tropics, nutrients are considered as the principal determinants. For this reason,
phytoplankton is considered as an indicator of nutrient pollution and eutrophy.
Nutrients influence phytoplankton growth in terms of absolute concentration and
in specific elemental ratios. The specific stoichiometric ratio (Redfield ratio) of N, P,
and Si (16:1:16) is important for the balanced growth of phytoplankton specifically
for diatoms (Turner et al. 2003). Recent studies have reported a shift in this ratio due
to disproportionate input of nutrients in lakes and rivers (Pandey et al. 2014b,
2016b). This shift in stoichiometry would alter the ecosystem structure and functioning in due course of time with long-term irrecoverable consequences (Pandey
et al. 2014b). The shifts in nutrient stoichiometry affect a diverse group of primary
producers, specifically diatoms which are R-strategic (high reproductive potential)
phytoplankton and show rapid change in their growth with changes in aquatic
ecosystems. Besides nutrient stoichiometry, the changes in ionic strength, pH,
light penetration, and temperature also affect the diversity and abundance of diatoms
(Potapova and Charles 2003).
Benthic diatoms are an important group of protists that show niche specialization
against nutrient pollution and light regime shifts. Because this group grows by
attaching to certain substrate, they can be used as a selected group to assess
responses especially in lotic systems. In response to atmospheric deposition of N
in oligotrophic alpine lakes and P enrichment in temperate lakes, species such as
Asterionella and Fragilaria have been reported to constitute the dominant group
(Saros et al. 2005). The Ganga River receives, along with point and nonpoint
sources, huge amount of N and P through atmospheric deposition (Pandey et al.
2013, 2014a, 2016a) suggesting that these inputs will also have strong effect on the
distribution of diatoms. In an earlier study conducted along 268 km stretch from
Allahabad to Varanasi downstream, we found a significant shift in diatom abundance
and diversity in concordance with the changing state of carbon and nutrient pollution
(Pandey et al. 2017). A significant decline in species diversity was observed with
decreasing N/P stoichiometry, and, except for a few species which are adapted to
high nutrient concentrations, the sites with nutrient-rich condition showed less
overall diversity of diatoms. Species adapted to nutrient pollution, especially to P
pollution, are accommodated in heterogeneous habitats through dominance transference (Fig. 10.1 and Table 10.1), which helps in the restoration of ecosystem
functioning under adverse conditions (Pandey et al. 2017). Using an appropriate
statistical tool, P-loving species can easily be grouped in one cluster (Fig. 10.2).
The N- and Si-limiting condition could shift the diatom assemblages from
P-sensitive to P-tolerant species. The freshwater diatom species have high content
of Si compared to the marine species (Conley et al. 1989). Thus, the relative
availability of dissolved silica (DSi) regulates the proportion of siliceous diatoms
in freshwater bodies (Conley 1997). Centric diatom bloom farmer, Cyclotella, a
highly silicified diatom, is of growing concern in eutrophied rivers (Tavernini et al.
2011) and is reported to enhance the export of biogenic silica and C to the river
sediment (Pandey et al. 2015a, b). The shift in environmental variables such as light
and nutrients changes diatom species composition (Bere and Tundisi 2011). A study
conducted in Varanasi region has shown increasing abundance of Cocconeis,
10 Ecosystem Responses to Pollution in the Ganga River: Key Issues to Address. . .
229
phytoplankton is considered as an indicator of nutrient pollution and eutrophy.
Nutrients influence phytoplankton growth in terms of absolute concentration and
in specific elemental ratios. The specific stoichiometric ratio (Redfield ratio) of N, P,
and Si (16:1:16) is important for the balanced growth of phytoplankton specifically
for diatoms (Turner et al. 2003). Recent studies have reported a shift in this ratio due
to disproportionate input of nutrients in lakes and rivers (Pandey et al. 2014b,
2016b). This shift in stoichiometry would alter the ecosystem structure and functioning in due course of time with long-term irrecoverable consequences (Pandey
et al. 2014b). The shifts in nutrient stoichiometry affect a diverse group of primary
producers, specifically diatoms which are R-strategic (high reproductive potential)
phytoplankton and show rapid change in their growth with changes in aquatic
ecosystems. Besides nutrient stoichiometry, the changes in ionic strength, pH,
light penetration, and temperature also affect the diversity and abundance of diatoms
(Potapova and Charles 2003).
Benthic diatoms are an important group of protists that show niche specialization
against nutrient pollution and light regime shifts. Because this group grows by
attaching to certain substrate, they can be used as a selected group to assess
responses especially in lotic systems. In response to atmospheric deposition of N
in oligotrophic alpine lakes and P enrichment in temperate lakes, species such as
Asterionella and Fragilaria have been reported to constitute the dominant group
(Saros et al. 2005). The Ganga River receives, along with point and nonpoint
sources, huge amount of N and P through atmospheric deposition (Pandey et al.
2013, 2014a, 2016a) suggesting that these inputs will also have strong effect on the
distribution of diatoms. In an earlier study conducted along 268 km stretch from
Allahabad to Varanasi downstream, we found a significant shift in diatom abundance
and diversity in concordance with the changing state of carbon and nutrient pollution
(Pandey et al. 2017). A significant decline in species diversity was observed with
decreasing N/P stoichiometry, and, except for a few species which are adapted to
high nutrient concentrations, the sites with nutrient-rich condition showed less
overall diversity of diatoms. Species adapted to nutrient pollution, especially to P
pollution, are accommodated in heterogeneous habitats through dominance transference (Fig. 10.1 and Table 10.1), which helps in the restoration of ecosystem
functioning under adverse conditions (Pandey et al. 2017). Using an appropriate
statistical tool, P-loving species can easily be grouped in one cluster (Fig. 10.2).
The N- and Si-limiting condition could shift the diatom assemblages from
P-sensitive to P-tolerant species. The freshwater diatom species have high content
of Si compared to the marine species (Conley et al. 1989). Thus, the relative
availability of dissolved silica (DSi) regulates the proportion of siliceous diatoms
in freshwater bodies (Conley 1997). Centric diatom bloom farmer, Cyclotella, a
highly silicified diatom, is of growing concern in eutrophied rivers (Tavernini et al.
2011) and is reported to enhance the export of biogenic silica and C to the river
sediment (Pandey et al. 2015a, b). The shift in environmental variables such as light
and nutrients changes diatom species composition (Bere and Tundisi 2011). A study
conducted in Varanasi region has shown increasing abundance of Cocconeis,
10 Ecosystem Responses to Pollution in the Ganga River: Key Issues to Address. . .
229