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M. Naeem et al.
possibly avoided owing to already present plenty of nutrients in the algal cells accumulated earlier. Besides, intense
microbial activity also releases a number of nutrients in a
water body rich in organic matter. Hence, dense populations
of algae appear in the upper layers. As the day advances,
upper water layers become saturated with oxygen as a result of algal photosynthesis. The presence of large population of planktons in upper water layers prevents light to
go to deeper areas. Lower layers, thus, suffer from oxygen
deficit that stimulates anaerobic activity of the aquatic organisms. The anaerobic systems may produce a number of
organic substances, which are intermediate metabolites of
the algal metabolism, thus further facilitating the luxuriant
growth of algae. In the evening as sunlight weakens, upper
water layers get cooled. Thus, oxygen-saturated water, having algal planktons, sinks down leading to mixing of water
layers at deeper areas. The added oxygen increases the microbial activity at deeper areas resulting into mineralization
of the organic matter that was partially decomposed during
the day. Thus, plenty of nutrients are present in the lower layers, which are partly derived from the bottom mud and rocks
and partly from decomposition of organic matter. Algal cells
rapidly take up and accumulate these nutrients. Experimental evidences suggests that a number of organic substances
released by microbial systems are intermediate products of
plant metabolism, which are taken up by algal cells as such
and incorporated in their metabolism (Landsberg 2000; Sellner et al. 2003). As sun comes up the following day intense
light promotes active photosynthesis and planktons rise to the
upper layers, which get again warmed up. The whole process
is repeated day by day and, thus, the algal bloom acquires a
permanent nature (Landsberg 2000; Sellner et al. 2003).
Eutrophication progresses following the six stages:
↓
LUXURIANT GROWTH OF ALGAE
↓
SUDDEN DEATH OF ALGAE
(Due to certain factors)
↓
FLOURISHING GROWTH OF BACTERIA
(O 2 consumption)
↓
LACK OF OXYGEN
↓
SUFFOCATION
(Death of aquatic organisms)
ADDITION OF MINERAL NUTRIENTS
(Specifically nitrates and phosphates)
16.2 Nutrient Loading During Eutrophication
Nitrogen and phosphorus are two major nutrients responsible for eutrophication. Nitrogen has a more complex pathway than phosphorus. Nitrogen can enter and leave the water
body in the form of free nitrogen gas through atmospheric
exchange. Carbon has been shown to diffuse into the water
column at rates sufficient to meet the needs of photosynthesizing cells (Molot and Dillon 1997). Phosphorus, on the
other hand, may enter the water body through the inflows,
precipitation, and dry fallout and from sediments; it may be
removed by sedimentation and through the outflow. The different pathways of nitrogen, phosphorus, and carbon cycles
make phosphorus the obvious choice for eutrophication control in the lake. A certain reduction of phosphorus input will
generally result in a greater reduction in algal biomass compared with the same reduction of nitrogen. Furthermore, the
reduction of nitrogen input without a proportional reduction
in phosphorus creates low N/P ratio, which favors nitrogenfixing nuisance algae, without any reduction in algal biomass (Babin et al. 1989). Hence, phosphorus content is considered the key variable for eutrophication control in a lake.
Total phosphorus content includes crystalline, occluded,
absorbed, particulate organic, soluble organic, and soluble
inorganic phosphorus. In terms of decreasing availability,
there are three biologically available phosphorus fractions of
phosphorus, viz. soluble reactive phosphorus (a mixture of
dissolved inorganic and organic phosphorus), soluble unreactive phosphorus (It includes dissolved phosphorus fed by
persulfate oxidation, which is available for phytoplanktons
by enzymatic hydralization that frees organically bound fractions) and labile phosphorus (associated with soil particles).
Biologically available phosphorus describes a mixture of
phosphorus fractions of different availability. Vollenweider
(1979) described the following phosphorus sources in terms
of decreasing biological availability, which should be considered important in nutrient control measures. These are
urban sewage plus certain industrial effluents greater than
erosional runoff and leaching from forests and agricultural
areas.
