed
decreases in discharge and sediment transport might be largely explained by
increasing river impoundments and water diversion (Milliman et al., 2008; Li
and Bush, 2015a). However, recent increases in the frequency and intensity of
extreme precipitation events observed in many parts of Asia (Min et al.,
2011) suggest that potential changes in monsoon rainfall regimes as a
consequence of climate change can amplify seasonal and year-to-year
variations in discharge and the transport of sediment and C even in dammed
river systems. Therefore, predicting future changes in riverine C fluxes in
increasingly human-modified Asian river systems would require a better
understanding of the complex interplay between anthropogenic perturbations
and the concurrent climate change.
Summary of water use and wastewater production in southern and
eastern Asia. Source of data: AQUASTAT, a global water information system
operated by the Food and Agriculture Organization (FAO;
http://www.fao.org/nr/water/aquastat/sets/index.stm; last access: 15 December 2017).
CountryPopulationRenewableDam capacity Municipal water Municipal wastewater surface water(2015)(2014)Produced Treated ×103km3 yr-1km3Yearkm3 yr-1Yearkm3 yr-1Yearkm3 yr-1YearBangladesh160 9961206.06.520133.620080.72000Bhutan77578.00.020080.02000Brunei4238.50.020100.22009Cambodia15 578471.50.120060.01994China1 407 3062739.0829.8201375.0201348.5201349.32014North Korea25 15576.213.620150.92005India1 311 0511869.0224.0200556.0201015.520114.42011Indonesia257 5641973.023.0201514.0200514.32012Japan126 573420.029.0199315.4200916.9201111.62011Laos6802333.57.820100.120030.120080.01995Malaysia30 331566.022.520153.920054.220092.62009Mongolia295932.70.320150.120090.120120.12006Myanmar53 8971157.015.520053.320000.01995Nepal28 514210.20.120150.02006Pakistan188 925239.227.820159.720083.120110.02002Papua New Guinea7619801.00.720100.22005Philippines100 699444.06.320066.220091.32011South Korea50 29367.116.219946.920057.820116.62011Singapore56190.120151.120050.520130.52013Sri Lanka20 71552.05.919960.820050.12009Thailand67 959427.468.320102.720075.120121.22012Timor11858.10.12004Vietnam93 448847.728.020101.220052.020120.22012S, SE, and E Asia3 964 38614027.11325.2201.6120.276.5World7 344 83752 952.77039.6464.1311.6187.1Contrasting effects of river impoundment on organic C transport and
CO2 emission
According to a recent estimate based on the Global Reservoir and Dam database
(GRanD), there may be about 16.7 million reservoirs larger than 0.01 ha
globally, with a combined storage capacity of 8069 km3 (Lehner et al.,
2011). Out of 6862 dams registered in GRanD with a total storage capacity of
6197 km3, 1906 dams located in Asia (excluding middle eastern Asia and
western Russia) store 1625 km3 of water, accounting for 26 % of the
global storage capacity. AQUASTAT, a global water information system operated
by the Food and Agriculture Organization (FAO), provides a similar estimate for
the reservoir storage capacity of southern and eastern Asia: 1325 km3
(Table 2; FAO, AQUASTAT). Over the last decades, rivers across Asia have been
increasingly impounded by dams of various types and sizes, and a recent boom in
constructing hydroelectric dams is posing an unprecedented challenge for the
sustainable management of the affected river basins (Grumbine et al., 2012;
Winemiller et al., 2016). River impoundments not only affect downstream flows
but also disrupt the ecological and biogeochemical connectivity of rivers
(Lehner et al., 2011; Winemiller et al., 2016; Maavara et al., 2017). A
growing number of dams have been decreasing both river flow and sediment
fluxes, sequestering over 100 billion metric tons of sediment and 1 to
3 billion metric tons of C in reservoirs constructed over the last 50 years
(Syvitski et al., 2005). Many Asian rivers, such as the Indus, Yangtze, and
Yellow, have seen the largest reductions in sediment export to the oceans
compared to the pre-dam era (Syvitski et al., 2005). Therefore, investigating
altered rates of C storage and losses in dammed Asian rivers is crucial for a
better understanding of human impacts on global riverine C fluxes.
Summary of pCO2 measured (M) or estimated (E) for the rivers
in South (S), Southeast (SE), and East (E) Asia in comparison with data
for the global rivers including Asian rivers. Only a single value per site
was used to obtain the mean and range of each Asian river system, so multiple
measurements at a site were averaged to provide one representative value.
