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Studies in Wisconsin and Minnesota have found that air diffuser systems
seem most effective in providing efficient, reliable winter aeration. To
save energy costs, the system can be run only on an as-needed basis.
Careful monitoring of dissolved oxygen levels throughout the winter can
be used to determine when. or even if.
aeration in a particular year is necessary Oftentimes, dangerously low
oxygen levels do not appear in Illinois lakes until late winter. In most
cases, if a lake’s average dissolved oxygen level was found to be
between 4 and 5 mg/L, start-up would he warranted. After you become more
familiar with your lake’s situation, start-up can be fine—tuned.
Turbulence from the rising air bubbles and uplifting of the slightly
warmer bottom waters will begin opening the ice within a few hours after
system start-up. Be aware that if the system is turned on when oxygen
concentrations already have fallen too low, mixing of anaerobic bottom
water with low—oxygen water just under the ice may cause the entire
lake to have oxygen levels too low for fish survival.
Surface spray units, impeller-aspirators, and pump-and-cascade systems
also can be used in the winter to keep
an area ice-free. Of the three, pump-and-cascade systems appear to be
the most reliable in averting
winter fishkills. They also can be moved from lake to lake. On small
lakes, their wintertime performance has compared favorably with air
diffuser systems.
Design Considerations
There are several technical issues to consider when
designing and installing an artificial
circulation system. For example, if the air diffuser is positioned too
far above the lake bottom, an anaerobic zone will remain below it.
However, if the diffuser is placed on or too near the lake bottom—or
if the system is oversized (mixing is too vigorous)—sediments may be
stirred up and resuspended in the lake. If the system is undersized,
mixing will be incomplete. In very large lakes, mixing will he limited
unless more than one device is used.
To Aerate—or Not to Aerate?
It’s a good idea to seek experienced professional
help when considering the installation of and in designing a
properly-sized aeration system. The first question to consider is
whether your lake can really benefit from a destratification/circulation
(or other) aeration system Would summer and/or winter operation be most
effective? Answering these questions requires background knowledge of
your lake’s physical and water quality characteristics. You
also should have established lake use goals
(e.g., what you’d like to use the lake for, how you’d like the lake
to look). Seek out the advice of unbiased water quality professionals—don’t
limit your advice to just the individual or company proposing to sell you
a system! By examining your lakes
characteristics together with your goals, you can then better determine
whether aeration, and what type
of system. might meet your objectives.
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Lake Notes . . .is
a series of publications produced by the Illinois Environmental
Protection Agency about issues confronting
Illinois’ lake resources. The objective of these
publications is to provide
lake and watershed
residents with a greater
understanding of environmental cause-and-effect relationships, and
actions we all can take
to protect our lakes.
This Lake Notes publication
was prepared by Holly Hudson and Bob Kirschner of the Northeastern
Illinois Planning Commission. Chicago. Illinois. Illustrations by
Holly Hudson. University
of Wisconsin—Extension. Wisconsin Department
of Natural Resources. and Illinois State Water Survey.
For more information about other publications in
this series and to request copies. please
contact: Illinois Environmental Protection
Agency, DWPC-Lake and Watershed Unit. P.O.
Box 19276. Springfield. Illinois. 62794-9276: 217/782-3362.
November 1997. Permission granted to
reprint with credit to
the Illinois Environmental Protection Agency and theNortheastern
Illinois Planning Commission.
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