225
16 Task of Mineral Nutrients in Eutrophication
ers or by benthic regeneration of organic matter. In estuary
habitats, the dominant primary producers are phytoplankton,
benthic microalgae, epiphytes, sea grasses, and other submerged aquatic vegetation. Primary productivity, in many estuaries, is measured by either
14
CO 2 uptake or O 2 evolution,
determining the photosynthetic rate. Rates of photosynthesis
and estimates of primary production of an estuary provide
databases for assessing long-term trends in trophic state. According to Nixon (1995), the trophic status of an estuary can
be classified as oligotrophic (< 100 g C m
−2
y
−1
), mesotrophic
(100–300 g C m
−2
y
−1
), eutrophic (300–500 g C m
−2
y
−1
), or
hypertrophic (> 500 g C m
−2
y
−1
).
16.1.2 Accelerated Eutrophication
Natural eutrophication takes place very slowly. But, human
activities have accelerated the process of eutrophication in
the past several decades. Huge amount of mineral nutrients
and organic matter are added in water reservoirs in the form
of sewage effluents, organic wastes, agricultural-field runoffs, excreta, and exudates of living beings. These effluents
contain plenty of phosphates, which are used in large quantities in soaps, detergents, and as water softeners. The aquatic
bodies acquire more nutrients from these effluents in a day
than it would do in a year under undisturbed natural conditions. Runoff from agricultural fields carries huge amount
of nitrates and phosphates applied as fertilizers, which promote luxuriant growth of algae and other aquatic plants in
the water bodies (Khan and Ansari 2005; Ansari et al. 2011).
Following the eutrophication, bacteria and blue green algae
fix atmospheric nitrogen, while the phosphates present in
rocks and detritus at the bottom are solubilized by microbial activity. Later, a rich microbial and animal population
also develops. Ultimately, silt and organic debris collects
in the bottom and the water body turns into a shallow and
muddy pond, then to a marsh and finally to a dry land. Thus,
the water body which could have served us as a reservoir of
fresh water, fishes, and other products for hundreds of years
becomes useless within a span of a few years only (Khan and
Ansari 2005; Ansari et al. 2011; Dokulil and Teubner 2011).
16.1.3 Algal Blooms or Water Blooms
In some eutrophic water bodies, dense population of planktonic algae develops, turning the water green within a short
span of time. The phenomenon is called as formation of algal
blooms or water bloom because of the fast growth rate of
algal population. Such waters are useless for human beings
as it is very difficult and costly to remove the microscopic
green plants. The entire mass of planktonic algae may often
die suddenly. It results in large quantities of dead and decaying organic matter, changing the taste and odor of water
unfavorably. So far, the scientists have not been able to solve
the problem of water blooms successfully. Nonetheless, the
problem arises as a consequence of rapid nutrient enrichment
of water body or accelerated eutrophication. Apart from nutrient enrichment of water bodies, water blooms arise because of some other factors also. Water blooms are usually
found associated with certain specific conditions such as:
• Water bodies where there is very little diversity in aquatic
species. Factually, algal blooms are usually composed of
only one or two species.
• High organic matter content of the water is usually associated with water blooms. This usually turns the pH on the
alkaline side.
• Little amount of nitrates or phosphates in the water body
is also associated with formation of algal blooms because
most of the nutrients remain lodged inside the algal cells.
In support, a high concentration of these nutrients has
been recorded prior to the formation of blooms (Landsberg 2000; Sellner et al. 2003; Ansari et al. 2011).
• Hot and humid climate and high light intensity usually
results in the development of algal blooms.
Presence of high concentrations of mineral nutrients to support the huge population of planktonic algae is the primary
requirement for the development of algal blooms. Addition of
nutrients to aquatic bodies show that even if enough nitrogen
and phosphorus is provided to an aquatic system, bloom formation fails to occur because almost all the added nutrients
are quickly absorbed by aquatic biota, specifically the Algae,
which accumulate large quantities of nitrogen and phosphorus in their cells in order to support their multiplication for
future cell generations (Landsberg 2000; Sellner et al. 2003).
Apart from high levels of dissolved organic material in
water in which bloom formation occurs, a number of algae
are known to produce organic secretions (Landsberg 2000;
Anderson et al. 2002; Sellner et al. 2003). The usual abundance of only one or two species in a water bloom may also
be ascribed to the inhibitory action of extracellular products,
which suppress the growth of other aquatic forms. Water
blooms are usually developed in hot and humid condition
with high light intensity. These climatic conditions promote
intense microbial activity, which result in quick decomposition of organic matter leading to abundant quantity of dissolved organic matter in the water body. Dissolved organic
matter, thus produced, releases plenty of nutrients for algal
growth. In tropical and subtropical regions, hot and humid
conditions also results in thermal stratification in aquatic
vegetation. With the sun heat, upper layers of water turn hot,
and being lighter these layers stay above the cooler layers
present beneath them. In the upper layers, oxygen is dissolved from the atmosphere; while, intense sunlight supports
active photosynthesis leading to luxuriant algal growth.
