Pond / Lake Health:
Effects of Destratification / Circulation
Dissolved Oxygen
The most common result of destratification is an improvement
in dissolved oxygen levels and consequent benefits on warm water
fish and water supply quality.
Fish
Destratification is generally considered beneficial for warm
water fish. Fish require adequate dissolved oxygen levels and
cannot survive in an oxygen-deficient hypolimnion. Warm water
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 1919 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 hypolimnion
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:
- algal cells will be mixed to deeper,
darker lake areas, decreasing the cells' time in sunlight and
thereby reducing their growth rate
- 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
- 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
- mixing of algae-eating zooplankton into
deeper, darker waters reduces their chances of being eaten by
sight-feeding fish; hence, if more zooplankton survive, their
consumption of algal cells also may increase.
While algal blooms have been reduced in
some lake destratification/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 cause a
noticeable change in algae abundance.
Back to Lake and Pond Health
Index
