trophic relationships in the ecosystem. "Bottom up" controls include those influencing the supply
of basic requirements for growth such as nutrients or light. In this view the algal species compete
for nutrients or available light and this competitive interaction coupled with specific growth
potential determines the amount and types of algal species. The term "bottom up" implies that
the base of the food web controls overlying or dependent components of the food web.
Another view is that food web control is "top down". In this view, the presence or absence of
different predators will control the types of herbivores. The types of herbivores then influence
herbivory which in turn influences the amount and types of algae dominating the ecosystem.
"Top down" implies that higher food web components (such as fish) control the growth and
types of the food items they depend upon.
The other factors such as temperature, climate, geology, etc. fall into the category of
independent factors not controlled by biological processes. All of the above factors are listed
Nuisance growth cannot occur where conditions cannot support luxuriant plant
growth. Therefore, the essential elements for growth must be present in excess. The
limiting nutrient must be found in sufficient concentration. Phosphorus is commonly
implicated as the limiting nutrient and is one of the most common components of
pollution. Any essential nutrient can theoretically be a limiting nutrient. Others
commonly implicated are N, C, Si, and Fe (mostly marine).
Light must be available. Excessive turbidity or other mechanisms blocking the light
will tend to limit growth even where nutrients are in excess. Seasonal trends are also
important in as much as some latitudes have longer summer days with less intense
light while other latitudes have shorter days with more intense light.
Herbivory by zooplankton or other animals can limit the standing crop of plant
biomass. This is the "sledgehammer" approach to plant control if, for example, a
herbivore such as the grass carp is introduced to remove aquatic plants. In the
plankton, a prolific population of planktivores in a good year can shift the dominant
Herbivory can also be selective, allowing less-grazed species a selective advantage.
Changes in predators on herbivores can change patterns of herbivory. This may
affect the selective pressure on the plant species, causing a shift in species
dominance. Although well-described, this is often ignored in such management
decisions as creel or size limits for fish populations. If ignored, the 'bottom up'
control assumption is implied.
Physical processes affecting the distributions and movements of supporting
resources are also important. The most important of these is water movements,
either flows or turbulence.
Factors Under Hydro-project Control. This course presents a number of means of
improving or managing water quality in reservoirs and tailwaters. When modifications to project
operation are made for water quality considerations these are usually related to the temperature