biological characteristics are determined in large part by physical
characteristics of the water column. Important physical characteristics
include temperature, light transparency, and wave action, as well as
the total abundance of inorganic nutrients, which is largely a watershed
characteristic. In addition, preceding populations influence successive
populations by assimilating critical nutrients. Populations also have
varying susceptibilities to grazing by zooplankton,
which vary seasonally in type and abundance. As physical, chemical,
and biological conditions in the lake change over time, some species
will be effectively eliminated from a lake because they cannot tolerate
the new conditions. Other species will be out-competed by organisms
that are better adapted to the new environment.
represent an important ecological pattern in lakes known as algal succession.
In most natural systems the seasonal succession of
is a recurrent, if not exactly repetitive, yearly cycle. A typical algal
succession is shown in Figure 19. Some species flourish for a period
of time and then give way to other species more compatible with changed
conditions, such as warmer water, more daylight, or lower concentrations
of phosphorus or nitrogen. Short-lived plankton communities are characterized
by these seasonal fluctuations; longer-lived organisms, such as fish,
must be tolerant of lake conditions all year.