ranging from cellular- to system-level processes and may lead to ecosystem-level
consequences if it persists for longer duration (Dai et al. 2013). These nonlinear
responses of ecosystems to environmental changes cause destabilization of nutrient
cycling and other ecosystem processes leading to delay in the recovery from
disturbances. Although various studies have emphasized the importance and relevance of understanding these processes in ecosystem restoration (Scheffer et al.
2001), identification and quantification of positive feedbacks for riverine ecosystems
are very difficult.
High input of oxygen-demanding substances leads to development of hypoxic/
anoxic zones which trigger positive feedbacks in aquatic environments. The
dissolved oxygen (DO) plays the most important regulatory role in aquatic ecosystem structure and functioning. The oxygen deficiency causes dramatic changes in
benthic communities and thus leads to noticeable changes on ecosystem functioning,
species richness, and abundance, ultimately leading to development of alternative
stable states. Some of the important feedbacks associated with benthic hypoxia/
anoxia include the denitrification, sediment-P release, and sediment-metal releases.
These functional shifts coupled with increased dissolved oxygen deficit (DOD) lead
the system toward alternate stable states and development of fragmented patches
with varying species composition, nutrient and metal concentrations, and shift in
trophic state and food webs (Scheffer et al. 2001; Pandey et al. 2019).
Studies conducted by Jaiswal and Pandey (2019d, 2019f) reveal a high rate of
denitrification, sediment-P release, and sediment-metal release at sites downstream
of point sources, tributary confluences, and downstream cities along the Ganga
River. These locations did show DO sw below 1.5 mg L
À1 indicating that the benthic
hypoxia has generated positive feedbacks leading to the release of sediment-bound
nutrients and metals to the overlying water. The release of nutrients and metals from
the sediment accelerates eutrophication enhancing the DOD further. Also, the
ecological communities and ecosystems respond to human perturbations in different
ways. For instance, some processes are enhanced but respond smoothly and gradually to changes in environmental conditions, whereas some variables show instantaneous response and others may remain inert until a threshold condition is reached
at which the ecosystem responds abruptly (Scheffer et al. 2009). Studies show that
the extracellular enzymes can be used as a suitable predictor of ecosystem responses
toward carbon, nutrient, and metal enrichment (Sinsabaugh et al. 2009; Jaiswal and
Pandey 2019a). Because, the carbon and nutrients stimulate microbial activities, a
concordant increase in the enzyme activity can be used as an indicator of eutrophy.
Opposite to this, the heavy metal pollution causes toxic effects leading to a sudden
decrease in the enzyme activity even in the presence of high concentration of carbon
and nutrients (Jaiswal and Pandey 2018, 2019a). Because the carbon chelates metals
leading to a reduction in toxicity, in ecosystems with prevalence of toxicants and
stimulants, the toxic impact is reduced to a certain extent. The shifts in these
responses can be used as alternative alert systems for understanding the health and
level of deterioration of human-impacted riverine ecosystems.
244
D. Jaiswal et al.
consequences if it persists for longer duration (Dai et al. 2013). These nonlinear
responses of ecosystems to environmental changes cause destabilization of nutrient
cycling and other ecosystem processes leading to delay in the recovery from
disturbances. Although various studies have emphasized the importance and relevance of understanding these processes in ecosystem restoration (Scheffer et al.
2001), identification and quantification of positive feedbacks for riverine ecosystems
are very difficult.
High input of oxygen-demanding substances leads to development of hypoxic/
anoxic zones which trigger positive feedbacks in aquatic environments. The
dissolved oxygen (DO) plays the most important regulatory role in aquatic ecosystem structure and functioning. The oxygen deficiency causes dramatic changes in
benthic communities and thus leads to noticeable changes on ecosystem functioning,
species richness, and abundance, ultimately leading to development of alternative
stable states. Some of the important feedbacks associated with benthic hypoxia/
anoxia include the denitrification, sediment-P release, and sediment-metal releases.
These functional shifts coupled with increased dissolved oxygen deficit (DOD) lead
the system toward alternate stable states and development of fragmented patches
with varying species composition, nutrient and metal concentrations, and shift in
trophic state and food webs (Scheffer et al. 2001; Pandey et al. 2019).
Studies conducted by Jaiswal and Pandey (2019d, 2019f) reveal a high rate of
denitrification, sediment-P release, and sediment-metal release at sites downstream
of point sources, tributary confluences, and downstream cities along the Ganga
River. These locations did show DO sw below 1.5 mg L
À1 indicating that the benthic
hypoxia has generated positive feedbacks leading to the release of sediment-bound
nutrients and metals to the overlying water. The release of nutrients and metals from
the sediment accelerates eutrophication enhancing the DOD further. Also, the
ecological communities and ecosystems respond to human perturbations in different
ways. For instance, some processes are enhanced but respond smoothly and gradually to changes in environmental conditions, whereas some variables show instantaneous response and others may remain inert until a threshold condition is reached
at which the ecosystem responds abruptly (Scheffer et al. 2009). Studies show that
the extracellular enzymes can be used as a suitable predictor of ecosystem responses
toward carbon, nutrient, and metal enrichment (Sinsabaugh et al. 2009; Jaiswal and
Pandey 2019a). Because, the carbon and nutrients stimulate microbial activities, a
concordant increase in the enzyme activity can be used as an indicator of eutrophy.
Opposite to this, the heavy metal pollution causes toxic effects leading to a sudden
decrease in the enzyme activity even in the presence of high concentration of carbon
and nutrients (Jaiswal and Pandey 2018, 2019a). Because the carbon chelates metals
leading to a reduction in toxicity, in ecosystems with prevalence of toxicants and
stimulants, the toxic impact is reduced to a certain extent. The shifts in these
responses can be used as alternative alert systems for understanding the health and
level of deterioration of human-impacted riverine ecosystems.
244
D. Jaiswal et al.