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Regional Report for the Prairies - Bill Charles
Originally Published in
August / October 2007 Issue
While anglers
across the Prairies are familiar with the phenomenon of
winter-kill, trout die-offs can suddenly occur in the summer
months. Although less common, it can be just as devastating to
trout populations in our lakes. As with winter-kill,
summer-kill is caused by a loss of dissolved oxygen and is
more common in heavily vegetated shallow lakes that contain
large volumes of decomposing organic matter.
High summer daytime heat in
combination with extended cloudy weather will cause a
reduction in the amount of dissolved oxygen in lakes. When the
day in dominated by full sun, photosynthesis results in high
dissolved oxygen levels later in the day. At night, oxygen
levels drop as the daytime production is consumed. Cloudy
weather further reduces the daytime oxygen accumulation. If
this pattern repeats itself over several days, there will be a
creeping reduction of dissolved oxygen. The situation can be
made worse if calm, windless conditions accompany cloudy hot
weather.
As many of
our Prairie trout lakes are found in agricultural areas,
phosphorus and nitrogen from the surrounding watershed can
produce dense algae blooms. Any sudden algae die-off will
result in a great increase in bacterial decomposition of the
dead plankton further reducing dissolved oxygen levels.
Lastly, sudden dramatic changes in lake stratification can
lead to the occurrence of a Summer-kill. Under normal
conditions, lakes stratify by temperature during the summer.
Simply stated, the water stratifies due to the difference in
density according to temperature. Summer stratification is
characterized by warm surface waters that hold enough
dissolved oxygen to support life. In contrast, the bottom
water layers commonly have extremely low oxygen levels due to
bacterial consumption of organic matter. This holds
particularly true for shallower lakes with heavy bottom
vegetation and shallow gradual sloping shorelines.
For lakes
with compromised surface oxygen levels, any event that results
in a movement of oxygen deficient bottom water can result in a
summer-kill. Premature turnover is not usually a problem early
in the summer or after September. In early summer,
stratification has just been established, and the bottom
levels of the lake will still contain a certain amount of
dissolved oxygen. Consequently, a sudden mixing of water
layers will not result in overall fatally low oxygen levels.
After the heat of July and August has passed, lake
temperatures begin to cool with the surface water holding
substantial oxygen reducing the likelihood of a dramatic
turnover event. Similarly, the fall decrease in water
temperature moderates distinct stratification allowing oxygen
to be introduced into the deepest water layers of the lake.
The most
common precipitating event leading to a mid-summer lake
inversion is a heavy thunderstorm accompanied by strong winds.
The sudden introduction of large amounts of cold water at the
surface can cause the lake’s stratified levels to ‘flip’,
mixing non-oxygenated bottom water throughout all layers of
the lake. Lakes with larger watersheds that receive high
levels of surface runoff are particularly susceptible to
inversions.
Summer use of
aeration systems can help to minimize the risk of a fish kill
by adding oxygen to the water and by helping to prevent summer
stratification.
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