denitrification as it generates electron source for denitrifying enzymes (Dodla et al.
2008). The increase in NH 4
+ efflux ultimately stimulates phytoplankton growth and
in turn increases hypoxia/anoxia at sediment-water interface making the recovery of
ecosystem even more difficult (Middelburg and Levin 2009).
About 85% of metal inputs to the river deposit in the bed sediments (Zhang et al.
2016). However, the benthic hypoxic/anoxic condition leads to enhanced efflux of
metals from sediment to overlying waters (Jaiswal and Pandey 2019d). The hypoxia/
anoxia affects the redox condition of sediments and in turn provides positive
feedback to sediment-metal release (Jaiswal and Pandey 2019d). The metals in the
sediment are adsorbed onto organic carbon, iron, and manganese oxides and clay
particles (Eggleton and Thomas 2004) in different ways such as occlusion in
amorphous materials, absorption at the surface of oxy-hydroxides of Fe and Mn,
complexation with organic matter, and incorporation with sulfides (Zhang et al.
2014). A high DOD sw causes reduction of nitrate and oxy-hydroxides of iron and
manganese leading to increased release of metals from riverbed sediment (Eggleton
and Thomas 2004). We found a high rate of sediment-metal release in the Ganga
River at sites associated with hypoxia/anoxia such as the locations close to drain
mouth, tributary confluences, and sites downstream of cities (Jaiswal and Pandey
2019d). Significant increase in Fe and Mn releases at DO < 2.0 mg L
À1 has been
reported by Banks et al. (2012) in an incubation experiment, by Fu et al. (2014) for
Jialu River, and by Liu et al. (2019) in microcosm experiments. Banks et al. (2012)
found increased dissolved fraction of Zn, Pb, Cd, and Cu from contaminated
sediments even at very short-time hypoxic conditions. Liu et al. (2019) have reported
significant increase in bioavailability of Zn, Pb, Cd, Cu, and Cr under severe hypoxic
condition (DO, 0–2.0 mg/l). For the Ganga River, an increase in benthic metal
bioavailability has been reported at locations with DO sw < 2.0 mg L
À1 (Jaiswal
and Pandey 2019d).
Enhanced release of metals at sediment-water interface has toxicological implications. Different forms of the metals show different degree of mobility, chemical
interactions, biological availability, and toxicity (Xu et al. 2017). The mobile and
bioavailable fractions of metals cause greater toxicity to aquatic organisms (Eggleton
and Thomas 2004). Thus, assessing the bioavailable fractions and the factors
affecting their concentration and release can be helpful in understanding transformations, transport, and impact of metals in the aquatic environment (Morelli and
Gasparon 2014). Various studies have shown the impact of metal toxicity on benthic
ecosystems in terms of increased mortality and biodiversity loss (Stark et al. 2004),
effect on colonization and dispersal (Stark et al. 2004), and reduced reproduction rate
and population growth (Vicente-Martorell et al. 2009). Increased fraction of bioavailable metals have been shown to cause more negative impacts on epibenthic and
pelagic invertebrates and fishes (Vicente-Martorell et al. 2009). Given that the
Ganga River is the home of a diversity of economically important fisheries (Rao
2001) and other aquatic organisms, a decrease in DO, development of hypoxic
zones, and consequent release of bioavailable metals will lead to death of these
organisms, influencing trophic cascade and the nutritional and livelihood security to
human consumers.
234
D. Jaiswal et al.
2008). The increase in NH 4
+ efflux ultimately stimulates phytoplankton growth and
in turn increases hypoxia/anoxia at sediment-water interface making the recovery of
ecosystem even more difficult (Middelburg and Levin 2009).
About 85% of metal inputs to the river deposit in the bed sediments (Zhang et al.
2016). However, the benthic hypoxic/anoxic condition leads to enhanced efflux of
metals from sediment to overlying waters (Jaiswal and Pandey 2019d). The hypoxia/
anoxia affects the redox condition of sediments and in turn provides positive
feedback to sediment-metal release (Jaiswal and Pandey 2019d). The metals in the
sediment are adsorbed onto organic carbon, iron, and manganese oxides and clay
particles (Eggleton and Thomas 2004) in different ways such as occlusion in
amorphous materials, absorption at the surface of oxy-hydroxides of Fe and Mn,
complexation with organic matter, and incorporation with sulfides (Zhang et al.
2014). A high DOD sw causes reduction of nitrate and oxy-hydroxides of iron and
manganese leading to increased release of metals from riverbed sediment (Eggleton
and Thomas 2004). We found a high rate of sediment-metal release in the Ganga
River at sites associated with hypoxia/anoxia such as the locations close to drain
mouth, tributary confluences, and sites downstream of cities (Jaiswal and Pandey
2019d). Significant increase in Fe and Mn releases at DO < 2.0 mg L
À1 has been
reported by Banks et al. (2012) in an incubation experiment, by Fu et al. (2014) for
Jialu River, and by Liu et al. (2019) in microcosm experiments. Banks et al. (2012)
found increased dissolved fraction of Zn, Pb, Cd, and Cu from contaminated
sediments even at very short-time hypoxic conditions. Liu et al. (2019) have reported
significant increase in bioavailability of Zn, Pb, Cd, Cu, and Cr under severe hypoxic
condition (DO, 0–2.0 mg/l). For the Ganga River, an increase in benthic metal
bioavailability has been reported at locations with DO sw < 2.0 mg L
À1 (Jaiswal
and Pandey 2019d).
Enhanced release of metals at sediment-water interface has toxicological implications. Different forms of the metals show different degree of mobility, chemical
interactions, biological availability, and toxicity (Xu et al. 2017). The mobile and
bioavailable fractions of metals cause greater toxicity to aquatic organisms (Eggleton
and Thomas 2004). Thus, assessing the bioavailable fractions and the factors
affecting their concentration and release can be helpful in understanding transformations, transport, and impact of metals in the aquatic environment (Morelli and
Gasparon 2014). Various studies have shown the impact of metal toxicity on benthic
ecosystems in terms of increased mortality and biodiversity loss (Stark et al. 2004),
effect on colonization and dispersal (Stark et al. 2004), and reduced reproduction rate
and population growth (Vicente-Martorell et al. 2009). Increased fraction of bioavailable metals have been shown to cause more negative impacts on epibenthic and
pelagic invertebrates and fishes (Vicente-Martorell et al. 2009). Given that the
Ganga River is the home of a diversity of economically important fisheries (Rao
2001) and other aquatic organisms, a decrease in DO, development of hypoxic
zones, and consequent release of bioavailable metals will lead to death of these
organisms, influencing trophic cascade and the nutritional and livelihood security to
human consumers.
234
D. Jaiswal et al.