River systemMean (range) of pCO2 (µatm) MethodReferenceBasin wideMain stemHeadwaterTributaryImpoundmentGlobal3100ERaymond et al. (2013),(0–100 000)GLORCIHa2400ELauerwald et al. (2015),(2019–2826)GLORICHa4196bE/MMarx et al. (2017),(32–93616)GLORICHaAsia (total)1754126311702116835Sum of freshwater(28–11 793)(35–10 977)(74–5076)(28–11 793)(128–8785)data belowS AsiaGanges89310834011685181, 224cE/MManaka et al. (2015b),(65–2620)(65–2184)(165–1222)(1035–2620)GLORICHaBrahmaputra664494292758EHuang et al. (2011), Manaka et al. (2015b),(28–6706)(65–6706)(208–513)(28–3678)Qu et al. (2017), GLORICHaIndus853941768EGLORICHa(117–7725)(117–7725)(165–3161)Krishna215218712305EGLORICHa(711–4098)(976–2536)(711–4098)Godavari8785d8785dEPrasad et al. (2013)Bhote Kosi592592EGLORICHa(35–5907)(35–5907)Various26853081609E/MPanneer Selvam et al. (2014)(420–10 977)(426–10 977)(420–692)Cochin(2975–6001)eEGupta et al. (2009) (saline estuary)Other estuaries5882 (293–18 492)ESarma et al. (2012) (saline estuary)SE AsiaMekong123511201367f1310882gE/MAlin et al. (2011), Li et al. (2013b),(110–4503)(703–1687)(110–4503)(864–899)Manaka et al. (2015a)Red River15891589MLe et al. (2017)(992–3129)(992–3129)Irrawaddy20072007EManaka et al. (2015a)(1222–3157)(1222–3157)Chao Phraya320228143357EManaka et al. (2015a)(925–5076)(2632–2996)(925–5076)Malaysian rivers12171217MWit et al. (2015)(1159–1274)(1159–1274)Indonesian rivers52625262MWit et al. (2015)(2400–8555)(2400–8555)E AsiaYellow2164210410832470555, 441hE/MRan et al. (2015a),(147–9659)(582–4770)(147–3546)(425–9659)(266–735)Ran et al. (2017a)Yangtze2366132279426801393EQu et al. (2015), Liu et al. (2016),(74–7718)(528–3800)(74–4472)(249–7718)(760–1908)Ran et al. (2017bi), GLORICHaPearl River1747246516711967EYao et al. (2007), Zhang et al. (2009),(231–5006)(2136–2833)(231–5006)(282–4808)Zou (2016), GLORICHaHan River257114806284670251MYoon et al. (2017)(101–11 793)(173–4089)(101–11 793)(128–454)
a GLORICH: Global River Chemistry
Database (Hartmann et al., 2014), from
which pCO2 for global rivers was calculated by Raymond et al. (2013; excluding data
obtained at pH < 5.4) and Lauerwald et al. (2015) excluding data obtained high pollution
levels. Marx et al. (2017) excluded pCO2 values above 100 000 µatm to avoid any
potential overestimation resulting from pH and alkalinity effects. b The
mean and range were calculated from the data set (GLORICH and additional
literature data) of pCO2 measured or estimated for streams draining
small watersheds < 20 km2 (Marx et al., 2017). c Dakpatthar
Barrage on the Yamuna, a Ganges tributary (cf. the other barrage at Rishikesh
on the main stem), using the headspace equilibration method (Park, unpublished
data). d Dowleiswaram Reservoir (only the mean of time series data was
included in calculating the regional mean and range). e Cochin estuary:
2 sites on the Periyar River and 11 sites on the estuary. f Measured at
Lancang headwater at Qinghai, China using the headspace equilibration method
(Park, unpublished data). g Impounded tributaries of the lower Mekong (Li
et al., 2013b). h Impoundments on a Yellow River
tributary (Ran et al.,
2017a). i Ran et al. (2017b) excluded pCO2 values calculated at
pH < 6.5.
The Yellow River in northern China provides an excellent example of
basin-scale impoundment impacts on sediment and C transport (Fig. 3). The
sediment load in the Yellow River peaked during the period 1800–1950
following a millennium of aggravating soil erosion in the Loess Plateau, but
dam construction and other human activities to control soil erosion have
reduced the annual sediment flux by 90 % over the last 60 years (Chen et
al., 2015; Wang et al., 2015). Particularly, silt check dams and reservoirs,
along with hillslope soil conservation measures such as terrace farming, made
large contributions to the observed reductions. More than 110 000 silt check
dams have been constructed in the Loess Plateau since the 1950s, trapping
approximately 21 billion tons of sediment in the reservoirs (Zhang et al.,
2016). A conservative estimate indicates that the rate of annual POC trapping
in more than 3000 large dams (excluding 110 000 silt check dams) within the
Yellow River basin can amount to 3.3–4.3 Tg C yr-1
(1 Tg = 1012 g; Zhang et al., 2013; Ran et al., 2014), which is
similar in magnitude to the total organic C export to the Bohai Sea
(4.1 Tg C yr-1; Ran et al., 2014; Fig. 3). Furthermore, as an
important attempt to control soil erosion, the Chinese government initiated
the largest ever revegetation program in history called the “Grain for Green
Project” from the late 1990s. The implementation of this project has made an
additional contribution to the decreasing trend of sediment flux (Wang et
al., 2015), with far-reaching impacts on soil organic C stocks and
riverine C fluxes (Feng et al., 2013). A great number of new silt check dams,
up to 163 000, are planned to be built on the Loess Plateau through
2020 (Zhang et al., 2016), so dams and other erosion control projects will
continue to have significant impacts on sediment and associated C dynamics in
the Yellow River basin in the future.
