Effects of Destratification/ Circulation
• Dissolved Oxygen: The most common
result of
destratification is an improvement in dissolved oxygen
levels—and consequent benefits on warmwater fish and
water supply quality.
• Fish: Destratification
is
generally considered beneficial for warmwater
fish. Fish require adequate dissolved oxygen levels and cannot survive
in an oxygen-deficient hypolimnion. Warmwater fish (e.g.,
.bass, bluegill) require a minimum dissolved
oxygen concentration of 5 mg/L,
and coldwater fish (e.g., trout) need 6-7 mg/L.
Destratification allows warm- water fish to inhabit the entire lake, and
enhances conditions for fish food organisms as well. However, because
destratification warms the deep waters, some coldwater fish species
may be eliminated or prevented from inhabiting
that lake.
• Water Supply Quality: A common result
of destratification is an
improvement in industrial
and drinking water supply quality (in fact, the first artificial
circulation system was used in 191 9 in a small water supply reservoir).
Under anoxic (without oxygen, anaerobic) conditions, lake bottom
sediments release metals (iron, manganese) and gases (hydrogen sulfide)—which
can cause taste and odor problems in drinking water. When the anoxic
hypolininion is eliminated, these problems are eliminated or greatly
reduced as well. Water treatment costs also decrease.
• Phytoplankton: The effects on phytoplankton(algae) are less predictable. Destratification may reduce
algae through one or more processes: 1) algal
cells will be mixed to deeper, darker lake areas, decreasing the cells’
time in sunlight and thereby reducing their growth rate, 2) some algae
species that tend to sink quickly and need mixing currents to remain
suspended (e.g., diatoms) may be favored over more buoyant species such
as the more noxious blue- greens, 3)
changes in the lake’s water chemistry (pH, carbon dioxide, alkalinity)
brought about by higher dissolved oxygen levels can
lead to a shift from blue- green to less noxious
green algae or diatoms, and 4) mixing of
algae-eating zooplankton into deeper, darker waters reduces their
chances of being eaten by sightfeeding fish; hence, if
more zooplankton survive, their consumption of
algal cells also may increase.
While algal blooms have been reduced in
some lake destrafi- cation/circulation projects, in other lakes
phytoplankton
populations have not changed or have actually increased. For shallow
lakes, it’s even less likely that complete circulation would result in
any of the above-mentioned benefits. This is because algae are less
likely to become light-limited in shallow lakes, nor would water
chemistry changes be as pronounced.
Phosphorus: Destratification
has the potential to reduce phosphorus (P) concentrations in some lakes.
During summer stratification when the hypolimnion is oxygen-poor, P
becomes more soluble (dissolvable) and is released from the bottom
sediments into the hypolimnion. Because stratified lakes can sometimes
partially mix, this allows greater amounts of P to “escape” into the
epilimnion. These increased P levels in the lake’s surface waters can
potentially stimulate an algal bloom. For similar reasons, algal blooms
often are seen at fall turnover. Because destratification increases the
bottom water’s oxygen content, it follows that P release from the
sediments should be reduced, which in turn can lead to decreased algae
abundance. However, the most suitable candidates for P reduction are
deep, stratified lakes where a majority of the lake’s P comes from
anoxic, hypolimnetic sediments (i.e., internal sources). In lakes where
the majority of P comes from external sources (such as watershed runoff,
the atmosphere, waterfowl, septic systems), a reduction in sediment P
release may not be enough to notice a change in algae abundance.
Winter Operation
Artificial circulation systems also can help prevent
winter fishkills in ice-covered lakes. Low dissolved oxygen levels
during winter occur because ice covering the lake prevents diffusion of
atmospheric oxygen into the water. Even though photosynthesis by some
algae and rooted aquatic plants occurs in the winter months under the
ice, bacterial decomposition of organic matter on the lake bottom can
consume
more oxygen than photosynthesis can replace. Furthermore. if enough snow
covers the ice or if the ice is opaque, sunlight will be unable to
penetrate and photosynthesis will stop. If under-ice oxygen levels
become too low before ice-out, a partial or total fishkiil will occur.
Shallow lakes are most susceptible to dissolved oxygen depletion since
they have a smaller amount of water as compared to deeper lakes.
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