large fish population. However, this fish population usually represents different
species. A lake low in productivity tends toward the more preferred game fish
such as trout and salmon, whereas a lake that is high in productivity is more
commonly inhabited by bass, pickerel, and catfish. The term oligotrophic has been
used to describe lakes of low productivity and eutrophic for lakes of high productivity. In between is a region called mesotrophic, indicating that there is a gradual
transition between oligotrophic and eutrophic. These terms are not intended to imply
that all eutrophic lakes are undesirable or that all oligotrophic lakes are desirable.
The desirability of a specific level of productivity is a function of the specific use of
the lake and a balance of various types is needed. The long-range problem is that as
lakes age the nutrients accumulate within the lake. New nutrients are brought into the
lake by allochthonous inputs, which are substances entering the lake from outside of
the water body. This includes siltation, organic matter from decaying leaves, and, of
course, anthropogenic sources such as sewage or treated sewage effluent. Autochthonous inputs are those that are generated within the lake. This mostly represents a
recycling of the nutrients within the lake since these are relatively constant. Thus, as
there is an increase in allochthonous sources, more nutrients are made available to
the organisms within the lake, and the productivity increases.
Based on Liebig’s law of the minimum, it is possible to control excess productivity by eliminating one of the essential nutrients. It is not necessary to eliminate all
of them. Since phosphorus is most commonly the limiting nutrient, most efforts have
been expended toward controlling phosphorus inputs to lakes in order to control
excess productivity. Whereas little can be done to control the natural process of
eutrophication, there can be control of the anthropogenic sources of the nutrients that
will encourage the unwanted higher productivity. In some instances, reduction of the
anthropogenic sources of nutrients has been shown to reverse the eutrophication
trend. In other instances the lowering of anthropogenic inputs merely slows down or
delays the rate of eutrophication. What works in one lake may not necessarily work
in another seemingly similar lake.
Since phosphorus is most frequently the limiting nutrient in a lake, more efforts
have been directed toward finding means of reducing phosphorus inputs to a lake.
This may include diversion of all storm runoff from gaining direct access into the
lake, treatment of wastewaters for phosphorus removal prior to discharge, and use of
land application wastewater techniques that have been shown to reduce phosphorus
content significantly. Another reason phosphorus has been chosen as the nutrient to
remove is that it may be fairly readily removed by chemical precipitation with iron,
aluminum, or calcium. The next most important nutrient for removal would be
nitrogen. Removal of nitrogen is somewhat more difficult; however, it can be
achieved by producing a nitrified treatment plant effluent and then creating denitrification (anaerobic or anoxic) conditions. This will convert the nitrate nitrogen to
atmospheric nitrogen, which will then escape to the atmosphere. However, there are
certain organisms, particularly blue-green algae, that are able to utilize or “fix”
nitrogen directly from the atmosphere into plant material. Since elementary nitrogen
makes up approximately 80% of the air, it can be expected that there will be
sufficient nitrogen available in the water body to provide for all of the nitrogen
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
279
species. A lake low in productivity tends toward the more preferred game fish
such as trout and salmon, whereas a lake that is high in productivity is more
commonly inhabited by bass, pickerel, and catfish. The term oligotrophic has been
used to describe lakes of low productivity and eutrophic for lakes of high productivity. In between is a region called mesotrophic, indicating that there is a gradual
transition between oligotrophic and eutrophic. These terms are not intended to imply
that all eutrophic lakes are undesirable or that all oligotrophic lakes are desirable.
The desirability of a specific level of productivity is a function of the specific use of
the lake and a balance of various types is needed. The long-range problem is that as
lakes age the nutrients accumulate within the lake. New nutrients are brought into the
lake by allochthonous inputs, which are substances entering the lake from outside of
the water body. This includes siltation, organic matter from decaying leaves, and, of
course, anthropogenic sources such as sewage or treated sewage effluent. Autochthonous inputs are those that are generated within the lake. This mostly represents a
recycling of the nutrients within the lake since these are relatively constant. Thus, as
there is an increase in allochthonous sources, more nutrients are made available to
the organisms within the lake, and the productivity increases.
Based on Liebig’s law of the minimum, it is possible to control excess productivity by eliminating one of the essential nutrients. It is not necessary to eliminate all
of them. Since phosphorus is most commonly the limiting nutrient, most efforts have
been expended toward controlling phosphorus inputs to lakes in order to control
excess productivity. Whereas little can be done to control the natural process of
eutrophication, there can be control of the anthropogenic sources of the nutrients that
will encourage the unwanted higher productivity. In some instances, reduction of the
anthropogenic sources of nutrients has been shown to reverse the eutrophication
trend. In other instances the lowering of anthropogenic inputs merely slows down or
delays the rate of eutrophication. What works in one lake may not necessarily work
in another seemingly similar lake.
Since phosphorus is most frequently the limiting nutrient in a lake, more efforts
have been directed toward finding means of reducing phosphorus inputs to a lake.
This may include diversion of all storm runoff from gaining direct access into the
lake, treatment of wastewaters for phosphorus removal prior to discharge, and use of
land application wastewater techniques that have been shown to reduce phosphorus
content significantly. Another reason phosphorus has been chosen as the nutrient to
remove is that it may be fairly readily removed by chemical precipitation with iron,
aluminum, or calcium. The next most important nutrient for removal would be
nitrogen. Removal of nitrogen is somewhat more difficult; however, it can be
achieved by producing a nitrified treatment plant effluent and then creating denitrification (anaerobic or anoxic) conditions. This will convert the nitrate nitrogen to
atmospheric nitrogen, which will then escape to the atmosphere. However, there are
certain organisms, particularly blue-green algae, that are able to utilize or “fix”
nitrogen directly from the atmosphere into plant material. Since elementary nitrogen
makes up approximately 80% of the air, it can be expected that there will be
sufficient nitrogen available in the water body to provide for all of the nitrogen
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
279