It remains an important research question whether POC trapped in ever-growing
reservoir sediments within the Yellow River basin would function as a sink or
source of CO2 for the atmosphere (Zhang et al., 2013; Ran et al.,
2015a, 2017a). As illustrated by the relatively high estimated rate
(27 %) of CO2 emission from the POC eroded from the entire
Yellow River basin (Ran et al., 2014; Fig. 3), the organic C trapped in
reservoir and stream sediments can become an important source of
CO2 under favorable conditions that would accelerate the rate of
the biodegradation of POC during fluvial transport and sediment storage.
Although the rate of CO2 evasion from the water surface can
increase in the impounded river reaches, as longer residence times tend to
create favorable environments for the microbial biodegradation of organic C
(Ittekkot et al., 1985; Ran et al., 2015a), the countering effect of enhanced
planktonic CO2 uptake in the euphotic reservoir surface has rarely
been compared against biodegradation (refer to the wide range of
pCO2 summarized in Table 3). Based on extensive CO2
evasion measurements in the river–reservoir–river continuum on the Loess
Plateau, Ran et al. (2017a) found that the Loess Plateau reservoirs acted as
relatively small sources or even sinks of C due largely to significantly
reduced surface turbulence and enhanced photosynthesis. Compared to the
standing waters of the reservoirs with enhanced primary production, rapidly
flowing waters along both the upstream and downstream reaches were larger C
sources for the atmosphere, exhibiting much higher pCO2 levels
and faster flow velocities, which
provide favorable conditions for an efficient gas evasion from the
aqueous boundary layer. Liu et al. (2016) also observed over 60 %
decreases in pCO2 along the eutrophic impounded reaches of the
Three Gorge Reservoir in the Yangtze River, but they argued that enhanced
primary production in the impounded reach would play a rather temporary and
minor role in controlling pCO2 dynamics compared to the
predominant influence of allochthonous C. The contrasting impoundment effects
observed in the Yellow River and other Chinese rivers suggest that a
basin-wide assessment of impoundment impacts on sediment C storage and
CO2 emissions should take into consideration concurrent changes in
primary production and organic matter biodegradation.
In the Lancang River (the upper Mekong River located in China), Li et
al. (2013a) observed dramatic increases in the abundance of phytoplankton and
a shift of the algal community toward Chlorophyta and
Cyanophyceae following the construction of cascade dams since 1995.
In a recent study that compared the emission rates of CO2 and CH4
across the upper riverine reach and six cascade dams along the Lancang River,
Shi et al. (2017) found that gas emission rates, particularly those of
CH4, were highest in the most upstream and second newest dam and that
the percent of organic C in the reservoir bottom sediment had decreased with the
increasing age of the dams. These results suggest that favorable conditions
created by river impoundments, such as increased water temperature and
retention time, can stimulate OM processing in both the impounded water and
trapped sediments, at least in the short term following the construction of
dams on high-POC mountainous rivers such as the Lancang.
Most major Indian rivers have been impounded by dams of various types and
sizes to meet domestic and industrial water demands during the dry period.
Because monsoonal rivers draining the Indian subcontinent account for the
largest share of the POC export by the Asian rivers (Ludwig et al., 1996;
Galy et al., 2015), altered discharges and sediment fluxes of these dammed
rivers can have significant implications for the global riverine export of
sediment and POC (Ittekkot et al., 1985; Krishna et al., 2015). However, the
effects of large dams on riverine C fluxes including CO2 have been
studied only in a few dammed rivers such as the Godavari (Sarma et al., 2011;
Prasad et al., 2013; Table 3). According to an earlier study conducted in the Krishna, dam
construction had decreased the sediment load from 67.7 Tg yr-1
measured at the upper reach to 4.11 Tg yr-1 at the river mouth (Ramesh
and Subramanian, 1988). Although many large dams and barrages have been
constructed on the main stem and tributaries of the Ganges, there has been no
systematic investigation of GHG emissions from the impounded reaches, except
for a few unpublished measurements (Table 3). Because of this paucity of
monitoring data, GHG emissions from impounded rivers add a considerable
uncertainty to the estimates of GHG emissions from the inland water systems
in the Indian subcontinent. For instance, Panneer Selvam et al. (2014)
extrapolated their flux measurements at 45 water bodies in South India to
estimate CO2 and CH4 emissions from all of India's inland
waters at 22.0 Tg CO2 and 2.1 Tg CH4 yr-1,
respectively. While they provided estimates of 2.37 Tg CO2 and
0.33 Tg CH4 yr-1 for the reservoirs and barrages, a follow-up
study offered larger estimates amounting to 3.08 Tg CO2 and
6.27 Tg CH4 yr-1 by considering additional literature data on
large rivers in northern India and reservoir downstream fluxes through
spillways and turbines (Li and Bush, 2015b). This example illustrates the
importance of adequate spatial coverage and downstream impacts for refining
regional-level estimates of GHG emissions from various impoundments.
