THE LAKE SYSTEM
The lake system is designed to adequately handle a 25-year storm event. This is a storm event, which involves approximately 12 inches of rain in a 72-hour period. The lake system is not designed just to receive and convey the storm water but to also store and improve the storm water quality. The lake system is divided into drainage basins. These basins usually consist of a series of interconnected lakes that receive storm water run-off from the surrounding area and store that storm water to a certain control elevation. While the storm water is staged in these basins it seeps into the surrounding ground and is stored for future use. (i.e. irrigation water) Additionally, sediments and impurities are allowed to settle out, be absorbed by aquatic plants or broken down by natural bacteria in the lake thus improving the storm water quality. Once the water has been stored up to the designed control elevation it will flow over a weir or control structure for that particular area of the community. The storm water then continues down stream through the system and eventually exits the community.
CHARACTERISTICS OF LAKES IN FLORIDA
Lakes in Florida, whether they are man-made or natural, have a life cycle. They start balanced, fresh and clear with natural clean-up mechanisms in place. As they age, increased nutrient levels can place a lake out of balance, which can overwhelm the lakes natural ability to keep itself clean. Algae blooms are the most common and immediate result of excessive nutrient levels. A comprehensive water management program is generally designed to slow down the aging process and to help prolong the peak period of desirable water quality, clarity and nutrient balance.
Every lake is a unique ecosystem. This is why it is important to understand the causes of the problems, as well as the effects. By analyzing specific lakes and their problems a balanced management program can be created.
The most significant factors that impact a lake’s condition are oxygen and nutrient levels as well as water temperature.
- OXYGEN– supports life in a lake’s ecosystem and provides a natural way to consume or break down organic matter and undesirable bacteria.
- NUTRIENTS– are essential to the life of a plant. However, in a water management system excessive amounts of these substances, that contain phosphorus and nitrogen (fertilizers and grass clippings), will increase the amount of aquatic weed and algae growth which if left untouched can cause severe problems.
- TEMPERATURE– differentials in surface and the bottom layer of water in a lake is called thermal stratification. The variations in water temperature cause differences in density, which will not allow warm and cold water to naturally mix. Warmer surface water induces algae growth.
CURRENT AQUATIC WEED MANAGEMENT PROGRAM
Now that we have outlined the components of the Community’s water management system and the characteristics of lakes in Florida we need to review the District’s current aquatic weed management program. When nutrient loading has created algae and noxious aquatic weed growth there are four common remedies that can be utilized. Mechanical harvesting, herbicide control, biological control and lake draw downs. The District currently manages its aquatic weed control through the use of a highly qualified lake maintenance contractor.
Though herbicide control is the most cost-effective way to deal with aquatic weed growth, it is a “target” type program. Unlike horticultural or agricultural herbicide programs there are not any preventative or pre-emergent type chemicals approved for use in aquatic weed control. However, there are some mechanical and biological means that can be implemented to help reduce aquatic weed growth.
Earlier we identified three significant factors that impact a lakes condition. The impacts of two of these factors, oxygen and temperature, can be significantly reduced through the use of aeration. The following are some of the biological benefits of an aeration system:
- Increased oxygen for fish populations
- Increased oxygen for improved bottom water quality
- Increased oxygen for rapid decomposition of organic matter
- Increased oxygen to help control undesirable bacteria
- Reduced water stagnation
- Increased circulation and mixing to reduce temperature differences in the water column
Aeration can be accomplished by a mechanical aerator or an underwater diffuser aerator. Mechanical aerators agitate water to produce water/ air contact, while underwater diffusers introduce air bubbles from a depth to achieve oxygen transfer and mixing. Diffuser type aeration systems are replacing mechanical aerators because of their low maintenance, reliability, safety, flexibility and overall efficiency. Diffuser aerators are also far more efficient at removing gases such as ammonia and carbon dioxide.