Humidity and the absence of air circulation prevent active evaporation, which prevents cooling of upper water layers. During formation of algal bloom, nutrient limitation is
16 Task of Mineral Nutrients in Eutrophication
ers or by benthic regeneration of organic matter. In estuary
habitats, the dominant primary producers are phytoplankton,
benthic microalgae, epiphytes, sea grasses, and other submerged aquatic vegetation. Primary productivity, in many estuaries, is measured by either
14
CO 2 uptake or O 2 evolution,
determining the photosynthetic rate. Rates of photosynthesis
and estimates of primary production of an estuary provide
databases for assessing long-term trends in trophic state. According to Nixon (1995), the trophic status of an estuary can
be classified as oligotrophic (< 100 g C m
−2
y
−1
), mesotrophic
(100–300 g C m
−2
y
−1
), eutrophic (300–500 g C m
−2
y
−1
), or
hypertrophic (> 500 g C m
−2
y
−1
).
16.1.2 Accelerated Eutrophication
Natural eutrophication takes place very slowly. But, human
activities have accelerated the process of eutrophication in
the past several decades. Huge amount of mineral nutrients
and organic matter are added in water reservoirs in the form
of sewage effluents, organic wastes, agricultural-field runoffs, excreta, and exudates of living beings. These effluents
contain plenty of phosphates, which are used in large quantities in soaps, detergents, and as water softeners. The aquatic
bodies acquire more nutrients from these effluents in a day
than it would do in a year under undisturbed natural conditions. Runoff from agricultural fields carries huge amount
of nitrates and phosphates applied as fertilizers, which promote luxuriant growth of algae and other aquatic plants in
the water bodies (Khan and Ansari 2005; Ansari et al. 2011).
Following the eutrophication, bacteria and blue green algae
fix atmospheric nitrogen, while the phosphates present in
rocks and detritus at the bottom are solubilized by microbial activity. Later, a rich microbial and animal population
also develops. Ultimately, silt and organic debris collects
in the bottom and the water body turns into a shallow and
muddy pond, then to a marsh and finally to a dry land. Thus,
the water body which could have served us as a reservoir of
fresh water, fishes, and other products for hundreds of years
becomes useless within a span of a few years only (Khan and
Ansari 2005; Ansari et al. 2011; Dokulil and Teubner 2011).
16.1.3 Algal Blooms or Water Blooms
In some eutrophic water bodies, dense population of planktonic algae develops, turning the water green within a short
span of time. The phenomenon is called as formation of algal
blooms or water bloom because of the fast growth rate of
algal population. Such waters are useless for human beings
as it is very difficult and costly to remove the microscopic
green plants. The entire mass of planktonic algae may often
die suddenly. It results in large quantities of dead and decaying organic matter, changing the taste and odor of water
unfavorably. So far, the scientists have not been able to solve
the problem of water blooms successfully. Nonetheless, the
problem arises as a consequence of rapid nutrient enrichment
of water body or accelerated eutrophication. Apart from nutrient enrichment of water bodies, water blooms arise because of some other factors also. Water blooms are usually
found associated with certain specific conditions such as:
• Water bodies where there is very little diversity in aquatic
species. Factually, algal blooms are usually composed of
only one or two species.
• High organic matter content of the water is usually associated with water blooms. This usually turns the pH on the
alkaline side.
• Little amount of nitrates or phosphates in the water body
is also associated with formation of algal blooms because
most of the nutrients remain lodged inside the algal cells.
In support, a high concentration of these nutrients has
been recorded prior to the formation of blooms (Landsberg 2000; Sellner et al. 2003; Ansari et al. 2011).
• Hot and humid climate and high light intensity usually
results in the development of algal blooms.
Presence of high concentrations of mineral nutrients to support the huge population of planktonic algae is the primary
requirement for the development of algal blooms. Addition of
nutrients to aquatic bodies show that even if enough nitrogen
and phosphorus is provided to an aquatic system, bloom formation fails to occur because almost all the added nutrients
are quickly absorbed by aquatic biota, specifically the Algae,
which accumulate large quantities of nitrogen and phosphorus in their cells in order to support their multiplication for
future cell generations (Landsberg 2000; Sellner et al. 2003).
Apart from high levels of dissolved organic material in
water in which bloom formation occurs, a number of algae
are known to produce organic secretions (Landsberg 2000;
Anderson et al. 2002; Sellner et al. 2003). The usual abundance of only one or two species in a water bloom may also
be ascribed to the inhibitory action of extracellular products,
which suppress the growth of other aquatic forms. Water
blooms are usually developed in hot and humid condition
with high light intensity. These climatic conditions promote
intense microbial activity, which result in quick decomposition of organic matter leading to abundant quantity of dissolved organic matter in the water body. Dissolved organic
matter, thus produced, releases plenty of nutrients for algal
growth. In tropical and subtropical regions, hot and humid
conditions also results in thermal stratification in aquatic
vegetation. With the sun heat, upper layers of water turn hot,
and being lighter these layers stay above the cooler layers
present beneath them. In the upper layers, oxygen is dissolved from the atmosphere; while, intense sunlight supports
active photosynthesis leading to luxuriant algal growth.
Humidity and the absence of air circulation prevent active evaporation, which prevents cooling of upper water layers. During formation of algal bloom, nutrient limitation is