16.2.1 Internal Loading of Phosphorus
In suitable conditions at the water sediment interface, nutrients contained in the sediments are released into the water
body. In a eutrophic lake, net oxygen consumption occurs in
the tropholytic zone below the compensation depth. As alternate terminal electron acceptors are consumed, the redox
potential decreases that tends to decrease with greater depth
in the water column and in sediments. Once the redox potential of the ferric-ferrous couple is reached (approximately
M. Naeem et al.
possibly avoided owing to already present plenty of nutrients in the algal cells accumulated earlier. Besides, intense
microbial activity also releases a number of nutrients in a
water body rich in organic matter. Hence, dense populations
of algae appear in the upper layers. As the day advances,
upper water layers become saturated with oxygen as a result of algal photosynthesis. The presence of large population of planktons in upper water layers prevents light to
go to deeper areas. Lower layers, thus, suffer from oxygen
deficit that stimulates anaerobic activity of the aquatic organisms. The anaerobic systems may produce a number of
organic substances, which are intermediate metabolites of
the algal metabolism, thus further facilitating the luxuriant
growth of algae. In the evening as sunlight weakens, upper
water layers get cooled. Thus, oxygen-saturated water, having algal planktons, sinks down leading to mixing of water
layers at deeper areas. The added oxygen increases the microbial activity at deeper areas resulting into mineralization
of the organic matter that was partially decomposed during
the day. Thus, plenty of nutrients are present in the lower layers, which are partly derived from the bottom mud and rocks
and partly from decomposition of organic matter. Algal cells
rapidly take up and accumulate these nutrients. Experimental evidences suggests that a number of organic substances
released by microbial systems are intermediate products of
plant metabolism, which are taken up by algal cells as such
and incorporated in their metabolism (Landsberg 2000; Sellner et al. 2003). As sun comes up the following day intense
light promotes active photosynthesis and planktons rise to the
upper layers, which get again warmed up. The whole process
is repeated day by day and, thus, the algal bloom acquires a
permanent nature (Landsberg 2000; Sellner et al. 2003).
Eutrophication progresses following the six stages:
↓
LUXURIANT GROWTH OF ALGAE
↓
SUDDEN DEATH OF ALGAE
(Due to certain factors)
↓
FLOURISHING GROWTH OF BACTERIA
(O 2 consumption)
↓
LACK OF OXYGEN
↓
SUFFOCATION
(Death of aquatic organisms)
ADDITION OF MINERAL NUTRIENTS
(Specifically nitrates and phosphates)
16.2 Nutrient Loading During Eutrophication
Nitrogen and phosphorus are two major nutrients responsible for eutrophication. Nitrogen has a more complex pathway than phosphorus. Nitrogen can enter and leave the water
body in the form of free nitrogen gas through atmospheric
exchange. Carbon has been shown to diffuse into the water
column at rates sufficient to meet the needs of photosynthesizing cells (Molot and Dillon 1997). Phosphorus, on the
other hand, may enter the water body through the inflows,
precipitation, and dry fallout and from sediments; it may be
removed by sedimentation and through the outflow. The different pathways of nitrogen, phosphorus, and carbon cycles
make phosphorus the obvious choice for eutrophication control in the lake. A certain reduction of phosphorus input will
generally result in a greater reduction in algal biomass compared with the same reduction of nitrogen. Furthermore, the
reduction of nitrogen input without a proportional reduction
in phosphorus creates low N/P ratio, which favors nitrogenfixing nuisance algae, without any reduction in algal biomass (Babin et al. 1989). Hence, phosphorus content is considered the key variable for eutrophication control in a lake.
Total phosphorus content includes crystalline, occluded,
absorbed, particulate organic, soluble organic, and soluble
inorganic phosphorus. In terms of decreasing availability,
there are three biologically available phosphorus fractions of
phosphorus, viz. soluble reactive phosphorus (a mixture of
dissolved inorganic and organic phosphorus), soluble unreactive phosphorus (It includes dissolved phosphorus fed by
persulfate oxidation, which is available for phytoplanktons
by enzymatic hydralization that frees organically bound fractions) and labile phosphorus (associated with soil particles).
Biologically available phosphorus describes a mixture of
phosphorus fractions of different availability. Vollenweider
(1979) described the following phosphorus sources in terms
of decreasing biological availability, which should be considered important in nutrient control measures. These are
urban sewage plus certain industrial effluents greater than
erosional runoff and leaching from forests and agricultural
areas.
16.2.1 Internal Loading of Phosphorus
In suitable conditions at the water sediment interface, nutrients contained in the sediments are released into the water
body. In a eutrophic lake, net oxygen consumption occurs in
the tropholytic zone below the compensation depth. As alternate terminal electron acceptors are consumed, the redox
potential decreases that tends to decrease with greater depth
in the water column and in sediments. Once the redox potential of the ferric-ferrous couple is reached (approximately