A diffuser type aerator works along the same principles of an aerator in an aquarium only on a much larger scale. An air compressor is located, in a noise proof box, on the bank of the lake. A series of weighted hoses transfer the air from the compressor to a series of diffuser stones that are placed on the bottom of the lake at pre-determined locations. The diffuser stones produce air bubbles that are an average of 3mm in size and which is easily absorbed into the water raising the dissolved oxygen or D.O. level in the lake. Additionally, the air bubbles expand and spread out as they rise. The displacement of the rising air bubbles causes a significant upward water current thus efficiently mixing the cooler bottom layer of water with the warmer surface layer of water at rates of up to 2,000 gallons per minute per diffuser. This mixing reduces the overall water body temperature, which leads directly to a reduction in algae and other aquatic weed growth. One diffuser type aerator, with 3 diffuser stones, can efficiently aerate up to 10 acres of lake.
BENEFICIAL AQUATIC PLANTS
The other factor that has a significant impact on a lake’s condition is nutrient loading. One of the primary sources of excessive nutrient loading is fertilizer run off from turf and landscape areas surrounding a lake. Another common source of excessive nutrient loading is grass clippings that are either washed into the lake during a rain event or are blown into the lake during the mowing operation. Public education and awareness regarding the adverse impacts of improper fertilizer applications and mowing techniques around lakes is done periodically. However, even with compliance of these best management practices, it is inevitable that some excessive nutrient loading will still be recognized. One very efficient way of reducing the negative impacts of excessive nutrient loading is the introduction of beneficial aquatic plants into the lake. Aquatic plants, like landscaping plants, require nutrients as food to survive and multiply. Some of the nutrients (fertilizers and grass clippings) that run off the surrounding lawns and golf courses during a rain event are absorbed by the beneficial aquatic plants surrounding the perimeter of a lake. The result is a healthy and thriving aquatic plant colony, reduced algae and aquatic weeds, and improved water quality as a result of long-term lowered nutrient levels. Secondary benefits from beneficial aquatic plants include increased habitat for fish and foul, erosion control, and improved overall aesthetics, especially during times of drought when the lake levels can drop well below the elevation of the surrounding turf areas and which would normally expose the lake bed (mud).
In the early 1990s, the governing agencies, including Hillsborough County, recognized the benefits of certain aquatic plants and began mandating their use during the construction of a lake or water body. Therefore, all lakes constructed within the community since the early 1990s have had a plant or littoral shelf built into the perimeter of the lakes in order to accommodate the planting and long term management of beneficial aquatic plants. The littoral shelf is nothing more than a flat slope leading from the immediate edge of the lake out approximately 20’. Beyond this point the remainder of the lake is generally excavated at a much steeper slope. The slope of the littoral shelf is generally a 6 to 1 slope or 1 foot of vertical drop for every 6 feet of horizontal travel. This allows for a water depth of approximately 3’ at the outer edge of the littoral shelf. The majority of the beneficial plants used in the community will survive in 6” to 2’ of water.
BIOLOGICAL AQUATIC WEED CONTROL
Triploid Grass Carp are an aquatic vegetation eating fish that may be utilized in some lakes as a primary or secondary means of controlling unwanted aquatic growth. A grass carp’s diet is rather limited and consists primarily of 10 to 12 different types of submersed or immersed aquatic weeds. For this reason, it must first be determined if the lake or water body is being impacted by any of the aquatic weeds that a grass carp is known to feed on. Grass carp can only be obtained by permit from the Fresh Water Fish and Game Commission. One of the requirements of the permit is to erect fish barriers around the connecting pipes and control structures to keep the fish within the lake or series of lakes that they were intended for use in. As of recent years the Game Commission has allowed these barriers to be constructed from low cost PVC, which can be painted to blend in with the surrounding environment.
It is important to note that Triploid Grass Carp are only considered to be productive feeders for a period of five years. Therefore, every five years the lakes that they were installed in will need to be restocked.
If you should have any questions, please feel free to contact the District office to discuss any questions you may have.