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Transparency is how easily light can pass through a substance.
In lakes this means how deep sunlight
penetrates through the water.
Plants and algae need sunlight
to grow, so they are only able to grow in areas of lakes where the sun penetrates. The top section of the lake that sunlight penetrates is called the
Euphotic zone.
The area around the edge of the lake
that is shallow enough for plants to receive light is called the littoral zone. The area of the lake that is too deep for plants to grow is called
the limnetic zone (Figure 1).
Water transparency depends on the amount of particles in the water. These particles can be algae or sediment from erosion, the more
particles - the less water transparency.
In other words, when the water
is murky or cloudy and contains a lot of particles, the light cannot penetrate as
deeply into the water column.
Water transparency is measured with a Secchi disk (how to use a
Secchi disk) .
A Secchi disk is a metal disk, 8 inches
in diameter that is lowered into the water on a cord. The
depth that the Secchi disk can no longer be seen through the water is the Secchi
depth.
When the water transparency is high,
the Secchi depth is high.
When the water transparency
is low and cloudy, the Secchi depth is low (Figure 2) (more
information on the relationship of Secchi disk to lake water quality).
Conclusions about the trophic state of a lake can be drawn by measuring the Secchi
depth (Figure 3) since water transparency is directly related to Total Phosphorous
and Chlorophyll-a.
An example of changes in transparency due to human impact
would be if there is construction going on near the lake shoreline.
The exposed dirt from digging gets blown into the lake by wind and washed
into the lake by rain. This dirt makes
the water cloudy in itself, but it also contains nutrients (phosphorus
and nitrogen)
that can cause algal blooms. Exposed
black dirt and eroded shorelines should be stabilized by natural vegetation (or
riprap if the slope is steep) to prevent impact on lake water quality (see Best Management Practices). |
Figure 1.
Different lake zones in reference to available sunlight.
Figure 2.
The relationship between light penetration and Secchi depth.
Figure 3. The relationship between Transparency (Secchi depth) and Lake Trophic State.
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