small fish, which may be consumed by large fish, which may be consumed by larger
vertebrates, including humans. But it is the small algae that are the base of this food
chain.
All biological systems require the presence of the proper balance of nutrients in
order to grow and reproduce. For the larger organisms, the smaller organisms
provide these nutrients. However, the algae, as the base of the food chain, must
gain their nutrients in the inorganic form from the surrounding water. Organisms that
rely on inorganic matter for growth are termed autotrophic, whereas those that
require organic matter as a source of food are called heterotrophic. Growth is a
function of the nutrients available (plus other limiting factors, such as temperature,
light, and so on), but these nutrients may vary both with location and time within a
lake. Specific organisms may vary in their specific nutrient requirements; however,
in general these organisms have a certain demand for the essential elements. Carbon
may be obtained from the solution of carbon dioxide. Hydrogen may be formed by
hydrolysis of water, or from bicarbonates dissolved in the water. Oxygen may be
produced in photosynthesis or may be secured from dissolved atmospheric oxygen.
Nitrogen is secured from dissolved nitrogenous materials including both ammonia
and nitrates. Phosphorus is usually derived from soil and animal excreta. Sulfur is
usually derived from the soil, but is also present in animal excreta. The needed ratio
of each individual material varies, but there is a general demand for approximately
60 parts of carbon to 15 parts of nitrogen to 1 part of phosphorus in most cellular
material. In addition to these stated growth factors, there may be many other trace
elements that are required in order to support adequate growth. Most frequently, the
requirements for these elements are so low that there is an adequate amount
available. Certain specific organisms may have special requirements. A typical
example of this is the diatoms, which require the presence of silicon in order to
manufacture their cell case, called a frustule.
Normally organisms such as algae will grow until one of the nutrients becomes
limiting. Then growth may be retarded or eliminated entirely. The limits are in the
ratio of the requirements; therefore, the limit is different for each element. This is
referred to as Liebig’s law of the minimum, which states that biological systems will
grow until they are limited by the nutrient that is present in the limiting concentration. In lakes the most common limiting nutrients are nitrogen and phosphorus;
however, in a few instances carbon has also been shown to be limiting. Most
frequently, phosphorus is the limiting constituent; however, this is not exclusively
so and there are many lakes in which nitrogen is the limiting factor.
Productivity is a measure of the utilization of inorganic material to produce plant
growth. Very frequently, productivity in lakes is measured by the amount of fish
available in the lake. Since the number of fish is indirectly related to the growth of
the other organisms lower in the food chain and, ultimately, to the amount of algal
growth, it may be seen that the productivity of fish is merely an indirect measure of
the amount of algae growth in the first place. Whether or not productivity is desirable
is a function of individual taste. A lake that is low in productivity will be clear and
conversely will have a small fish population. On the other hand, a productive lake
will be turbid because of the high concentration of plankton, but this will support a
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D. B. Aulenbach et al.
vertebrates, including humans. But it is the small algae that are the base of this food
chain.
All biological systems require the presence of the proper balance of nutrients in
order to grow and reproduce. For the larger organisms, the smaller organisms
provide these nutrients. However, the algae, as the base of the food chain, must
gain their nutrients in the inorganic form from the surrounding water. Organisms that
rely on inorganic matter for growth are termed autotrophic, whereas those that
require organic matter as a source of food are called heterotrophic. Growth is a
function of the nutrients available (plus other limiting factors, such as temperature,
light, and so on), but these nutrients may vary both with location and time within a
lake. Specific organisms may vary in their specific nutrient requirements; however,
in general these organisms have a certain demand for the essential elements. Carbon
may be obtained from the solution of carbon dioxide. Hydrogen may be formed by
hydrolysis of water, or from bicarbonates dissolved in the water. Oxygen may be
produced in photosynthesis or may be secured from dissolved atmospheric oxygen.
Nitrogen is secured from dissolved nitrogenous materials including both ammonia
and nitrates. Phosphorus is usually derived from soil and animal excreta. Sulfur is
usually derived from the soil, but is also present in animal excreta. The needed ratio
of each individual material varies, but there is a general demand for approximately
60 parts of carbon to 15 parts of nitrogen to 1 part of phosphorus in most cellular
material. In addition to these stated growth factors, there may be many other trace
elements that are required in order to support adequate growth. Most frequently, the
requirements for these elements are so low that there is an adequate amount
available. Certain specific organisms may have special requirements. A typical
example of this is the diatoms, which require the presence of silicon in order to
manufacture their cell case, called a frustule.
Normally organisms such as algae will grow until one of the nutrients becomes
limiting. Then growth may be retarded or eliminated entirely. The limits are in the
ratio of the requirements; therefore, the limit is different for each element. This is
referred to as Liebig’s law of the minimum, which states that biological systems will
grow until they are limited by the nutrient that is present in the limiting concentration. In lakes the most common limiting nutrients are nitrogen and phosphorus;
however, in a few instances carbon has also been shown to be limiting. Most
frequently, phosphorus is the limiting constituent; however, this is not exclusively
so and there are many lakes in which nitrogen is the limiting factor.
Productivity is a measure of the utilization of inorganic material to produce plant
growth. Very frequently, productivity in lakes is measured by the amount of fish
available in the lake. Since the number of fish is indirectly related to the growth of
the other organisms lower in the food chain and, ultimately, to the amount of algal
growth, it may be seen that the productivity of fish is merely an indirect measure of
the amount of algae growth in the first place. Whether or not productivity is desirable
is a function of individual taste. A lake that is low in productivity will be clear and
conversely will have a small fish population. On the other hand, a productive lake
will be turbid because of the high concentration of plankton, but this will support a
278
D. B. Aulenbach et al.