Comparison of pCO2 measured or estimated for major Asian
rivers, including three rivers selected for a detailed review – the Ganges,
Mekong, and Yellow River. The blue dotted line indicates the ambient air
pCO2 level around 400 atm. The horizontal black and red lines of the
box plots are the median and mean value, respectively. Each box covers the
25th to 75th percentile, whereas the whiskers represent the 10th and 90th
percentile. The number of data points included in the total Asian rivers is
1240 (main stem: 261, headwater: 199, tributary: 750, impoundment: 30), in the
Ganges 63 (main stem: 14, headwater: 30, tributary: 17, impoundment: 2), in the
Mekong 59 (main stem: 19, headwater: 1, tributary: 37, impounded tributary:
2), and in the Yellow River 215 (main stem: 14, headwater: 22, tributary: 164,
impoundment: 15). Refer to Table 3 for more details on data sources.
A significant reduction in primary production observed in the Godavari
River was ascribed to the removal of nitrogen and phosphorus by several dams
constructed within the basin (Das, 2000). Ramesh et al. (2015) reported
an increasing retention of particulate OM in the dams and reservoirs of the
Godavari and Krishna. However, other studies conducted in the Godavari
estuary have found that during the peak discharge periods in the monsoon
season, the estuary receiving discharge waters from an upstream dam exhibited
extraordinarily high levels of pCO2 up to 33 000 µatm
compared to the dry season values lower than 500 µatm, presumably
due to enhanced bacterial decomposition of the organic C released from the
upstream dam in the highly eutrophic estuary (Sarma et al., 2011; Prasad et
al., 2013). On the other hand, less rainfall during dry years can also result
in large downstream impacts of impoundments through an increased production
of labile OM by freshwater algae in the upstream dam and algal blooms in the
estuary (Pradhan et al., 2014). Further research is required to establish how
seasonal and interannual variations in climatic and trophic conditions in
dammed Indian rivers alter the balance between autotrophy and heterotrophy
and hence CO2 emissions along the “discontinuous”
river–reservoir–estuary continuum as found in the Godavari basin.
In accordance with the serial discontinuity concept (Ward and Standford,
1983; Fig. 2), multiple dams constructed on large Asian rivers such as the
Mekong and Yellow River create standing water conditions that may shift
stream metabolisms and pCO2 dynamics from the patterns observed for
freely flowing reaches. The observed contrasting impoundment effects on
CO2 emissions across different Asian river systems might have resulted
from an interplay between planktonic CO2 uptake, organic matter
biodegradation, and sediment C sequestration (Liu et al., 2016; Maavara et
al., 2017). The balance between the competing processes affecting the actual
level of pCO2 in reservoir waters may change not only seasonally
(Prasad et al., 2013; Liu et al., 2016) but also with the increasing age of
dams (Barros et al., 2011). While large pulses of GHGs may be released from
the flooded vegetation and soil OM during the initial flooding phase (Abril
et al., 2005; Chen et al., 2009; Hu and Cheng, 2013; Deshmukh et al., 2016,
2018), sedimentation can accumulate a growing amount of C in reservoir
sediments, greatly decreasing the rate of CO2 release from aging
reservoirs (Barros et al., 2011). Large pulse emissions of CO2 and
CH4 have been measured in the years following the construction of the
Three Gorges Dam on the Yangtze River (Chen et al., 2009) and the Nam Theun 2
on the large tributary feeding into the middle reach of the Mekong River
(Deshmukh et al., 2016, 2018). A recent report on drought-enhanced emissions
of GHGs in an old hydroelectric reservoir in Korea suggested that stochastic
emissions during extreme climatic events can reverse the trend of declining C
emissions from aging reservoirs through the offsetting effect of extreme events on
the C accumulated in reservoir sediments over timescales of years to decades
(Jin et al., 2016). The paucity of pCO2 measurements in dammed Asian
rivers (Table 3; Fig. 4) does not allow for any generalization of long-term
impoundment effects on sediment C storage and CO2 emissions; this
requires
more long-term investigations of seasonal and year-to-year variations in
metabolic processes and pCO2 levels across a wide range of impounded
inland water systems.
Effects of water pollution on riverine metabolisms and CO2
emissions
Across Asia, rapidly urbanizing river basins are highly polluted with poorly
treated or untreated wastewater. Using AQUASTAT data, Evans et al. (2012)
estimated the annual wastewater generation in Asia around the year 2000 at
142 km3, of which only an estimated 33–35 % was treated before
being discharged to streams and rivers. We used the latest data available on
the AQUASTAT webpage to provide more up-to-date estimates of water withdrawal
and wastewater production, focusing on southern and eastern Asia (FAO,
AQUASTAT). In 2010, the annual municipal water withdrawal in 24 southern and
eastern Asian countries was 201.6 km3, accounting for 43.4 % of the
global municipal withdrawal (464.1 km3; Table 2). The total volume of
municipal wastewater generated each
year within urban areas of these countries was 120.2 km3. The generated
wastewater included domestic, commercial, and industrial effluents and storm
water runoff, accounting for 38.6 % of the global municipal wastewater
production (311.6 km3). Based on the AQUASTAT and other published data,
Mateo-Sagasta et al. (2015) estimated that each year more than 330 km3
of municipal wastewater is produced globally. Although the volume of
municipal wastewater generated constitutes only ∼ 0.9 % of the
renewable surface water available in these Asian regions
(14 027.1 km3), both poorly treated and untreated wastewater can have
disproportionately large impacts not only on the water quality and ecological
integrity of downstream aquatic ecosystems (Meybeck and Helmer, 1989; Evans
et al., 2012) but also on the riverine GHG emissions (Yoon et al., 2017).
