Lake. The long term effect of this decrease in available nutrients would require long-term study and
careful consideration of changes that may take place elsewhere in the watershed for Clarks Hill Lake.
Habitat, as most people already understand, is often viewed as the place, space or environment
in which an organism lives. Many of our management concerns are for habitat. Habitat is often confused
with niche , a concept that is much more abstract and is defined as characteristic of the organism or
population. Eugene Odum compared the niche to an organism's profession and the habitat to its work
environment. In aquatic systems, habitat is often defined or bounded by physical and chemical
properties of the lake or stream. In lakes these properties include depth, temperature, light intensity,
light quality, dissolved oxygen and nutrient concentrations, and sometimes other chemical
concentrations or factors such as oxidation-reduction potential or pH. Because some of these habitat
boundaries coincide closely with the physical and chemical limnological zones discussed earlier in the
course, terms that can be applied to most lakes exist for these habitats.
The epilimnion of a lake is often similar in size and limits to the photic or euphotic zone , a zone
which exists from the surface to the depth of approximately 1% of surface light intensity. This
correspondence should not be surprising if one considers the effect of light absorbance on temperature
and thermal energy distribution in a lake. The euphotic zone, however, can be affected by many factors
which alter the absorbance of light, organisms included. And while the euphotic zone is primarily
important for plant life in lakes, it is also important to animals whose feeding and reproductive behaviors
depend on specific light environments. The euphotic zone is highly variable spatially and temporally, light
being quicker to respond to external influences than temperature.
The euphotic zone has been defined by the depth of 1% of incident light and this rather arbitrary
limit betrays an early belief by limnologists that plants could not effectively utilize light intensities less than
1% of surface illumination. At lesser depths photosynthesis could actually add dissolved oxygen to the
lake. Numerous exceptions to this generality are know today.
Surface illumination is dependent upon many factors, latitude, date, time-of-day, climatic
conditions. The solar constant, the constant maximum light intensity arriving to the earth at the outer
edge of the atmosphere, is approximately equal to 1.94 cal. cm-2 minute-1. Some of this light does not
enter a lake due to the factors mentioned earlier such as absorbance or reflectance by the atmosphere
or water surface. Assuming that only 1% of the incident light (0.02 cal. cm-2 minute-1) defines the
euphotic zone, then photosynthesis (the process by which light is used to convert inorganic molecules
into organic compounds) at that depth would be assumed not to exceed respiration (the process by
which organic compounds are decomposed biochemically to yield energy and inorganic molecules).
The depth at which photosynthesis was equal to respiration is considered the compensation depth, or
the depth at which photosynthetically produced food exactly compensates for the loss due to
respiration. Today, we know that some organisms are capable of utilizing light intensities less than 1%


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