Determining Water Clarity
(or visibility)
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Visibility in a body of water is the distance
through which you can still see something.
The distance you can see through water
can be affected by several things.
A few things that affect water clarity are
listed on the next slide.
Go on to the next slide.
Things that affect clarity
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Amount of algae in the water
Dirt from erosion
Churned up sediment from the bottom of
the body of water
Amount of small organisms living in the
water
Take a moment to write these things down
on your activity report.
Testing Visibility
Visibility through a body of water can
be tested with a device called a
“Secchi (seh-key) disc.”
 A Secchi disc is pictured here.
 The disc is attached to a
rope and lowered into
water until it is no
longer visible.
Go on to the next slide
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Using a Secchi Disc
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There is a Secchi disc apparatus set
up by the entrance to the laboratory.
You will use the device to determine
the depth of visibility in the water
sample provided.
Go on to the next slide
Why measure visibility in water?
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The visibility through a sample of
water can provide us with valuable
information about the ecosystem of
the body of water.
Lake Ontario is currently suffering
from an overpopulation of zebra
mussels that eat the microorganisms
in the water.
Go on to the next slide
Secchi disk activity
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You will be doing this activity with
the Secchi disk by the door to the
lab.
Take out the binder in your drawer
and turn to the Secchi disk activity
section.
Once you complete the Secchi disk
activity, you may go on to the next
slide.
An Underwater Ecosystem
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Algae, or “phytoplankton,” has a
significant affect on the amount of
visibility in water.
The next slide has a diagram of a
food chain in Lake Ontario. Write
down the creatures that zebra
mussels eat, and what eats zebra
mussels.
Go on to the next slide
Trout, Salmon
Warm water
Sport fish
Large, exotic
Zooplankton
Alewife
Juvenile Fish
Zooplankton
(Tiny animals)
Gobies
Sturgeon
Zebra Mussels
Quagga Mussels
Phytoplankton
(Microscopic
Plants)
Sunlight
Nutrients
Use this chart to find
out some information
about all of the
different lake
organisms. You can
write down your
observations on your
worksheet.
A disturbed balance
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The picture to the right is
a zebra mussel.
The zebra mussel is a
species that is not native
to the great lakes. The
best explanation is that
the zebra mussels came
into the lakes on ships.
Go on to the next slide.
Zebra Mussels
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Zebra mussels do not
have many natural
predators in the Great
Lakes.
The lake sturgeon,
which is pictured to the
right, is one of the only
predators of zebra
mussels in Lake
Ontario.
Finish writing your
observations on the
worksheet.
Once you are done, go
on to the next activity!
Trout are freshwater fish that are related to salmon. They are a part
of a subdivision of the salmon family called “Salmoninae.” The Lake
Trout used to be the primary predator of Lake Ontario, but almost
reached extinction in the lake due to over-fishing and environment
destruction.
Return to
Food Chain
The Alewife is a small fish that is second in command in
the Lake Ontario food chain. It feeds on zooplankton
(Tiny aquatic animals)
Return to
Food Chain
The pictures on this slide are some of the large zooplankton that
can be found in Lake Ontario. Zooplankton are sometimes called
“brine shrimp.” They can be visible with the naked eye or
microscopic.
Return to
Food Chain
Gobies are an aggressive
species of bottom dwelling fish
that are not native to Lake
Ontario. They were brought
here in the ballast of cargo
ships.
Lake Sturgeon can grow to be very
large in size. (six to eight feet) They are
an endangered species because they
used to be hunted heavily for their
caviar. (eggs)
Return to
Food Chain
Zooplankton are microscopic creatures that are
one of the building blocks of the Lake Ontario
food chain. The population of zooplankton can
fluctuate extremely rapidly based on the
chemistry of the water. If there is pollution in
the water, the zooplankton are some of the first
to feel it.
Return to
Food Chain
The creature on the left is a
zebra mussel. The Zebra
Mussel originated in the
Caspian Sea, which is
Bordered by Russia. It was
brought to the United States by
cargo ships.
The image on the right is a Quagga
Mussel. The Quagga mussel prefers
cold and deep waters. It too was
brought to the United States from
Russia in the ballast tanks of
oceanliners.
Return to
Food Chain
Phytoplankton are
microscopic
plants. They are
responsible for the
basis of food
production in Lake
Ontario.
Return to
Food Chain
Go On To
Next Slide
Sunlight is needed for plants to
do photosynthesis to make
food.
Return to
Food Chain