Compared to extensive studies conducted in polluted rivers and estuaries in
Europe and North America (Frankignoulle et al., 1998; Borges et al., 2006;
Hartmann et al., 2007; Borges and Abril, 2011; Griffith and Raymond, 2011;
Amann et al., 2012; Joesoef et al., 2015), few efforts have been made to
measure pCO2 in polluted Asian rivers, except for some large rivers
and estuaries in East Asia (Zhai et al., 2005; Chou et al., 2013; Ran et al.,
2015b; Yoon et al., 2017). These studies, together with a small number of
studies that used water chemistry data to estimate pCO2 levels in
major Asian rivers such as the Mekong (Li et al., 2013b), the Yangtze (Ran et
al., 2017b), the Ganges-Brahmaputra (Manaka et al., 2015b), and Indian
estuaries (Gupta et al., 2009; Sarma et al., 2012), underscored the
importance of anthropogenic OM and nutrients for riverine CO2 dynamics,
particularly along lower river reaches and estuaries draining highly
populated areas. When published data on pCO2 were compared between
headwater streams and tributaries feeding into the middle and lower reaches
of major Asian rivers, tributary pCO2 levels (mean:
2116 µatm) tended be higher than those for headwaters of Asian
rivers (mean: 1170 µatm; Table 3; Fig. 4). The ranges of pCO2 in the three river systems reviewed in detail also differed
between headwaters and lower main stem reaches and their tributaries,
displaying some river-specific patterns as described in the following
paragraphs.
With the basin-wide average pCO2 around 2164
(147–9659) µatm (Table 3), the Yellow River has been evaluated as
a source of CO2 for the atmosphere (Ran et al., 2015a, b, 2017a;
Fig. 3). The total emission of CO2 from the basin-wide fluvial network
was estimated at 4.7–7.9 Tg yr-1, with > 70 %
emitting from the tributaries draining the Loess Plateau (Ran et al., 2014,
2015b; Fig. 3). In recent decades water pollution has become an increasingly
important watershed management issue for the basin inhabited by a population
> 100 million, particularly in the middle and lower reaches
flanked by large industrial complexes and irrigated farmlands (Li et al.,
2006; Zhang et al., 2013; Lu et al., 2015). Drainage waters from croplands
containing high loads of OM have increased DOC concentrations in the middle
reach of the Yellow River, while wastewater discharged from large regional
population centers has been evaluated as the major source of DOC and POC in
the lower reach, particularly in winter (Zhang et al., 2013). As a combined
result of higher loads of pollutants discharged from local sources to
tributaries and higher flow diluting pollutant concentrations in the
main stem, tributaries appear to be more polluted than the main stem Yellow
River, exhibiting the highest levels of pCO2 among the three compared
river systems (Fig. 4). Higher concentrations of DOC and POC in more polluted
lower reaches and their tributaries might lead to enhanced in-stream
biodegradation of allochthonous C by labile OM fractions of anthropogenic
origin, but altered rates of biodegradation and primary production have not
yet been measured in any reach of the Yellow River. Along with the question
about impoundment effects on sediment C, the role of organic pollution in
riverine metabolisms and CO2 emissions along lower reaches is crucial
for understanding the fate of organic C derived from various sources in the
Yellow River basin, including C stocks stored in reservoir and floodplain
sediments.
It is very difficult to evaluate the overall pollution impacts on C dynamics of
the main stem Mekong River, as measurements of the nutrient and C cycles have
been made for short reaches of the lower Mekong in Laos and Cambodia (Alin et
al., 2011; Ellis et al., 2012; Martin et al., 2013) and in the Mekong Delta
(Borges et al., 2018). The annual DIC flux of the Mekong was estimated at
3.95 Tg, with DIC and alkalinity both negatively correlated with discharge
(Jeffrey E. Richey, unpublished data). This
trend, which has also been observed in the Pearl (Zhang et al., 2007) and
Yellow River (Ran et al., 2015a), has been attributed to the dominance of a
weathering-based source in the dry season that is diluted by less ion-rich
rainwater during the high-flow periods (Cai et al., 2008). The mean
reported pCO2 value for the lower Mekong River is 1235 µatm
(Table 3). Using a model based on pH and alkalinity, Li et al. (2013b)
reported a similar mean pCO2 for the lower Mekong: 1090
(224–5970) µatm. Alin et al. (2011) measured similar values
(∼ 1200 µatm) at eight main stem locations in Laos and
Cambodia. The seasonal trend in pCO2 opposes the alkalinity and DIC
trends, peaking in the flood season and being lowest in the dry season, similar to
several previous studies in tropical river systems (Sarma et al., 2011;
Borges et al., 2018). The level of pCO2 in the lower Mekong tends to
increase downstream, with an average of 812 µatm near Chiang Saen,
Thailand increasing toward 1670 µatm in the Mekong Delta (Li et
al., 2013b). The potential effects of polluted tributaries on main stem CO2
emissions were indicated by high pCO2 values approaching
2000–3000 µatm in tributaries draining highly populated areas
such as the Tonle Sap near Phnom Penh and local tributaries feeding into the
Mekong Delta (Li et al., 2013b; Table 3). In Phnom Penh, a combined drainage
system delivers untreated municipal wastewater and storm runoff either
directly or through four natural wetlands surrounding the city used for
natural purification to the Tonle Sap and Mekong (Irvine et al., 2006).
