Any dynamic system is composed of interacting elements or components. All of the ecological
systems that occupy our interest are dynamic systems. Lakes and ponds are no exceptions. They can
be viewed in a variety of ways. In one example, ecosystems are thermodynamic systems. Open
systems exchange both energy and materials across their boundaries. All ecological systems (except
for the microcosm) are open systems. The accounting necessary for application of thermodynamic
principles to ecosystems is a complex but lively field of inquiry.
In another example, ecosystems are information systems where the currency of interest is not
material or energy but information. The information is observed and described (modeled) using
sophisticated mathematics. Ecological diversity is a simple example of such an approach applied to
community structure.
At their simplest, ecosystems can be considered machines in which materials are placed into
motion via energy flow through the system. The form of the materials is determined by the physical
structures and organisms in the system. Ponds and lakes, too, can be viewed as mechanisms in
which sunlight enters the water causing thermal changes and activating plants which absorb
chemicals during growth. Their later consumption by animals or decomposition releases
energy and materials. This apparent cycle, simply viewed here, is the centerpiece of nutrient cycling a
recent topic of hot limnological discussion. But the components of this system are always the same,
energy transfer allowing movement and conversion of materials. Complex water quality models such as
CEQUAL employ this approach.
Again, energy inputs can be shown, for example, to drive the conversion of water from liquid to
vapor which is transported to other locations in the system where the water is transformed to water
again through another energy exchange. This hydrologic cycle is central to all aquatic systems and often
determines the physical and energetic limits to the system in question. Other, different kinds of energy
exchanges will enable inorganic materials to become transformed into organic structures. Then the
decomposition of those structures will release some energy and enable the incorporation of those
materials into other organic forms or into inorganic form. These systems have great complexity and yet
still perform the same basic functions as the most simple conceptual system. However, those complex
systems are much closer to reality and in order to use them, we must contend with the complexity.
Most often limnologists consider the system boundary of the reservoir system as the shoreline.
But to paraphrase a comment by Barbara Speziale, every time one of us waters a lawn or flushes a
toilet, the system boundary is redefined.


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