229
16 Task of Mineral Nutrients in Eutrophication
plant growth and development. Since all plant enzymes are
made of proteins, N is needed for all of the enzymatic reactions in a plant. N is a major part of the chlorophyll molecule
and is, therefore, necessary for photosynthesis. N is a necessary component of several vitamins. It improves the quality
and quantity of dry matter in leafy vegetables and protein in
grain crops (Black 1973; Marschner 2002).
16.4.2 Sources and Functions of Phosphorus
Phosphorus (P) is available to plants as orthophosphate ions
(HPO 4
2– and H 2 PO 4
– ). In photosynthesis and respiration, P
plays the major role in energy storage and transfer in the
form of ADP (adenosine diphosphate), ATP (adenosine triphosphate), DPN (diphosphopyridine nucleotide), and TPN
(triphosphopyridine nucleotide). P is part of the RNA and
DNA structures, which are the major components of genetic
information in the cell. Seeds have the highest concentration
of P in a mature plant; P is required in large quantities in
young cells, such as shoots and root tips, where metabolism
is high and cell division is rapid. Additionally, P aids in root
development, flower initiation, and seed and fruit development. P has also been shown to reduce disease incidence in
some plants and improve the quality of certain crops (Raghothama 1999; Marschner 2002; Taiz and Zeiger 2006).
16.4.3 Role of Nitrogen and Phosphorus in
Eutrophication
The loading of N and P to the rivers, lakes, and oceans is
very strongly influenced by human population densities, the
population densities of livestock, and the land use (Omernik
1977; Reckhow et al. 1980; Jones et al. 1984; Cole et al.
1993; Caraco 1995; Howarth et al. 1995, 1996; Jaworski
et al. 1997; Smith et al. 1997). Of the many mineral resources required for plant growth, inorganic N and P are the two
principal nutrients that have been found to limit the growth
of terrestrial plants (Schlesinger 1991; Vitousek and Howarth
1991; Sharpley et al. 2003). However, this nutrient limitation
of plant biomass is not restricted to terrestrial ecosystems
alone. The supply rate of N and P also strongly influences
the growth of algae and vascular plants in the freshwater and
marine ecosystem (Vollenweider 1968; Hecky and Kilham
1988; Howarth 1988; Smith 1998). Water bodies having
relatively large supplies of nutrients are termed eutrophic
(well nourished), and those having poor nutrient supplies are
termed oligotrophic (poorly nourished). Waters having intermediate nutrient supplies are termed mesotrophic.
The concept of nutrient limitation can be considered the
basis of eutrophication research. It implies that: (1) one key
nutrient should be the primary limiting factor for plant growth
in a given ecosystem, (2) the growth of plants in a given ecosystem should be proportional to the rate of supply of this
key nutrient, and (3) the control of eutrophication should be
accomplished by restricting the loading of this key nutrient
to the ecosystem (Smith 1998). Nitrogen is an element that
can limit algal biomass in natural waters. The forms of nitrogen that are generally available for aquatic plant growth are
nitrate and ammonia. In addition, certain algae and bacteria
in natural waters can use dissolved nitrogen gas as the nitrogen source. Normally, ten times as much nitrogen is required
as phosphorus for growth of algae (on a mass basis). Natural
water bodies typically contain at least this relative quantity
of algal-available nitrogen over phosphorus. During the past
decades, several techniques have been developed to assess
the relative significance of nitrogen versus phosphorus as
growth-limiting elements for algae in natural waters. These
include algal assay procedures and examination of changes
in concentrations of available nutrient forms during algal
blooms. According to these procedures, generally freshwater lakes tend to be phosphorus-limited, while marine waters tend to be nitrogen-limited. There are exceptions to the
generalizations, which are usually associated with gross
pollution of the water body owing to the activities of man,
such as from agricultural drainage and discharge of domestic
wastewaters. Although phosphorus may not be limiting the
algal growth in general because of large phosphorus inputs,
the control of phosphorus inputs can decrease the available
phosphorus concentration in a water body, making the phosphorus limiting in a water body. For most freshwater bodies,
there is sufficient evidence to conclude that phosphorus is
the major limiting element for aquatic plant growth. Nitrogen ranks the second in this regard. Because the influence
of the human activities, excessive nitrogen, phosphorus, and
other nutrients are loaded into water bodies like lake, reservoirs, embouchure and bay, which result in the fast growth
of algae and other planktons that deteriorate the water quality (Western 2001). Although nitrogen and phosphorus exert
primary control over the growth of total algal biomass in
most surface waters, trace elements, vitamins, and organic
growth factors may also control the types of algae present.
The relative roles of nitrogen and phosphorus in the eutrophication process are usually reversed from those typically
found in fresh water.
In conditions of restricted circulation and massive nutrient inputs, more or less classical eutrophication problems do
occur in marine water. In such situation, there may be a large
growth of algae. Eventually, the algae die and subsequently
settle through the thermocline. Decomposition of the dead
algae depletes the oxygen in the bottom area of the water
bodies. The depletion of dissolved oxygen results in largescale destruction of benthic organisms in the area, many of
which may be of great commercial importance, including
lobster, crabs, flounder, and other flat fish. In this case nitro-
16 Task of Mineral Nutrients in Eutrophication
plant growth and development. Since all plant enzymes are
made of proteins, N is needed for all of the enzymatic reactions in a plant. N is a major part of the chlorophyll molecule
and is, therefore, necessary for photosynthesis. N is a necessary component of several vitamins. It improves the quality
and quantity of dry matter in leafy vegetables and protein in
grain crops (Black 1973; Marschner 2002).
