linkages and measure the organic pollution load and health status of the river (Tare
et al. 2003; Dwivedi et al. 2018). The sediment oxygen demand (SOD) has been
reported to share >50% of total oxygen demand (MacPherson et al. 2007) but is a
more neglected cause of increasing DOD in the Ganga River. The SOD has two
components: biological sediment oxygen demand (BSOD), which is associated with
benthic respiration and microbial decomposition of organic matter, and chemical
sediment oxygen demand (CSOD), the oxygen consumed in the oxidation of Fe, Mn,
and NH 4
+ (Higashino et al. 2004). A study conducted along a 518-km-long segment
of the middle stretch of the Ganga River and downstream of two point sources
showed high SOD and DOD sw and development of hypoxia/anoxia at many locations of the river (Jaiswal and Pandey 2019b). This merits attention because wastewater treatment technologies generally address removal of BOD only. The
increasing load of other chemicals in the river is also causing a greater risk to DO
and needs proper management.
Episodic development of hypoxic/anoxic zones is considered as a signal of
critical health of an aquatic ecosystem as the DO less than 2 mg L
À1 can cause
lethal to sublethal effects on benthic organisms and fish (Diaz and Rosenberg 2008).
Hypoxia/anoxia-induced ecosystem consequences include loss of biodiversity,
changes in organic matter processing and nutrient cycling, decreased resistance to
invasion and ecosystem functioning, and ultimately reduced resistance to natural and
human perturbations (Solan et al. 2004; Carstensen et al. 2014). Further, the hypoxic/anoxic conditions affect secondary production which ultimately influences the
food web (Carstensen et al. 2014). These conditions lead to ~75% reduction in
burrow productions which reduces the sediment oxygenation exacerbating the
problem further (Middelburg and Levin 2009).
The benthic hypoxia/anoxia generates positive feedbacks such as sediment-P
release, sediment-metal release, and denitrification (Conley et al. 2002; Eyre and
Ferguson 2009) which result in massive alteration in nutrient and metal cycling
(Hu et al. 2001; Villnäs et al. 2012). In a study conducted in the Ganga River, we
found a high rate of sediment-P release at sites with high dissolved oxygen deficit
(Jaiswal and Pandey 2019b; Pandey et al. 2019). The phosphate bounded to iron
oxides in oxygenated sediments is released to the overlying waters when the oxides
are reduced under hypoxic/anoxic condition (Middelburg and Levin 2009). The
increased flux of P from bed sediment alleviates P limitation (Conley et al. 2002)
and propels phytoplankton production (Cardinale 2011) leading to DOD which
delays the recovery further (Jaiswal and Pandey 2019b). The oxygen deficit in the
benthic region accelerates denitrification (Eyre and Ferguson 2009). Studies have
reported severe hypoxia/anoxia at the Bay of Bengal, and it has been urged that even
a small change in oxygen level will lead to a drastic change in nitrogen balance
(Bristow et al. 2017). We have reported a high rate of denitrification downstream of
point sources and tributary confluences at sites where DO sw was below 2.0 mg L
À1
(Jaiswal and Pandey 2019f; Pandey et al. 2019). The study sites with high denitrification also have high concentrations of total organic carbon (TOC), nitrate, and
total nitrogen (TN) which are known to enhance the rate of denitrification (PiñaOchoa and Alvarez-Cobelas 2006). The organic carbon concentration affects
10 Ecosystem Responses to Pollution in the Ganga River: Key Issues to Address. . .
233
et al. 2003; Dwivedi et al. 2018). The sediment oxygen demand (SOD) has been
reported to share >50% of total oxygen demand (MacPherson et al. 2007) but is a
more neglected cause of increasing DOD in the Ganga River. The SOD has two
components: biological sediment oxygen demand (BSOD), which is associated with
benthic respiration and microbial decomposition of organic matter, and chemical
sediment oxygen demand (CSOD), the oxygen consumed in the oxidation of Fe, Mn,
and NH 4
+ (Higashino et al. 2004). A study conducted along a 518-km-long segment
of the middle stretch of the Ganga River and downstream of two point sources
showed high SOD and DOD sw and development of hypoxia/anoxia at many locations of the river (Jaiswal and Pandey 2019b). This merits attention because wastewater treatment technologies generally address removal of BOD only. The
increasing load of other chemicals in the river is also causing a greater risk to DO
and needs proper management.
Episodic development of hypoxic/anoxic zones is considered as a signal of
critical health of an aquatic ecosystem as the DO less than 2 mg L
À1 can cause
lethal to sublethal effects on benthic organisms and fish (Diaz and Rosenberg 2008).
Hypoxia/anoxia-induced ecosystem consequences include loss of biodiversity,
changes in organic matter processing and nutrient cycling, decreased resistance to
invasion and ecosystem functioning, and ultimately reduced resistance to natural and
human perturbations (Solan et al. 2004; Carstensen et al. 2014). Further, the hypoxic/anoxic conditions affect secondary production which ultimately influences the
food web (Carstensen et al. 2014). These conditions lead to ~75% reduction in
burrow productions which reduces the sediment oxygenation exacerbating the
problem further (Middelburg and Levin 2009).
The benthic hypoxia/anoxia generates positive feedbacks such as sediment-P
release, sediment-metal release, and denitrification (Conley et al. 2002; Eyre and
Ferguson 2009) which result in massive alteration in nutrient and metal cycling
(Hu et al. 2001; Villnäs et al. 2012). In a study conducted in the Ganga River, we
found a high rate of sediment-P release at sites with high dissolved oxygen deficit
(Jaiswal and Pandey 2019b; Pandey et al. 2019). The phosphate bounded to iron
oxides in oxygenated sediments is released to the overlying waters when the oxides
are reduced under hypoxic/anoxic condition (Middelburg and Levin 2009). The
increased flux of P from bed sediment alleviates P limitation (Conley et al. 2002)
and propels phytoplankton production (Cardinale 2011) leading to DOD which
delays the recovery further (Jaiswal and Pandey 2019b). The oxygen deficit in the
benthic region accelerates denitrification (Eyre and Ferguson 2009). Studies have
reported severe hypoxia/anoxia at the Bay of Bengal, and it has been urged that even
a small change in oxygen level will lead to a drastic change in nitrogen balance
(Bristow et al. 2017). We have reported a high rate of denitrification downstream of
point sources and tributary confluences at sites where DO sw was below 2.0 mg L
À1
(Jaiswal and Pandey 2019f; Pandey et al. 2019). The study sites with high denitrification also have high concentrations of total organic carbon (TOC), nitrate, and
total nitrogen (TN) which are known to enhance the rate of denitrification (PiñaOchoa and Alvarez-Cobelas 2006). The organic carbon concentration affects
10 Ecosystem Responses to Pollution in the Ganga River: Key Issues to Address. . .
233