DC41Pondowner -
Lakes face two basic kinds of environmental problems: threats to water quality from the deterioration of the land comprising the lake’s watershed and poor quality water flowing or seeping into the lake from another source upstream. Direct effluent discharge from municipalities, agriculture and industries are a major source of water pollution. Watershed runoff carrying insecticides, landscaping and lawn fertilizers, grass clippings, septic tanks leachate and road and parking lot drainage also seriously foul lakes and accelerate the otherwise slow natural process of eutrophication.
Pollution causes an enrichment of the lakes with various nutrients including nitrogen and phosphorus and increased suspended solids. These nutrients support abnormally high levels of biological productivity including algae and aquatic weed growth and bottom muck accumulation. The worsening conditions create an excessive demand on the dissolved oxygen content of the lake.
Natural aeration occurs in two ways to meet a lake’s varying chemical and biological oxygen demand. First, horizontal and vertical density currents occur because most lakes stratify into layers with the warmer water on top in the epilimnion and colder water below in the hypolimnion. This separation of layers allows only the top layer access to oxygen from the atmosphere while the lower layer become stagnant and low in oxygen. Secondly, the annual seasonal cycle alternately warms and cools the epilimnion changing its density and causing the two layers to intermix and oxygenate the entire lake. Most lakes turn over twice per year in the spring and fall but very shallow lakes may remain destratified throughout the year via normal wind generated currents. In urban and agricultural communities natural aeration may not be sufficient to meet the lake’s oxygen demand.
Artificial lake aeration is a technique that adds oxygen to a water body to supplement natural aeration. The practice of aerating lakes began in the 1940s when the concept was first employed to prevent winter fish kills. Over the years, significant testing, research and refinement of various aeration methods has occurred. Today, the most advanced methods include the introduction of air to the bottom of lakes through a series of flexible membrane diffusers or, in lakes less than four feet deep, the utilization of mechanical splashers or fountains that add oxygen near the surface. Aeration systems are recommended when a water body receives urban or agricultural runoff, has a history of low oxygen levels that result in summer and winter fish kills or a blue-green type algae problem.
Bottom diffusers work by lifting low oxygen or anaerobic bottom water to the surface where it is exposed to the atmosphere. At the surface it can lose carbon dioxide and hydrogen sulfide and adsorb oxygen before leaving the surface and circulating back to the bottom of the lake. An air compressor is set in a metal cabinet on a suitable pad or pole near the shoreline. A self-weighted hose is run from the compressor to the bottom of the lake where it is connected to one or more diffusers. The diffuser’s bubbles are used to entrain the bottom water and lift it toward the surface thus creating an upward circulation. A properly designed system will distribute oxygen throughout the lake. Bottom diffuser systems are highly economical to use as they pump air instead of water and rely on the natural force of the rising air bubbles to circulate the entire water column.
Installing an aeration system requires water testing, lake dimensions, watershed review, experience and knowledge to size the system correctly. Under-powered aeration systems can easily cause more harm than good by raising up nutrient rich water without the capability of re-oxygenating it. Decreased dissolved oxygen levels, fish kills and increased algae growth could result. Lake aeration systems are designed for, and should be run, twenty-four hours per day to maintain necessary oxygen levels throughout the water column. Do not under-budget an aeration system design. For blue-green algae control in a normal depth lake, a target turnover rate equal to .75 to 1.3 gallons per minute is recommended depending on diffuser placement depth and other factors. This rate will generally but not always control blue-green algae problems.
Besides algae control, aeration will greatly improve sport fisheries by allowing fish to expand their range into formerly oxygen deprived portions of the lake. Destratification resulting from increased circulation will permit sport fish to survive and flourish in the bottom areas of the lake. Beneficial insects and other food sources will begin to grow there as a result of the increased oxygen levels.
Another benefit of aeration is control of certain bacteria. Many different kinds of bacteria are found in surface waters but pathogenic bacteria are not normally present. But disease-causing organisms can occasionally enter water from an outside source, which is nearly always of fecal nature. Pathogenic bacteria do not normally reproduce in lakes but usually survive in a transient state making them susceptible to the effects of artificial circulation. These pathogens may be killed with artificial circulation by the resulting increase in oxygen toxicity, the oxidation of fecal or mucous matter, increase in pH or exposure to ultraviolet light when brought to the lake’s surface.
In summary, the advantages of bottom aeration are:
1. Stops low oxygen fish kills, stress and related diseases
2. Improves fisheries by expanding oxygenated habitat
3. Reduces excessive nutrient levels and associated algae growth
4. Increases dissolved oxygen levels
5. Ends thermal stratification
6. Eliminates foul odors from undesirable dissolved gasses
7. Reduces aquatic midge and mosquito insect hatches
8. Destroys pathogenic bacteria
9. Clarifies and circulates water
10. Stops muck accumulation
11. Safe for swimmers and boaters, no electricity in the water
12. Highly efficient with very low operating and maintenance costs
Definitions:
Epilimnion: The upper water portion of a lake above the thermocline
Hypolimnion: The lower water portion of a lake below the thermocline that is stagnant, fairly uniform in temperature and usually low dissolved oxygen except during lake turnover
Thermocline: A narrow layer of water between the Epilimnion and Hypolimnion
Eutrophication: A process whereby water becomes rich in dissolved nutrients and low in dissolved oxygen
Dissolved oxygen: The most crucial measurement of a lake’s health and water quality, a minimum oxygen level of 6.0 parts per million is required at the bottom for a healthy freshwater aquatic ecosystem
Anaerobic: An environment devoid of air or free oxygen
Carbon dioxide: A gas produced by organisms by the process of respiration
Hydrogen sulfide: An odorous and poisonous gas produced by anaerobic bacteria
Pathogenic bacteria: A microorganism capable of causing disease
Sources: Restoration and Management of Lakes and Reservoirs, Second Edition
Lakesmarts, the First Lake Management Handbook
Aquatic Systems, Inc., Water Analysis Definitions
Clean-Flo Labs, Effect of Aeration/Circulation of Lakes Upon Disease Bacteria
