of surface intensity. Among these are photosynthetic bacteria, cyanobacteria (blue-green algae) and
some green algae.
Where adequate light illuminates the bottom of a lake, rooted vegetation can become established,
depending on sediment type and nutrient availability, and the zone where this condition exists along the
shore of a lake is called the littoral zone . In the open-water, deeper areas of the lake, different
organisms often are adapted for this limnetic zone or pelagic zone . The term `pelagic' often is used
for the open ocean areas of the world whereas the term `limnetic' is restricted to use in freshwater
As already discussed, light quality (wavelength) also changes with depth with the general trend of
short wavelengths penetrating deeper than longer wavelengths. To compensate for this certain
organisms living in deeper waters have developed the ability to preferentially utilize those wavelengths of
light that are present. This ability is called chromatic adaptation.
The diagram illustrating longitudinal gradients in reservoirs shows the major distinction between
impoundments and natural lakes. Most reservoirs have longitudinal gradients in water quality. Natural
lakes have similar gradients but those gradients are not always longitudinal nor are they usually as
dramatic as in reservoirs. Reservoirs are conceptually divided into three functional zones. The riverine
zone is located close to major inflows and extends into the lacustrine or limnetic zone longitudinally
until the horizontal (advective ) influence of the river or other inflow has become less than the influence
of lake or limnetic processes which are characteristic of lentic environments. Between the limnetic or
lacustrine zone and the riverine zone is a region in which a transition is made between the dominant
influences on lake water quality. This is called the transition zone and it varies in magnitude and
position between lakes as well as temporally within any lake. These temporal variations arise from lake
level changes, from variations in river flow rates, from thermal trends (which are often seasonal) and
from shifts in sediment load or other loading materials.
Reservoir biota or aquatic organisms often have apparent preferences for certain habitats. This
fact is reflected in their distribution among depths or habitat zones in reservoirs. As a group, fish are
found in nearly all potential habitats except those such as the sediments or sands. Yet certain types of
fish specialize for preferred habitats. As a group the fish belong to the functional classification of
nekton, organisms which are capable of controlling their own positions and are capable of doing this in
spite of water currents. Fish are the major components of the nekton although some insect larvae may
be considered as part of this functional group as well. Organisms which are unable to control their
position in the presence of water movements are called plankton.
The terminology associated with the biota is extensive and diverse. It reflects size, function,
habitat and other characteristics. Benthic organisms inhabit the bottom of the lake. Meroplankton are
planktonic for only part of their life cycle and inhabit some other habitat at other times. Neuston are
organisms, usually microscopic, which associate with the surface film of a lake. They are sometimes


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