BIOLOGICAL ZONES
Scientists divide lakes into zones related
to biological activity. The commonly used
terms are littoral, sub-littoral, and pelagic.
Littoral and sub-littoral zones are defined
as those portions of the lake in which plants
grow. One authority suggests that the littoral
zone includes that area where plants emerge
above the surface or float on it, and the
sub-littoral zone is that area where only
fully-submerged plants occur. Hereafter,
unless separation is necessary for clarity,
the two are combined and referred to as the
littoral zone. I believe it's impossible to
overemphasize the importance of the littoral
zone to fly-fishers. In general this is where
the useful action is found. Plant life in the
littoral zone provides food and shelter to many
insects and small fish, the food-forms of larger
fish.
Unfortunately, at least from the classification
point of view, some stillwaters, or areas of
others, do not divide neatly into these zones.
These are lakes where the nature of the benthos
(bottom) or chemical composition inhibits plant
growth even though there is sufficient light.
Examples are gravel, marl, or clay bottoms and
stillwaters with very low nutrient levels. Like
the profundal zone described below, this doesn't
mean such areas are devoid of food-forms. So,
to avoid pointless verbiage in later chapters,
I include these shallow regions in the littoral
zone.
Figure 1: Zonation of stillwaters; for illustrative
purposes the thermocline is over-thick. Also, the
line above the profundal zone is often considered
the 1% incident-light level, that is, by the time
the light reaches this level it has been reduced
to 1% of its intensity at the surface.
The pelagic zone is the open-water area of a
lake above depths that are too deep for plant
growth. It's further subdivided into photic and
aphotic zones. The photic zone extends from the
lake surface to a depth where incident light
has been reduced to 1% of its surface value,
below that is the aphotic zone. Another phrase
used to describe the deep region of a lake that
is devoid of plant life is the profundal zone.
Note, however, that while barren of plants the
profundal zone is not devoid of life. Under
certain circumstances it may provide the bulk
of a trout's food.
ANNUAL TEMPERATURE CYCLE
Most fly-fishers are familiar with the
annual temperature cycle of lakes, so I
will be relatively brief. Water is most
dense at 39 deg F (4 deg C). Ice, being
less dense, floats. Where it's cold enough
in winter, the ice and the water immediately
below it are less dense than the remainder of
the lake water and so a temperature inversion
occurs, i.e., colder water resting over warmer
water. In the spring the ice melts and the
surface water gradually reaches 39 deg F
(4 deg C). At this time the entire lake is
at the same temperature and, given sufficient
wind, a thorough mixing at all depths follows.
This is called the spring turnover.
During the summer the surface water continues
to warm. If the lake is deep enough, and if
there is sufficient wind, currents are generated
that mix the warm and cold water, but only to a
certain depth. This action creates a thermal
stratification. The water is warm on top and
cold on the bottom. At some depth, which varies
considerably with environmental and physical
conditions, a narrow transition zone appears;
the well-known thermocline (Figure 1). Under
some conditions the thermocline may be the only
comfortable area for certain species, both predator
and prey. The thickness of this transition zone
is highly variable, from a few inches to several
yards. Nor is it static, moving downward in the
summer and upward as fall approaches, and in
shallow lakes may disappear intermittently
depending on wind conditions. Finally, the
thermocline is not even always horizontal. Certain
wind-induced currents can cause it to tilt, meaning
that it can be closer to the surface at one end of
a lake than at the other.
An associated phenomenon is the chemocline -
a transition zone between upper regions
oxygenated by wind action and oxygen-depleted
(anoxic) depths. Chemoclines occur in lakes
that for reasons of size/depth ratios or
location never turn over. Trout won't live
below a chemocline, and so, when on the "bottom,"
they will always be suspended above it.
Cooling surface temperatures in the autumn
provoke the final phase of the cycle. The
cool water sinks forcing the warmer water
below to the top. The water slowly reaches
a uniform temperature everywhere; then the
wind can thoroughly mix it again. This is
the fall turnover. Clearly the cycle depends
on geographic location. For example, some
northern lakes that don't freeze have a
fall turnover period that extends through
to spring.
The temperature cycle is clearly important
to fly-fishers. An example is the effect
of spring turnover (fall turnover is
generally after the end of the fishing
season) on nutrient-rich lakes. Mixing
may stir-up large quantities of plant
detritus from the bottom, turning the
stillwater into a virtually unfishable
soup - a good time to avoid. ~ PCM
More next time.
Credits: Excerpt from Stillwater Fly Fishing,
Tools & Tactics By Paul C. Marriner, published
by Gale's End Press. We appreciate use
permission.
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