Measurements of pCO2 along three freshwater channels in the Mekong
Delta ranged between 1895 and 2664 µatm during the high-flow
periods of December 2003 and October 2004 (Borges et al., 2018) and exceeded
the range of 703–1597 µatm observed in the upstream reach during
a similar period (September–October 2004 and 2005) by Alin et al. (2011).
As suggested by Borges et al. (2018), anthropogenic pollution sources in the
densely populated and cultivated areas along lower reaches may release more
CO2 and biodegradable OM compared to the upstream reach.
As summarized in Table 3, many studies of aquatic CO2 dynamics in
India have been conducted in estuaries and coastal areas (Mukhopadhyay et
al., 2002; Biswas et al., 2004; Gupta et al., 2009; Sarma et al., 2012;
Samanta et al., 2015), except for the secondary data on pCO2
calculated using C system equations (Pierrot et al., 2006) and water quality
data collected in various headwaters (Sarin et al., 1989; Bickle et al.,
2003; Chakrapani and Veizer, 2005) and lower reaches (Manaka et al., 2015b)
of the Ganges-Brahmaputra. The values of pCO2 estimated for some
headwaters, lower reaches, and tributaries of the Ganges basin (mean: 893;
range: 65–2620 µatm) were relatively low compared to other Asian
rivers (Table 3; Fig. 4). In a study of the human impacts on C dynamics in
the Cochin estuary in southern India, Gupta et al. (2009) ascribed monsoonal
pCO2 increases of up to 6000 µatm to the enhanced
decomposition of the OM released from anthropogenic sources upstream.
Particular attention has been paid to the emission of CO2 and
CH4 from the Indian part of the deltaic region of the Ganges-Brahmaputra
system that includes estuaries with contrasting biogeochemical features: the
anthropogenically impacted estuary of the Hooghly (the largest Indian
distributary of the Ganges emptying into the Bay of Bengal) and the
mangrove-dominated estuaries of Sundarbans, the world's largest mangrove
ecosystem (Mukhopadhyay et al., 2002; Biswas et al., 2004; Dutta et al.,
2015; Samanta et al., 2015). The Hooghly estuary was found as net
heterotrophic, with the fugacity of CO2 (fCO2) varying
from ∼ 400 to 700 µatm (Mukhopadhyay et al., 2002). On an
annual scale, the Hooghly estuary acted as a source of CO2 (-2.78 to
84.4 mmol m-2 d-1) to the atmosphere (Mukhopadhyay et al.,
2002). The estuaries of Sundarbans were reported as a source of CO2
(314.6 mmol m-2 d-1) to the atmosphere (Dutta et al., 2015).
Samanta et al. (2015) reported a large annual DIC export of
(3.1–3.7) × 1012 g from the Hooghly estuary to the Bay of
Bengal exceeding the input of DIC through the river (freshwater). They
attributed the estuarine production of DIC to some biogeochemical processes
within the estuary including OM biodegradation and carbonate dissolution.
Drawing on wastewater discharge and DIC concentrations, Samanta et al. (2015)
estimated that direct anthropogenic sources of DIC within the Hooghly basin
might account for only 2–3 % of the river water DIC concentrations. It
remains unclear how much the biodegradation of organic C released from
anthropogenic sources could contribute to downstream DIC generation and
CO2 emissions in the Hooghly and other Indian estuaries.
