reduce the EE activities. As most of the human-impacted rivers receive a high
amount of toxicants along with carbon and nutrients (Jaiswal and Pandey 2019a),
a deviation has also been reported regarding the carbon sequestration in these rivers.
Studies have reported a reduced rate of decomposition and C mineralization even at
low concentration of metals (Nwachukwu and Pulford 2011). In a study, we found a
contrasting result regarding the carbon decomposition and sequestration along the
middle stretch of the Ganga River (Verma et al. 2019). In the sites rich in carbon and
nutrients and where metal concentrations did not exceed the toxic threshold
(ƩTHM < 360 μg g
À1 ), an increase in CO 2 emission was observed. These sites
were also reported with high EE activities as organic C enhances microbial activity.
However, at sites with high metal concentration (ƩTHM > 360 μg g
À1 ), a significant
reduction in CO 2 emission was recorded (Jaiswal and Pandey 2019e). This could be
linked with the fact that increased metal concentration negatively influences the
microbial activity and carbon degrading enzymes (β-D-glucosidase and FDAase)
leading to reduction in microbial ability to metabolize carbon sources (Jaiswal and
Pandey 2018). A high C/N ratio was also reported at these sites, further indicating
higher accumulation of C relative to release (Verma et al. 2019). Overall, our studies
with the Ganga River clearly indicate that metal pollution in eutrophic lotic systems
enhances C storage relative to flux (Jaiswal and Pandey 2019e; Verma et al. 2019).
These results indicate that the metals cause physiological constraints in carbon
breakdown and consequently enhance C accumulation. If the heavy metal concentration continues to increase, as expected in future, the CO 2 emission and C
degradation may not be proportionate to the amount of carbon the human-impacted
rivers are receiving. This will lead to enhanced C accumulation relative to flux in
anthropogenically impacted large rivers.
10.6 Alternative Alert Systems
10.6.1 Extracellular Enzymes
Biomonitors play an important role in identifying shifts in ecosystem structure and
functioning, recognizing the causal factors, and understanding the consequences.
Unlike terrestrial ecosystems, where most of the shifts are quantitatively detectable
(Oliver et al. 2015), scientists often face a number of limitations in identifying
specific and universal biomonitors for changes in water quality and trophic status
of riverine ecosystems (Lafont 2001). The major challenges for lotic ecosystems are
the recurrence of variable and multiple anthropogenic perturbations, climate change,
hydrological forcing, and connectivity with other domains such as watershed and
airshed, which influence the universality and specificity of a biomonitoring tool
(Pearson et al. 2016).
Studies generally use variables such as biological oxygen demand (BOD), chlorophyll a biomass (Gholizadeh et al. 2016), phycocyanin (Ahn et al. 2007),
microinvertebrates (Turley et al. 2016), and diatom indices (Potapova et al. 2004;
236
D. Jaiswal et al.
amount of toxicants along with carbon and nutrients (Jaiswal and Pandey 2019a),
a deviation has also been reported regarding the carbon sequestration in these rivers.
Studies have reported a reduced rate of decomposition and C mineralization even at
low concentration of metals (Nwachukwu and Pulford 2011). In a study, we found a
contrasting result regarding the carbon decomposition and sequestration along the
middle stretch of the Ganga River (Verma et al. 2019). In the sites rich in carbon and
nutrients and where metal concentrations did not exceed the toxic threshold
(ƩTHM < 360 μg g
À1 ), an increase in CO 2 emission was observed. These sites
were also reported with high EE activities as organic C enhances microbial activity.
However, at sites with high metal concentration (ƩTHM > 360 μg g
À1 ), a significant
reduction in CO 2 emission was recorded (Jaiswal and Pandey 2019e). This could be
linked with the fact that increased metal concentration negatively influences the
microbial activity and carbon degrading enzymes (β-D-glucosidase and FDAase)
leading to reduction in microbial ability to metabolize carbon sources (Jaiswal and
Pandey 2018). A high C/N ratio was also reported at these sites, further indicating
higher accumulation of C relative to release (Verma et al. 2019). Overall, our studies
with the Ganga River clearly indicate that metal pollution in eutrophic lotic systems
enhances C storage relative to flux (Jaiswal and Pandey 2019e; Verma et al. 2019).
These results indicate that the metals cause physiological constraints in carbon
breakdown and consequently enhance C accumulation. If the heavy metal concentration continues to increase, as expected in future, the CO 2 emission and C
degradation may not be proportionate to the amount of carbon the human-impacted
rivers are receiving. This will lead to enhanced C accumulation relative to flux in
anthropogenically impacted large rivers.
10.6 Alternative Alert Systems
10.6.1 Extracellular Enzymes
Biomonitors play an important role in identifying shifts in ecosystem structure and
functioning, recognizing the causal factors, and understanding the consequences.
Unlike terrestrial ecosystems, where most of the shifts are quantitatively detectable
(Oliver et al. 2015), scientists often face a number of limitations in identifying
specific and universal biomonitors for changes in water quality and trophic status
of riverine ecosystems (Lafont 2001). The major challenges for lotic ecosystems are
the recurrence of variable and multiple anthropogenic perturbations, climate change,
hydrological forcing, and connectivity with other domains such as watershed and
airshed, which influence the universality and specificity of a biomonitoring tool
(Pearson et al. 2016).
Studies generally use variables such as biological oxygen demand (BOD), chlorophyll a biomass (Gholizadeh et al. 2016), phycocyanin (Ahn et al. 2007),
microinvertebrates (Turley et al. 2016), and diatom indices (Potapova et al. 2004;
236
D. Jaiswal et al.