Similarly, the Si/N ratio below 1 leads to reducing the proportion of diatoms/
siliceous algae in the phytoplankton assemblage and consequently causes a shift in
the higher trophic levels (Gilpin et al. 2004). Further, the Si-limiting condition leads
to enhance non-diatom algal growth (Pandey et al. 2017). The excessively higher
concentration of N and P compared to Si is causing a dramatic shift in the phytoplankton composition, changing the pattern of community dominance toward green
or blue-green algae (Teubner and Dokulil 2002) including those in the Ganga River
as indicated also by high concentration of phycocyanin at nutrient-rich sites (Pandey
et al. 2016b). Further, as the system moves toward eutrophy, feedbacks at sedimentwater interface may increase P supply and consequently promote the growth of
P-favored harmful algal species (Pandey et al. 2017). Thus, the changing pattern of
nutrient limitation and the resulting competition for resources in phytoplankton
would decrease the proportion of less adapted algal species, increasing the share of
non-siliceous diatoms in the community and consequently decreasing the C sequestration and compromising the ecological assimilation capacity of the river.
The atmospheric deposition (AD) of N and P has increased tremendously in
various parts of the world (Galloway et al. 2008) and is continuing to rise in the
Ganges basin (Siddiqui et al. 2019b). Since N and P are the major component of AD,
a potential shift in AD-N/P ratio will alter N/P ratios of surface waters which would
shift phytoplankton composition. Many of the temperate European and North
American lakes have been reported to be suffering with this problem (Bergstrom
and Jansson 2006). As the Ganga River receives large but disproportionate input of
nutrients through point and nonpoint sources including atmospheric deposition, at
many locations, the river experiences shifts in N/P/Si stoichiometry and the proportion of specific nutrient availability (Pandey et al. 2016b). A watershed-scale study
from Devprayag to Ganga Sagar (Pandey et al. 2016a) has reported that at polluted
sites, the ratio of N/P remained below 16:1 indicating that P is no more a limiting
nutrient in the river and concordantly the abundance of dominant diatom genera has
also changed. Diatom species such as Diatoma vulgaris, Fragilaria intermedia, and
Gomphonema parvulum were found abundantly at sites characterized by high P,
whereas species such as Cocconeis placentula, Cyclotella meneghiniana, and
Cymbella affinis were found at sites with low phosphorus concentration (Pandey
et al. 2017). The results further revealed a relatively lower proportion of Si
(Si/P < 16:1) in the Ganga River than the required ratio to meet cellular Si for
diatoms. A higher N/Si ratio shows that if this condition is continued, it will lead to
Si limitation in the long-term future (Pandey et al. 2016a).
A study of Mississippi River has shown that excessive N and P input as compared
to Si is causing severe eutrophication in the river (Turner et al. 2003). Shifts in
stoichiometric ratios affect the quantity as well as quality of primary production.
With increasing N input, an increase in the cellular N/P ratio of terrestrial and aquatic
plants has been reported (Elser et al. 2009). This change in cellular N/P ratio affects
various metabolic processes ultimately leading to a cascade of effects ranging from
shift in growth of individual organism to alteration in species composition and
community functioning (Pandey et al. 2017). A classic example of this type of
ecosystem response is the shift in population of Daphnia (a freshwater zooplankton).
10 Ecosystem Responses to Pollution in the Ganga River: Key Issues to Address. . .
239
siliceous algae in the phytoplankton assemblage and consequently causes a shift in
the higher trophic levels (Gilpin et al. 2004). Further, the Si-limiting condition leads
to enhance non-diatom algal growth (Pandey et al. 2017). The excessively higher
concentration of N and P compared to Si is causing a dramatic shift in the phytoplankton composition, changing the pattern of community dominance toward green
or blue-green algae (Teubner and Dokulil 2002) including those in the Ganga River
as indicated also by high concentration of phycocyanin at nutrient-rich sites (Pandey
et al. 2016b). Further, as the system moves toward eutrophy, feedbacks at sedimentwater interface may increase P supply and consequently promote the growth of
P-favored harmful algal species (Pandey et al. 2017). Thus, the changing pattern of
nutrient limitation and the resulting competition for resources in phytoplankton
would decrease the proportion of less adapted algal species, increasing the share of
non-siliceous diatoms in the community and consequently decreasing the C sequestration and compromising the ecological assimilation capacity of the river.
The atmospheric deposition (AD) of N and P has increased tremendously in
various parts of the world (Galloway et al. 2008) and is continuing to rise in the
Ganges basin (Siddiqui et al. 2019b). Since N and P are the major component of AD,
a potential shift in AD-N/P ratio will alter N/P ratios of surface waters which would
shift phytoplankton composition. Many of the temperate European and North
American lakes have been reported to be suffering with this problem (Bergstrom
and Jansson 2006). As the Ganga River receives large but disproportionate input of
nutrients through point and nonpoint sources including atmospheric deposition, at
many locations, the river experiences shifts in N/P/Si stoichiometry and the proportion of specific nutrient availability (Pandey et al. 2016b). A watershed-scale study
from Devprayag to Ganga Sagar (Pandey et al. 2016a) has reported that at polluted
sites, the ratio of N/P remained below 16:1 indicating that P is no more a limiting
nutrient in the river and concordantly the abundance of dominant diatom genera has
also changed. Diatom species such as Diatoma vulgaris, Fragilaria intermedia, and
Gomphonema parvulum were found abundantly at sites characterized by high P,
whereas species such as Cocconeis placentula, Cyclotella meneghiniana, and
Cymbella affinis were found at sites with low phosphorus concentration (Pandey
et al. 2017). The results further revealed a relatively lower proportion of Si
(Si/P < 16:1) in the Ganga River than the required ratio to meet cellular Si for
diatoms. A higher N/Si ratio shows that if this condition is continued, it will lead to
Si limitation in the long-term future (Pandey et al. 2016a).
A study of Mississippi River has shown that excessive N and P input as compared
to Si is causing severe eutrophication in the river (Turner et al. 2003). Shifts in
stoichiometric ratios affect the quantity as well as quality of primary production.
With increasing N input, an increase in the cellular N/P ratio of terrestrial and aquatic
plants has been reported (Elser et al. 2009). This change in cellular N/P ratio affects
various metabolic processes ultimately leading to a cascade of effects ranging from
shift in growth of individual organism to alteration in species composition and
community functioning (Pandey et al. 2017). A classic example of this type of
ecosystem response is the shift in population of Daphnia (a freshwater zooplankton).
10 Ecosystem Responses to Pollution in the Ganga River: Key Issues to Address. . .
239