16.4.2 Sources and Functions of Phosphorus
Phosphorus (P) is available to plants as orthophosphate ions
(HPO 4
2– and H 2 PO 4
– ). In photosynthesis and respiration, P
plays the major role in energy storage and transfer in the
form of ADP (adenosine diphosphate), ATP (adenosine triphosphate), DPN (diphosphopyridine nucleotide), and TPN
(triphosphopyridine nucleotide). P is part of the RNA and
DNA structures, which are the major components of genetic
information in the cell. Seeds have the highest concentration
of P in a mature plant; P is required in large quantities in
young cells, such as shoots and root tips, where metabolism
is high and cell division is rapid. Additionally, P aids in root
development, flower initiation, and seed and fruit development. P has also been shown to reduce disease incidence in
some plants and improve the quality of certain crops (Raghothama 1999; Marschner 2002; Taiz and Zeiger 2006).
16.4.3 Role of Nitrogen and Phosphorus in
Eutrophication
The loading of N and P to the rivers, lakes, and oceans is
very strongly influenced by human population densities, the
population densities of livestock, and the land use (Omernik
1977; Reckhow et al. 1980; Jones et al. 1984; Cole et al.
1993; Caraco 1995; Howarth et al. 1995, 1996; Jaworski
et al. 1997; Smith et al. 1997). Of the many mineral resources required for plant growth, inorganic N and P are the two
principal nutrients that have been found to limit the growth
of terrestrial plants (Schlesinger 1991; Vitousek and Howarth
1991; Sharpley et al. 2003). However, this nutrient limitation
of plant biomass is not restricted to terrestrial ecosystems
alone. The supply rate of N and P also strongly influences
the growth of algae and vascular plants in the freshwater and
marine ecosystem (Vollenweider 1968; Hecky and Kilham
1988; Howarth 1988; Smith 1998). Water bodies having
relatively large supplies of nutrients are termed eutrophic
(well nourished), and those having poor nutrient supplies are
termed oligotrophic (poorly nourished). Waters having intermediate nutrient supplies are termed mesotrophic.
The concept of nutrient limitation can be considered the
basis of eutrophication research. It implies that: (1) one key
nutrient should be the primary limiting factor for plant growth
in a given ecosystem, (2) the growth of plants in a given ecosystem should be proportional to the rate of supply of this
key nutrient, and (3) the control of eutrophication should be
accomplished by restricting the loading of this key nutrient
to the ecosystem (Smith 1998). Nitrogen is an element that
can limit algal biomass in natural waters. The forms of nitrogen that are generally available for aquatic plant growth are
nitrate and ammonia. In addition, certain algae and bacteria
in natural waters can use dissolved nitrogen gas as the nitrogen source. Normally, ten times as much nitrogen is required
as phosphorus for growth of algae (on a mass basis). Natural
water bodies typically contain at least this relative quantity
of algal-available nitrogen over phosphorus. During the past
decades, several techniques have been developed to assess
the relative significance of nitrogen versus phosphorus as
growth-limiting elements for algae in natural waters. These
include algal assay procedures and examination of changes
in concentrations of available nutrient forms during algal
blooms. According to these procedures, generally freshwater lakes tend to be phosphorus-limited, while marine waters tend to be nitrogen-limited. There are exceptions to the
generalizations, which are usually associated with gross
pollution of the water body owing to the activities of man,
such as from agricultural drainage and discharge of domestic
wastewaters. Although phosphorus may not be limiting the
algal growth in general because of large phosphorus inputs,
the control of phosphorus inputs can decrease the available
phosphorus concentration in a water body, making the phosphorus limiting in a water body. For most freshwater bodies,
there is sufficient evidence to conclude that phosphorus is
the major limiting element for aquatic plant growth. Nitrogen ranks the second in this regard. Because the influence
of the human activities, excessive nitrogen, phosphorus, and
other nutrients are loaded into water bodies like lake, reservoirs, embouchure and bay, which result in the fast growth
of algae and other planktons that deteriorate the water quality (Western 2001). Although nitrogen and phosphorus exert
primary control over the growth of total algal biomass in
most surface waters, trace elements, vitamins, and organic
growth factors may also control the types of algae present.
The relative roles of nitrogen and phosphorus in the eutrophication process are usually reversed from those typically
found in fresh water.
In conditions of restricted circulation and massive nutrient inputs, more or less classical eutrophication problems do
occur in marine water. In such situation, there may be a large
growth of algae. Eventually, the algae die and subsequently
settle through the thermocline. Decomposition of the dead
algae depletes the oxygen in the bottom area of the water
bodies. The depletion of dissolved oxygen results in largescale destruction of benthic organisms in the area, many of
which may be of great commercial importance, including
lobster, crabs, flounder, and other flat fish. In this case nitro-