Science of Water : Concepts and Applications

172 The Science of Water: Concepts and Applications
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Favorable temperature
Favorable DO content
Suffi cient water level
The biotic and abiotic factors in an aquatic ecosystem that reduce growth include:
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Predators
Disease
Parasites
Pollution
Competition for space and food
Unfavorable stream conditions (i.e., low water levels)
Lack of food
With regard to stability in a freshwater ecosystem, the higher the species diversity, the greater
the inertia and resilience of the ecosystem. At the same time, when the species diversity is high
within a stream ecosystem, a population within the stream can be out of control because of an
imbalance between growth and reduction factors, with the ecosystem still remaining stable at the
same time. With regard to instability in a freshwater ecosystem, recall that imbalance occurs when
growth and reduction factors are out of balance. For example, when sewage is accidentally dumped
into a stream, the stream ecosystem, via the natural self-purifi cation process, responds and returns
to normal. This process is described as follows:
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Raw sewage is dumped into the stream.
It decreases the oxygen available as the detritus food chain breaks down the sewage.
Some fi sh die at the pollution site and downstream.
Sewage is broken down and washes out to the sea and is fi nally broken down in the
ocean.
The oxygen levels return to normal.
Fish populations that were deleted are restored as fi sh about the spill reproduce and the
young occupy the real estate formerly occupied by the dead fi sh.
Populations return to “normal.”
A shift in balance in a stream’s ecosystem (or in any ecosystem) similar to the one just described
is a common occurrence. In this particular case, the stream responded (on its own) to the imbalance
the sewage caused and through the self-purifi cation process returned to normal. Recall that
succession is the method by which an ecosystem either forms itself or heals itself. Thus, we can say
that a type of succession has occurred in the polluted stream described above, because, in the end,
it healed itself. More importantly, this healing process is a good thing; otherwise, there would have
been few streams left on Earth suitable for much more than the dumping of garbage.
In summary, through research and observation, ecologists have found that the succession
patterns in different ecosystems usually display common characteristics. First, succession brings
about changes in the plant and animal members present. Second, organic matter increases from
stage to stage. Finally, as each stage progresses, there is a tendency toward greater stability or
persistence. Remember, succession is usually predictable. This is the case unless humans interfere.
STREAM GENESIS AND STRUCTURE
Consider the following:
Early in spring on a snow and ice-covered high alpine meadow, the time and place, the water cycle
continues. The cycle’s main component, water, has been held in reserve—literally frozen, for the long