The fact that in all three river systems pCO2 tended to be higher
along lower reaches and tributaries than in headwater streams (Table 3;
Fig. 4) might seem contradictory to the findings of some recent global
syntheses that compared pCO2 levels between low-order streams and
rivers (Lauerwald et al., 2015; Marx et al., 2017). These syntheses assumed
that the stream pCO2 level might be determined by the relative
contributions from terrestrial processes such as soil respiration and
weathering and in-stream processes including biodegradation and
photodegradation. It follows then that the contribution of terrestrially
derived pCO2 may overwhelm the in-stream contribution, at least
in low-order streams and rivers, resulting in a general trend of downstream
decreases in pCO2. These assumptions, together with a finding of
gradual downstream decline in the rate of the biodegradation of riverine OM
due to an increasing dominance of recalcitrant components with increasing
stream order and retention time (Catalán et al., 2016), are based on the
river continuum concept and do not consider downstream variations in the
consumption and replenishment of stream OM pools associated with enhanced
algal production in impounded reaches and/or pulsatile inputs of
anthropogenic OM delivered by urban streams as described in Fig. 2. In a
global synthesis of data collected at 1182 sites, which did not include
highly polluted river sites in Europe and the three Asian regions reviewed
here, Lauerwald et al. (2015) found a slightly higher mean pCO2
(2471 µatm) in streams and small rivers compared to the mean value
for large rivers (2299 µatm). As Lauerwald et al. (2015)
acknowledged, however, pCO2 may continue to increase along the
lower reaches of large rivers in response to inputs of labile OM and
dissolved CO2 from floodplains (Abril et al., 2014; Borges et al.,
2015) and pollution sources in croplands and urban areas (Kempe, 1982;
Frankignoulle et al., 1998). Although it is very difficult to evaluate the
relative contributions of autochthonous, soil-derived, and anthropogenic OM
fractions to enhanced biodegradation along lower reaches, some studies have
examined the effects of domestic and industrial wastewater on the chemical
composition and lability of organic C in the riverine and estuarine waters of
some polluted Asian rivers (Guo et al., 2014; Samanta et al., 2015). By
comparing fluorescence excitation–emission matrices (EEMs) of DOM between
the branches and tributaries of the Yangtze River estuary, Guo et al. (2014)
found that labile DOM components delivered by the Huangpu River, a highly
polluted tributary, exerted a disproportionately large influence on the
biodegradability of DOM in the Yangtze estuary. These chemical analyses,
together with direct underway measurements that revealed extraordinarily high
pCO2 levels along the polluted river and estuarine reaches of
some Chinese rivers (Zhai et al., 2005; Chou et al., 2013; Wang et al.,
2017), suggest that labile OM fractions of anthropogenic origin can boost the
microbial processing of bulk riverine OM, enhancing CO2 emissions
from polluted waterways.
Longitudinal shift in the relationship between Chl a and
pCO2 observed between the upstream and downstream reaches of the Han River
receiving highly polluted urban tributaries (modified from the Supplement of
Yoon et al., 2017).
Unusually high pCO2 levels observed in some eutrophic rivers and
estuaries across the three Asian regions cannot be explained by widely used
metabolic continuum models that would be more relevant for rather “natural”
inland water systems (Hotchkiss et al., 2015; Catalán et al., 2016). Many
studies conducted in polluted European rivers reported frequent occurrences
of extreme algal blooms and altered metabolic rates in the eutrophic reaches
that had rarely been observed in flowing water systems under minimal to low
human influences (Meybeck and Helmer, 1989; Hilton et al, 2006; Garnier and
Billen, 2007). A recent report on CO2 outgassing from a highly urbanized
river system in Korea suggested a potential regime shift in riverine
metabolic processes by showing a shift in the relationship between Chl a
and pCO2 from the upstream reach less enriched in nutrients and
CO2 to the eutrophic downstream reach receiving highly polluted urban
tributaries carrying WWTP effluents (Yoon et al., 2017; Fig. 5). This case
study was conducted in a heavily impounded and populated river basin (the Han
River) where the middle reach is impounded by cascade dams and the lower
reach receives loads of OM and nutrients delivered by urban streams draining
the Seoul metropolitan area with a population > 20 million.
Therefore, multiple dams along the middle reach and pulsatile inputs of OM
and nutrients along the lower reach may create discontinuities in metabolic
processes and CO2 emissions along the longitudinally connected reaches,
providing an excellent example of the anthropogenic land-water-scape depicted
in Fig. 2. In accordance with the findings of large spatial and seasonal
variations in the balance between autotrophy and heterotrophy in eutrophic
European rivers (Garnier and Billen, 2007), the enhanced bacterial degradation of
OM of both allochthonous and autochthonous origin in the eutrophic lower
reach receiving high loads of organic pollutants might increase the level of
pCO2 substantially despite the longitudinal increase in primary
production with a widening channel toward the river mouth, shifting the regime
of riverine metabolisms away from those found in the less eutrophic upstream
reach (Fig. 5).
Summary and future research needs
This review identified alarming regional trends concerning dam construction
booms and the rapid pace of urbanization across three reviewed Asian regions,
both of which can significantly alter riverine metabolisms and C dynamics.
