227
16 Task of Mineral Nutrients in Eutrophication
at 120 mv) (Kortmann and Rich 1994), both soluble ferrous
iron and soluble phosphate accumulate. If the redox potential
continues to decrease, sulfate is reduced to sulfide at ~ 75
mv) (Kortmann and Rich 1994), which can remove iron and
permanently reduce phosphate binding capacity by interacting readily with ferrous iron to produce ferrous sulfide
(FeS). If FeS precipitates to form pyrite (FeS 2 ), ferrous iron
is no longer susceptible to oxidation to ferric iron with the
return of aerobic conditions. The relationships among sulfur,
iron, and phosphorus binding capacity raises questions about
potential impacts from increased sulfate loading by algicide
applications (copper sulfate), alum (aluminum sulfate) treatments, and acid rain (sulfuric acid). Holdren and Armstrong
(1980) have quoted scientific values of sediment phosphorus
release rates from several lakes in the USA regarding aerobic
(0–13 mg P/m
2
/day) and anaerobic (0–50 [max. 150] mg
P/m
2
/day) conditions.
16.2.2 Phosphorus Mobilization and Transport
Two different mechanisms are expected to occur simultaneously or within a short time regarding phosphorus mobilization: (1) phosphorus bound to particles or aggregates of
particles in the aquatic sediment must be transferred to the
pool of dissolved phosphorus in the water and (2) processes
which transport the dissolved phosphorus to the lake water
must be in order. Important processes regarding phosphorus
mobilization are desorption, dissolution, ligand exchange,
and enzymatic hydrolysis. These processes are affected by
a number of environmental factors, of which redox potential, pH and temperature are the most important ones. Essential phosphorus transport mechanisms are diffusion,
wind-induced turbulence, bioturbation, and gas convection. Dissolution and diffusion of phosphorus, controlled by
Redox potential, are considered as the dominant mechanisms
for phosphorus release from stagnant hypolimnetic bottom
areas of water bodies. All the mobilization and transport processes can theoretically contribute to the overall phosphorus
release from sediments in shallow lakes. At high temperatures, microanaerobic zones are formed very rapidly, and
redox potential controlled liberation of phosphate can occur
to well-aerated water. Wind-induced turbulences often have
a dominating role among the phosphorus transport processes
(Nurnberg 1994).
16.2.3 Nitrogen Transformations
Transformations between various nitrogen compounds
in the nitrogen cycle of aquatic ecosystems are important
for potential management of lakes. Most phytoplanktons,
which create nuisance bloom conditions are capable of nitrogen fixation and are not dependant on combined forms
of dissolved nitrogen. Of the combined forms of nitrogen,
the most important ones are ammonia and nitrate. Nitrogen
fixation, occurring in blue green algae, is inhibited by high
content of cellular ammonia that results from decomposition of organic matter. Ultimate sources of ammonia include
nitrogen fixation and nitrate assimilation in the aquatic and
paralimnetic ecosystem components. Under aerobic conditions, ammonia is first oxidized to nitrite and then to nitrate
in a two-step process called nitrification. Under anaerobic
conditions, conversion of ammonia to nitrate does not occur
and ammonia often accumulates at the bottom of the lakes.
Total oxygen demand includes respiratory demand and nonrespiratory demand (e.g., chemosynthesis). Nitrate is the
first alternate terminal electron acceptor used in anaerobic
respiration when oxygen is exhausted. As long as nitrate
remains available, the redox potential remains above the
threshold value that is required for iron reduction and subsequent release of sediment-phosphorus from ferric hydroxyphosphate complexes present in the sediments. Enhancing
nitrification of ammonia to nitrate, and subsequent use of
nitrate in denitrification can stabilize the redox potential and
reduce internal phosphorus loading.
16.3 Source of Mineral Nutrients in
Eutrophication
There are many sources of nutrients causing eutrophication
of lakes and other water reservoirs. Water quality of these
water bodies is affected directly or indirectly by the anthropogenic activities undergoing in the entire drainage area of
the water body. A lake or water reservoir may also be naturally eutrophied owing to nutrient-rich runoff from the agricultural land situated in a fertile area with nutrient-enriched
soils. In many lakes and reservoirs, wastewater is the main
source of nutrients, as untreated wastewater or the wastewater treated only by conventional mechanical/biological
methods still contains nitrogen (25−40 mg L
−1
) and phosphorus (6−10 mg L
−1
). Both nitrogen and phosphorus can
be removed from water reservoirs using well-known techniques. Phosphorus may be removed by addition of a chemical that precipitates phosphate through a chemical reaction.
