EXERCISE 7
Inorganic Nutrients: Nitrogen/
Phosphorus/ and Other Nutrients
Compounds of nitrogen, and especially those of
phosphorus, are major cellular components of
organisms. Since the availability of these elements may be less than the biological demand,
environmental sources can regulate or limit the
productivity of organisms in freshwater ecosystems. Other elements such as iron and sulfur are
essential cellular constituents but are required in
relatively low concentrations in relation to
availability in fresh waters. The major basic
cations, calcium, magnesium, sodium, and potassium, usually are required in very low quantities,
but their concentrations in fresh water can
influence the osmoregulation of organisms.
It should be remembered that chemical concentration in water is a static variable, that is, the
chemical mass (weight) of an element or compound per unit volume at a particular place or
time within the lake. Certain nutrients, such as
magnesium and sodium, are relatively conservative in concentration in that their solubility is
high, they usually are abundant in relation to
metabolic demands, and their concentrations are
relatively unaffected by metabolically altered
reduction-oxidation conditions of the water.
Concentrations of nitrogen and phosphorus
compounds, on the other hand, are highly dynamic because they may be utilized, stored,
transformed, and excreted rapidly and repeatedly
by the various aquatic organisms.
Measurements are further complicated by the
chemical form in which the element occurs. Ionic
concentrations are often exceedingly low, requir81
ing great care in collection and analysis of water
samples to avoid contamination. In general, all
equipment and glassware should be cleaned
thoroughly with acid, such as 6N HCI, and only
high-quality glass-distilled water and analytical
grade reagents should be used.
Storage of water samples before analysis
should be avoided. If storage is necessary, the
time must be minimized if reliable results are to
be obtained. Prompt freezing of water samples or
the addition of certain preservatives may be
satisfactory for certain analyses but not for
others. Storage methods will be discussed specifically in the following procedures.
It is often meaningful to the biologist, from the
standpoint of availability, to separate analyses of
total nutrient concentrations into those fractions
that are dissolved in inorganic and organic states
from those that occur in particulate form. This
separation is done generally by filtration through
glass fiber filters, taking great care to make
certain that contamination is not introduced
from the filters and filtration apparatus. Separate
chemical analyses for the inorganic and the
organic components then are performed on each
of these fractions.
The following discussion of chemical analyses
assumes that samples were obtained with suitable sampling apparatus. By far the most commonly used device is a nonmetallic Van Dorn
sampler (Fig. 7.1). This device (Van Dorn, 1956)
has the important advantages of being completely nonmetallic at points of contact with the
Inorganic Nutrients: Nitrogen/
Phosphorus/ and Other Nutrients
Compounds of nitrogen, and especially those of
phosphorus, are major cellular components of
organisms. Since the availability of these elements may be less than the biological demand,
environmental sources can regulate or limit the
productivity of organisms in freshwater ecosystems. Other elements such as iron and sulfur are
essential cellular constituents but are required in
relatively low concentrations in relation to
availability in fresh waters. The major basic
cations, calcium, magnesium, sodium, and potassium, usually are required in very low quantities,
but their concentrations in fresh water can
influence the osmoregulation of organisms.
It should be remembered that chemical concentration in water is a static variable, that is, the
chemical mass (weight) of an element or compound per unit volume at a particular place or
time within the lake. Certain nutrients, such as
magnesium and sodium, are relatively conservative in concentration in that their solubility is
high, they usually are abundant in relation to
metabolic demands, and their concentrations are
relatively unaffected by metabolically altered
reduction-oxidation conditions of the water.
Concentrations of nitrogen and phosphorus
compounds, on the other hand, are highly dynamic because they may be utilized, stored,
transformed, and excreted rapidly and repeatedly
by the various aquatic organisms.
Measurements are further complicated by the
chemical form in which the element occurs. Ionic
concentrations are often exceedingly low, requir81
ing great care in collection and analysis of water
samples to avoid contamination. In general, all
equipment and glassware should be cleaned
thoroughly with acid, such as 6N HCI, and only
high-quality glass-distilled water and analytical
grade reagents should be used.
Storage of water samples before analysis
should be avoided. If storage is necessary, the
time must be minimized if reliable results are to
be obtained. Prompt freezing of water samples or
the addition of certain preservatives may be
satisfactory for certain analyses but not for
others. Storage methods will be discussed specifically in the following procedures.
It is often meaningful to the biologist, from the
standpoint of availability, to separate analyses of
total nutrient concentrations into those fractions
that are dissolved in inorganic and organic states
from those that occur in particulate form. This
separation is done generally by filtration through
glass fiber filters, taking great care to make
certain that contamination is not introduced
from the filters and filtration apparatus. Separate
chemical analyses for the inorganic and the
organic components then are performed on each
of these fractions.
The following discussion of chemical analyses
assumes that samples were obtained with suitable sampling apparatus. By far the most commonly used device is a nonmetallic Van Dorn
sampler (Fig. 7.1). This device (Van Dorn, 1956)
has the important advantages of being completely nonmetallic at points of contact with the