(Yadav and Pandey 2017b; Jaiswal and Pandey 2018, 2019a) indicating that extracellular enzymes can be used as an alternative alert system against increasing human
pressure.
10.6.2 Elemental Stoichiometry
Elemental stoichiometry, the mass balance of key elements (C, N, P, and Si) in an
ecosystem (Elser et al. 2009), has a central role in the theory of resource ratio
competition between alga (Makulla and Sommer 1993), consumer-driven nutrient
recycling (Elser et al. 2009), and food chain efficiency (Sterner et al. 1998). The N/P/
Si Redfield ratio (16:1:16) is essential for balanced growth of phytoplankton specifically for diatoms (Turner et al. 2003). The changes in relative proportion of these
nutrients define which nutrient to limit phytoplankton growth (Elser et al. 2009). A
major factor to influence this ratio in the Ganga River is the increasing use of N and P
fertilizers to meet the demand of food of overpopulated Ganges basin. About
10 million tons of chemical fertilizers are applied in the Ganges basin, which
represent 45% of India’s total annual fertilizer consumption. These N and P fertilizers reach to the river through atmospheric deposition, leaching, and runoff in the
form of highly mobile NO 3
À and PO 4
3À ions. Unlike human-induced increases in
the concentration of N and P at global scale, the Si concentration in most cases is
either stable or declining. Hydrologic shifts in the watershed may reduce Si concentration by as much as 50% (Correll et al. 2000). A shift in this ratio causes cellular
nutrient imbalances and induces a change in phytoplankton composition, biogeochemical cycles, carbon sequestration, biological diversity, and trophic cascades
(Elser et al. 2009; Pandey and Yadav 2015). Therefore, the N/P/Si ratio is considered
as a sensitive indicator of aquatic health and food web structure. The absolute
concentration of nutrients and their stoichiometric ratios can be used together as a
comprehensive predictor of eutrophy across broad landscapes as represented by
large rivers.
Studies show that the disproportionate nutrient loading and management efforts
have changed the canonical N/P stoichiometric ratios in many aquatic ecosystems of
the world including India (Pandey and Yadav 2015; Pandey et al. 2016a). The
anthropogenic causations that enhance N and P input do not generally lead to a
proportionate increase in the concentration of Si in rivers. Indeed, some of the
anthropogenic activities such as river damming decrease the amount of Si reaching
to the coast. Further, low flow season induced increase in riverine primary productivity and nutrient uptake, and subsequently the sedimentation of diatoms leads to
loss of adsorbed silicate from the water column (Conley 1997). Since Si is essential
for the growth of diatoms, a deviation in the supply of Si may change the magnitude
of diatom-driven C sequestration. Studies have shown that a decrease in N/P ratio
causes a shift in the dominance in phytoplankton assemblage toward diazotrophic
cyanobacteria (Elser et al. 2000; Pandey et al. 2017). A shift in N/P ratio toward
<16:1 changes the phytoplankton community and promotes P-favored taxa.
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D. Jaiswal et al.
pressure.
10.6.2 Elemental Stoichiometry
Elemental stoichiometry, the mass balance of key elements (C, N, P, and Si) in an
ecosystem (Elser et al. 2009), has a central role in the theory of resource ratio
competition between alga (Makulla and Sommer 1993), consumer-driven nutrient
recycling (Elser et al. 2009), and food chain efficiency (Sterner et al. 1998). The N/P/
Si Redfield ratio (16:1:16) is essential for balanced growth of phytoplankton specifically for diatoms (Turner et al. 2003). The changes in relative proportion of these
nutrients define which nutrient to limit phytoplankton growth (Elser et al. 2009). A
major factor to influence this ratio in the Ganga River is the increasing use of N and P
fertilizers to meet the demand of food of overpopulated Ganges basin. About
10 million tons of chemical fertilizers are applied in the Ganges basin, which
represent 45% of India’s total annual fertilizer consumption. These N and P fertilizers reach to the river through atmospheric deposition, leaching, and runoff in the
form of highly mobile NO 3
À and PO 4
3À ions. Unlike human-induced increases in
the concentration of N and P at global scale, the Si concentration in most cases is
either stable or declining. Hydrologic shifts in the watershed may reduce Si concentration by as much as 50% (Correll et al. 2000). A shift in this ratio causes cellular
nutrient imbalances and induces a change in phytoplankton composition, biogeochemical cycles, carbon sequestration, biological diversity, and trophic cascades
(Elser et al. 2009; Pandey and Yadav 2015). Therefore, the N/P/Si ratio is considered
as a sensitive indicator of aquatic health and food web structure. The absolute
concentration of nutrients and their stoichiometric ratios can be used together as a
comprehensive predictor of eutrophy across broad landscapes as represented by
large rivers.
Studies show that the disproportionate nutrient loading and management efforts
have changed the canonical N/P stoichiometric ratios in many aquatic ecosystems of
the world including India (Pandey and Yadav 2015; Pandey et al. 2016a). The
anthropogenic causations that enhance N and P input do not generally lead to a
proportionate increase in the concentration of Si in rivers. Indeed, some of the
anthropogenic activities such as river damming decrease the amount of Si reaching
to the coast. Further, low flow season induced increase in riverine primary productivity and nutrient uptake, and subsequently the sedimentation of diatoms leads to
loss of adsorbed silicate from the water column (Conley 1997). Since Si is essential
for the growth of diatoms, a deviation in the supply of Si may change the magnitude
of diatom-driven C sequestration. Studies have shown that a decrease in N/P ratio
causes a shift in the dominance in phytoplankton assemblage toward diazotrophic
cyanobacteria (Elser et al. 2000; Pandey et al. 2017). A shift in N/P ratio toward
<16:1 changes the phytoplankton community and promotes P-favored taxa.
238
D. Jaiswal et al.