Pond and lakes are a valuable natural resource. They add to the beauty of the landscape, provide recreation, and are a habitat for fish and wildlife and an additional water source if needed. However, the good health of a pond is held in a delicate balance. A pond's condition deteriorates when the bottom environment cannot support animal life. The bottom is the area that runs out of oxygen first, it is where the most oxygen is used and it is the farthest from the surface where it is replenished. Without oxygen a lake or ponds self purification capability is not only reduced, it is reversed. The small animals, snails, worms, bacteria, etc., which help keep a pond clean cannot live and the pond's nutrients are then recycled from the sediment. This forms a layer of muck at the bottom which serves as a fertilizer for weed and excessive algae growth. It could also cause large fish kills.
Benefits of Pond Bed Aeration
Aeration means adding air to the water. To restore a lake to health, it is essential to get oxygen down to the lake bottom. Lake Bed Aeration™ not only adds oxygen to the surface water but to the water at the bottom of the lake as well. Once the lake is full of oxygen near the bottom, beneficial aquatic insect larvae, snails, fresh water shrimp, and other fish food can begin to live on the bottom and littoral zone (light zone).
Lake Eutrophication begins when the BOD (Biological Oxygen Demand) of a lake cannot be met. When too much pollution enters a lake, plant and algae growth dies and sinks to the bottom, resulting in an overload of organic sludge. Lower forms of life on the lake bed die and this debris rots. Anaerobic bacteria, which need no oxygen, give off deadly poisonous gases, such as hydrogen sulfide, ammonia, and methane. These gases, as they rise through the water, unite with and bind up and dissolved oxygen remaining in the water. Fish will then die from lack of oxygen. By pumping compressed air out onto the lake bed diffuser, the rising air bubbles bring the bottom water to the surface. Large volumes of water release pollutant gases to the air and pick up more oxygen while on the surface. If oxygen is present at the lake bed, dead organisms will not accumulate but will quickly be consumed by aerobic bacteria, thus providing for a healthier lake environment.
Effects of De-Stratification
- Dissolved Oxygen: The most common result of de-stratification is an improvement in dissolved oxygen levels and consequent benefits on warm water fish and water supply quality.
- Fish: De-stratification 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. De-stratification allows warm-water fish to inhabit the entire lake, and enhances conditions for fish food organisms as well. However, because de-stratification warms the deep waters, some coldwater fish species may be eliminated or prevented from inhabiting that lake.
- Water Supply Quality: A common result of de-stratification 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. De-stratification 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 de-stratification/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: De-stratification 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 de-stratification 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.