These graphs are useful in examining the yearly and long-term
fluctuations of Secchi transparency within an individual lake.
The values are calculated by subtracting the long-term average
from each year's summer average Secchi value. Secchi depth
is one of the best parameters for detecting trends in lake
trophic status as long as you can assume that changes in clarity
are primarily due to changes in the amount of algae in the
water. For some lakes, increased turbidity (loss of clarity)
may be due to sediment particulates from river inflows, shoreline
erosion, or even resuspension of bottom sediments in shallow
can be affected by changes in the:
amount of nutrients
? aquatic macrophyte
(precipitation, runoff, lake levels, temperature, sunlight).
Long-term patterns can sometimes be seen in lakes (consistently
increasing or decreasing values over time). Sometimes cyclical
patterns are seen which may be related to the above factors.
Because so many factors can affect water clarity it
can be difficult to determine what's happening to a lake but
looking at Secchi data is usually the best we've got.
More nutrients usually mean more algae, unless the lake has
already been "over fertilized". The biggest sources of nutrients
entering our lakes are from direct nutrient wash-in from the
watershed during rainstorms and from the mixing of high nutrient,
low oxygen bottom water up into surface waters during windstorms
and during spring and fall turnover.
eat algae and fish eat zooplankton. Walleye, bass, northerns
and muskies eat "zoops" in their larval and juvenile stages.
Minnows and panfish eat them throughout their life cycles.
Fish especially like a species of zooplankton, called Daphnia
that are the most efficient grazers of algae. When Daphnia
reach their peak in summer they can effectively "clear" the
water of algae. Too many zooplankton-eating fish can really
diminish the Daphnia population.
restoration method actually involves trying to increase the
number of zooplankton in the lake to maximize this clearing
effect. This involves netting or even poisoning (rotenone
treatment) the small, zooplankton eating fish. Or, large,
fish-eating predators can be added to the lake to accomplish
the same thing. The idea is to increase the number of zooplankton
so more algae are eaten and therefore the lake clears up.
These techniques are called "biomanipulation". They have been
successful in many urban lakes in Europe and North America
although they require very careful long-term management and
can be expensive. There is no cheap
quick fix for excessive nutrient loadings from the watershed!
and pondweeds also need nutrients to grow. Although bottom-rooted
lake plants get most of their nutrients from the sediments
they can compete with algae for water column nutrients. Perhaps
more importantly, they also shade the water column, which
decreases the amount of light available to algae. These plants
also provide a refuge or a place to hide for zooplankton so
they can avoid being eaten by fish. Healthy shallow lakes
or shallow zones in deeper lakes often have
flourishing macrophyte communities. In these zones you can
often see that the water is actually quite clear among the
day-to-day variations and seasonal changes in weather have
big effects on algae. Changes in physical factors such as
temperature and light as well as indirect effects on nutrients
and on other biological communities in the lake are all influenced
Why Is it Important?
secchi disk depth provides a low "tech" method for assessing
the clarity of a lake. A Secchi disk is a circular plate
divided into quarters painted alternately black and white.
The disk is attached to a rope and lowered into the water
until it is no longer visible. Secchi disk depth, then,
is a measure of water clarity. Higher Secchi readings
mean more rope was let out before the disk disappeared
from sight and indicates clearer water. Lower readings
indicate turbid or colored water. Clear water lets light
penetrate more deeply into the lake than does murky water.
This light allows photosynthesis to occur and oxygen to
be produced. The rule of thumb is that light can penetrate
to a depth of about 2 - 3 times the Secchi disk depth.
is affected by algae, soil particles, and other materials
suspended in the water. However, Secchi disk depth is primarily
used as an indicator of algal abundance and general lake productivity.
Although it is only an indicator, Secchi disk depth is the
simplest and one of the most effective tools for estimating
a lake's productivity.
for Natural Variation
disk readings vary seasonally with changes in photosynthesis
and therefore, algal growth. In most lakes, Secchi disk readings
begin to decrease in the spring, with warmer temperature and
increased growth, and continue decreasing until algal growth
peaks in the summer. As cooler weather sets in and growth
decreases, Secchi disk readings increase again. (However,
cooler weather often means more wind. In a shallow lake, the
improved clarity from decreased algal growth may be partly
offset by an increase in concentration of sediments mixed
into the water column by wind.) In lakes that thermally stratify,
Secchi disk readings may decrease again with fall turnover.
As the surface water cools, the thermal stratification created
in summer weakens and the lake mixes. The nutrients thus released
from the bottom layer of water may cause a fall algae bloom
and the resultant decrease in Secchi disk reading.
also may affect readings. Erosion from rainfall, runoff, and
high stream velocities may result in higher concentrations
of suspended particles in inflowing streams and therefore
decreases in Secchi disk readings. On the other hand, temperature
and volume of the incoming water may be sufficient to dilute
the lake with cooler, clearer water and reduce algal growth
rates. Both clearer water and lower growth rates would result
in increased Secchi disk readings.
color of the water also affects the readings. In most lakes,
the impact of color may be insignificant. But some lakes are
highly colored. Lakes strongly influenced by bogs, for example,
are often a very dark brown and have low Secchi readings even
though they may have few algae.
Impact of Pollution
tends to reduce water clarity. Watershed development and poor
land use practices cause increases in erosion, organic matter,
and nutrients, all of which cause increases in suspended particulates
and algae growth. Secchi disk depth is usually reported in
feet to the nearest tenth of a foot, or meters to the nearest
tenth of a meter. Secchi disk readings can be used to determine
a lake's trophic status. Though trophic status is not related
to any water quality standard, it is a mechanism for "rating"
a lake's productive state since unproductive lakes are usually
much clearer than productive lakes.