Nature Bulletin No. 77 August 3, 1946
Forest Preserve District of Cook County
Clayton F. Smith, President
Roberts Mann, Supt. of Conservation
In August we have our "Dog Days", so named by the ancient Romans
because the bright blue star Sirius, the Dog Star, is then in ascension:
rising with the sun. In the Dog Days, snakes went blind, dogs went mad,
and humans were subject to fevers and malignant influences -- all
foolish superstitions arising from the fact that Rome was a big
unhealthy city surrounded by swamps.
We apply "Dog Days" to the hot humid period in August. In the Middle
West it is good "corn weather". About this time, scum forms on the
surface of the water in swamps, ponds and streams. Fly-fishermen curse
it. Mothers refuse to let their kids go swimming in it. What is this
There are two general kinds, both composed of small living plants. One
kind is called duckweed or "duck meat" the smallest of the flowering
plants. Each plant consists of a tiny, floating leaf, some as small as a
pinhead, with 2 or 3 short thread-like roots dangling in the water. They
multiply rapidly in hot weather. They have a tiny flower and set seed.
Sometimes they completely cover small bays or even whole ponds.
They drift with the wind and may pile up a foot deep on a windward
shore. They are an important food for ducklings and certain adult
species -- notably teal.
The other kind of scum is made up of a variety of still simpler, smaller
Plants: algae, the most primitive form of plant life. Algae form scum
because most of them grow in tangled masses of filaments which trap
sufficient bubbles of the pure oxygen they produce in bright sunlight to
make them rise to the surface. Some contain enough fat to float them.
Lake land plants, algae can thrive and grow only if they get sunshine
and various substances dissolved in water. They form the base of a
pyramid of food chains supporting practically everything that lives in
water. Protozoa and water fleas live on algae; these are eaten by insect
larvae and worms; these in turn are eaten by crayfish and minnows,
which are eaten by such fish as bass and crappies.
Algae scum is the aquatic "staff of life. "
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Update: June 2012