Some recent studies have reported significant but contrasting impoundment
effects on GHG emissions and sediment C storage in dammed rivers. As
illustrated by large pulse emissions of CO2 and CH4 in the years
following the construction of the Three Gorges Dam on the Yangtze River and
cascade dams and the Nam Theun 2 in the lower Mekong River basin, flooded
soils and vegetation can become major sources of GHGs during the initial
years following dam construction. Long-term changes in GHG emissions and
sediment C storage might vary with dam location, initial conditions of the
flooded area, and land use changes occurring within the watersheds. As
summarized in Table 3, there have been only a small number of pCO2
measurements in dammed Asian rivers and almost no study that tracked
long-term changes in CO2 dynamics in impounded reaches and downstream
rivers, making it very difficult to constrain the factors crucial for the spatial
and temporal variations in pCO2 along the impounded reaches. Unlike in
Europe and North America where very few large dam projects have been
commissioned over recent decades, the current booms of mega-dam
construction across Asia appear to induce ever-increasing perturbations to
riverine C fluxes, demanding more systematic assessments of impoundment
impacts on riverine organic C transport and GHG emissions. Specifically,
these assessments require a basin-wide examination of temporal variations in
the rates of primary production and organic matter biodegradation in line
with pCO2 variations across impounded and upstream and downstream reaches
along the “discontinuity distance” (Ward and Standford, 1983) to predict
how impoundments alter the balance between autotrophy and heterotrophy and
hence the air–water exchange of CO2 both in the short-term diel cycle
and through seasonal and interannual variations. Long-term studies can also
evaluate how frequent droughts associated with regional climate change can
reverse the gradually decreasing GHG emissions from impounded river reaches
through the enhanced decomposition of the C stored in the reservoir bottom
sediment.
Although rapid urbanization across Asia is aggravating eutrophication and
organic pollution in many large rivers draining metropolitan areas with a
limited capacity for wastewater treatment infrastructure, the scarcity of
high-quality monitoring data represents a huge challenge for a thorough
assessment of the current status of riverine metabolisms and CO2
outgassing from the impacted rivers. Some exploratory studies conducted in
highly urbanized watersheds in East Asia (e.g., Wang et al., 2017; Yoon et
al., 2017) have questioned whether the conventional conceptual framework
perceiving riverine C fluxes as a gradual longitudinal continuum can address
the large cross-scale variations and pulsatile patterns of riverine CO2
outgassing observed in the highly modified river systems. Given the large
share of the reviewed Asian regions (∼ 40 %) in global
municipal wastewater production and disproportionately poor wastewater
treatment infrastructure, it remains largely unknown whether our current
understanding of biogeochemical processes in urban river systems in Europe
and North America can help explain the idiosyncratic features of OM composition
and turnover in streams and rivers contaminated with high loads of raw sewage
and nutrients. Building on conceptual and mathematical models developed for
highly eutrophic river systems in Europe and North America (e.g., Garnier and
Billen, 2007), we need to develop new integrative frameworks to explain
river-specific responses to the unprecedented pace and scale of urbanization
and water pollution. These integrative frameworks need to consider concurrent
multiple environmental changes, including dams and climatic variability, and
extreme events as confounding factors that can either boost or dampen
pollution-induced pulses of CO2 emissions from highly polluted urban
rivers. A key future challenge in predicting CO2 emissions from highly
polluted, eutrophic river systems would be to constrain shifting balances
between interrelated riverine metabolic processes, as illustrated by the
regime shift in the relationship between Chl a and pCO2 observed
between the less impacted upstream and eutrophic downstream reaches of a
highly urbanized river system (Fig. 5).
How to overcome the overall scarcity and spatially uneven availability of
high-quality data, guided by an integrative conceptual framework reflecting
observed regional trends, might be the number one research priority in
providing a scientifically robust assessment of the current status of the
human impacts on C fluxes in Asian river systems. Given the inadequate
research capacity in many developing Asian countries, more efforts should be
given to build collaborative research networks that can provide researchers
with practical guides and standardized methodologies for designing and
conducting field monitoring of riverine C fluxes at multiple spatial and
temporal scales. These efforts need to pay more attention to emerging local
issues in addition to the common regional patterns associated with river
impoundment and pollution. As observed in the Tibetan Plateau, urbanization
and dam construction have been expanding to the upstream headwater reaches of
large rivers such as the Mekong, Yangtze, and Yellow River. The rapid
expansion of anthropogenic perturbations, coupled with idiosyncratic local
climates and ecosystems, can amplify changes in riverine metabolic processes
and C dynamics. To better assess the interactive effects of concurrent
multiple environmental changes and human-induced perturbations to the
riverine networks of interacting land and water patches, we suggest that the
long-standing concept of river
continuum assuming “natural” states should be critically examined by field
measurements and complemented with alternative perspectives of
“discontinuity” or “discontinuous continuity” (sensu Poole, 2002) in
riverine metabolisms and C fluxes created in impounded and eutrophic reaches
of rivers draining increasingly urbanizing watersheds across Asia and
globally.
Data are available and can be requested from the
corresponding author (jhp@ewha.ac.kr).
All authors contributed to data acquisition, the discussion of concepts and
research topics, and paper preparation. The paper was written
through the concerted efforts of all authors and coordinated by JHP.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “Human impacts on
carbon fluxes in Asian river systems”. It is not associated with a
conference.
Acknowledgements
This work was supported by the Asia-Pacific Network for Global Change
Research (CRRP2016-01MY-Park) and the National Foundation of Korea (2017R1D1A1B06035179). We
thank Tae Kyung Yoon for providing a figure modified from the
Supplement to his publication and Dr. Gwenaël Abril and two anonymous reviewers for proving
insightful comments on the paper.
Edited by: Gwenaël Abril
Reviewed by: two anonymous referees
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