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 divided into epineuston and hyponeuston (above and below the surface film). Pleuston are
organisms which live on the surface but penetrate through the surface (duckweed, for example). There
are many more such distinctions. Had enough?
These organisms all act as agents of exchange between the physical environment especially with
regard to light, and the chemical environment especially with regard to oxygen and nutrients. And the
ecosystem perspective is the best way to make sense of the diversity and complexity of these lake
systems. In this perspective, materials and energy are viewed as moving, interacting components of a
lake, both affected by the organisms and affecting the organisms.
Biota may modify the way materials are distributed in lakes. For example, much of the
phosphorus in some lakes is in organismal form. This means that if we removed the organisms (the
plankton, the fish, etc.) then we would remove the nutrient as well. It also means that other forms of the
material are absent in other parts of the lake (the sediments for example) and that inputs from the
watershed or other sources do not significantly exceed the rate of growth of organisms. An example of
such a system might be, for example, an oligotrophic lake like Jocassee, a pumped-storage reservoir in
the Blue Ridge escarpment of South Carolina. Phosphorus, when it enters such a lake, is rapidly
accumulated into biomass. In such situations, these nutrients might be considered the limiting factor for
biological growth. That is to say, that with all other factors held constant, increasing the supply of the
limiting nutrient is all that is required for increasing production of biomass.
Functionally, growth of biota is often divided into several categories. Primary production is that
biomass which is the direct result of photosynthesis. This activity is limited to plants, either
phytoplankton or macrophytes, and some photosynthetic bacteria. Organisms that eat primary
producers are termed secondary producers or primary consumers . Many zooplankton and all cows
(except, of course those which are fed meat by-products) are primary consumers. So are grass carp,
manatees and some insects. Organisms which eat other consumers are termed predators or
carnivores.
This has the semblance of organization and it has been formalized with trophic structure - or who
eats whom. And this system is termed trophic dynamics. Ecologically, this is important because it
organizes how materials and energy are exchanged between functional organismal groups. This system
is important to lake management because it allows models of biota to be constructed for prediction of
effects of management strategies. Trophic dynamics has no relation, however, to lake trophic status and
the distinction is easily shown.
Lake trophic status is based on the idea that as lakes age they tend to become shallower in
depth, succeeding to less limnetic habitat and more wetland habitat and finally to forest habitat. The idea
of lake succession was originally described for northern glacial lakes but applies in some ways to
reservoirs as well. A lake may begin as a basin filled with water with low concentrations of nutrients and
sparse biota (an oligotrophic lake). With time, inflows deposit more nutrients and biota take
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