Nitrogen is usually removed by biological means employing the microbial activities. Removal of nitrogen costs more
money and is also technically more difficult compared to
phosphorus. Drainage water from agricultural land also
contains phosphorus and nitrogen. It usually contains much
more nitrogen than phosphorus because phosphorus is usually bound to soil particles. Extensive use of fertilizers results in significant concentrations of nutrients particularly
that of nitrogen, in agricultural runoff. If eroded soil reaches
the lake, both phosphorus and nitrogen present in the soil
16 Task of Mineral Nutrients in Eutrophication
at 120 mv) (Kortmann and Rich 1994), both soluble ferrous
iron and soluble phosphate accumulate. If the redox potential
continues to decrease, sulfate is reduced to sulfide at ~ 75
mv) (Kortmann and Rich 1994), which can remove iron and
permanently reduce phosphate binding capacity by interacting readily with ferrous iron to produce ferrous sulfide
(FeS). If FeS precipitates to form pyrite (FeS 2 ), ferrous iron
is no longer susceptible to oxidation to ferric iron with the
return of aerobic conditions. The relationships among sulfur,
iron, and phosphorus binding capacity raises questions about
potential impacts from increased sulfate loading by algicide
applications (copper sulfate), alum (aluminum sulfate) treatments, and acid rain (sulfuric acid). Holdren and Armstrong
(1980) have quoted scientific values of sediment phosphorus
release rates from several lakes in the USA regarding aerobic
(0–13 mg P/m
2
/day) and anaerobic (0–50 [max. 150] mg
P/m
2
/day) conditions.
16.2.2 Phosphorus Mobilization and Transport
Two different mechanisms are expected to occur simultaneously or within a short time regarding phosphorus mobilization: (1) phosphorus bound to particles or aggregates of
particles in the aquatic sediment must be transferred to the
pool of dissolved phosphorus in the water and (2) processes
which transport the dissolved phosphorus to the lake water
must be in order. Important processes regarding phosphorus
mobilization are desorption, dissolution, ligand exchange,
and enzymatic hydrolysis. These processes are affected by
a number of environmental factors, of which redox potential, pH and temperature are the most important ones. Essential phosphorus transport mechanisms are diffusion,
wind-induced turbulence, bioturbation, and gas convection. Dissolution and diffusion of phosphorus, controlled by
Redox potential, are considered as the dominant mechanisms
for phosphorus release from stagnant hypolimnetic bottom
areas of water bodies. All the mobilization and transport processes can theoretically contribute to the overall phosphorus
release from sediments in shallow lakes. At high temperatures, microanaerobic zones are formed very rapidly, and
redox potential controlled liberation of phosphate can occur
to well-aerated water. Wind-induced turbulences often have
a dominating role among the phosphorus transport processes
(Nurnberg 1994).
16.2.3 Nitrogen Transformations
Transformations between various nitrogen compounds
in the nitrogen cycle of aquatic ecosystems are important
for potential management of lakes. Most phytoplanktons,
which create nuisance bloom conditions are capable of nitrogen fixation and are not dependant on combined forms
of dissolved nitrogen. Of the combined forms of nitrogen,
the most important ones are ammonia and nitrate. Nitrogen
fixation, occurring in blue green algae, is inhibited by high
content of cellular ammonia that results from decomposition of organic matter. Ultimate sources of ammonia include
nitrogen fixation and nitrate assimilation in the aquatic and
paralimnetic ecosystem components. Under aerobic conditions, ammonia is first oxidized to nitrite and then to nitrate
in a two-step process called nitrification. Under anaerobic
conditions, conversion of ammonia to nitrate does not occur
and ammonia often accumulates at the bottom of the lakes.
Total oxygen demand includes respiratory demand and nonrespiratory demand (e.g., chemosynthesis). Nitrate is the
first alternate terminal electron acceptor used in anaerobic
respiration when oxygen is exhausted. As long as nitrate
remains available, the redox potential remains above the
threshold value that is required for iron reduction and subsequent release of sediment-phosphorus from ferric hydroxyphosphate complexes present in the sediments. Enhancing
nitrification of ammonia to nitrate, and subsequent use of
nitrate in denitrification can stabilize the redox potential and
reduce internal phosphorus loading.
16.3 Source of Mineral Nutrients in
Eutrophication
There are many sources of nutrients causing eutrophication
of lakes and other water reservoirs. Water quality of these
water bodies is affected directly or indirectly by the anthropogenic activities undergoing in the entire drainage area of
the water body. A lake or water reservoir may also be naturally eutrophied owing to nutrient-rich runoff from the agricultural land situated in a fertile area with nutrient-enriched
soils. In many lakes and reservoirs, wastewater is the main
source of nutrients, as untreated wastewater or the wastewater treated only by conventional mechanical/biological
methods still contains nitrogen (25−40 mg L
−1
) and phosphorus (6−10 mg L
−1
). Both nitrogen and phosphorus can
be removed from water reservoirs using well-known techniques. Phosphorus may be removed by addition of a chemical that precipitates phosphate through a chemical reaction.
Nitrogen is usually removed by biological means employing the microbial activities. Removal of nitrogen costs more
money and is also technically more difficult compared to
phosphorus. Drainage water from agricultural land also
contains phosphorus and nitrogen. It usually contains much
more nitrogen than phosphorus because phosphorus is usually bound to soil particles. Extensive use of fertilizers results in significant concentrations of nutrients particularly
that of nitrogen, in agricultural runoff. If eroded soil reaches
the lake, both phosphorus and nitrogen present in the soil