lake has distinct zones of biological communities linked to the physical
structure of the lake (Figure 10).
The littoral zone
is the near shore area where sunlight penetrates all the way to the sediment
and allows aquatic
to grow. Light levels of about 1% or less of surface values usually define this
depth. The 1% light level also defines
zone of the lake, which is the layer from the surface down to the depth where
light levels become too low
In most lakes, the sunlit euphotic zone occurs within
However, in unusually transparent lakes, photosynthesis may
occur well below the thermocline into
the perennially cold hypolimnion.
For example, in western Lake Superior near Duluth, MN, summertime algal
photosynthesis and growth can persist to depths of at least 25 meters,
while the mixed layer, or epilimnion,
only extends down to about 10 meters. Ultra-oligotrophic Lake
Tahoe, CA/NV, is so transparent that algal growth historically extended
to over 100 meters, though its mixed layer only extends to about 10
meters in summer. Unfortunately, inadequate management of the Lake
Tahoe basin since
about 1960 has led to a significant loss of transparency due to increased
algal growth and increased sediment inputs from stream and shoreline erosion.
The higher plants in the littoral zone, in addition to being a food
source and a substrate for
algae and invertebrates, provide a habitat for fish and other organisms
that is very different from the open water environment.
zone is the open water area where light does not generally penetrate
all the way to the bottom. The bottom sediment, known as the benthic
zone, has a surface layer abundant with organisms. This upper
layer of sediments may be mixed by the activity of the benthic organisms
that live there, often to a depth of 2-5 cm (several inches) in rich organic sediments.
Most of the organisms in the benthic
zone are invertebrates, such as Dipteran insect
larvae (midges, mosquitoes, black flies, etc.) or small crustaceans.
The productivity of
this zone largely depends upon the organic content of the sediment,
the amount of physical structure, and in some cases upon the rate
of fish predation. Sandy substrates contain relatively little organic
matter (food) for organisms and poor protection from predatory fish.
Higher plant growth is typically sparse in sandy sediment, because
the sand is unstable and nutrient deficient. A rocky bottom has a
high diversity of potential habitats offering protection (refuge)
from predators, substrate for attached algae (periphyton on
rocks), and pockets of organic "ooze" (food). A flat mucky bottom
offers abundant food for benthic organisms but is less protected
and may have a lower diversity of structural habitats, unless it
is colonized by higher plants.
THOSE THAT GO WHERE THEY CHOOSE
LARGER ZOOPLANKTON AND INSECTS
THOSE THAT GO WHERE THE WATER TAKES THEM
LIVING THINGS = PLANKTON
animals - zooplankton
algae - phytoplankton
bacteria - bacterioplankton
DEAD STUFF = DETRITUS
internal - produced within lake
external - washed in from watershed
THOSE THAT LIVE ON THE LAKE BOTTOM
|BENTHOS = ANIMALS
molluscs - clams, snails
other invertebrates -
higher plants -
attached algae -
BACTERIA & FUNGI
of algae, fungi