A BETTER WATER FOR A BETTER LAKE

Introduction

In a lake, since it is quite deep, sufficient light cannot penetrate down to the bottom to promote plant growth. The bottom, for the most part, is bare. With time runoff from the land and decay from dead organisms adds to the sediments at the bottom of the lake. The lake becomes shallower, eventually allowing light to penetrate to depths that enable green plants to grow on the bottom. The bottom soon becomes covered with vegetation, which adds more organic matter to the bottom, making the lake even shallower. Next, plants start to emerge above the surface of the water. The decay of these emergent plants fills the pond even more. Gradually, the open space of water closes. As the area dries, shrubs will begin to grow, then trees. Soon the lake will be filled in completely. A change in plant life brings a change in organisms inhabiting the area. Eventually the entire ecosystem is changed.

Phytoplankton forms the base of most aquatic food chains. The phytoplankton mainly live in the upper few meters of a lake. Phytoplankton are producers, and require light for photosynthesis.

Consumers are found at all depths of a lake. Herbivores hang out at the top layers, grazing on phytoplankton. Carnivores spend their time in various depths, perhaps as much as several hundred meters deep. These consumers, along with the decomposers at the bottom, need oxygen. Yet photosynthesis does not occur at such depths. How does oxygen get to the bottom of deep lakes? It does so by an interesting process called overturn.

As ice on a lake melts in the spring, the cold water that is formed sinks to the bottom. Gradually the bottom of the lake is filled with cold water. This mixing of water carries oxygen to the bottom. Water mixing from bottom to top also carries nutrients to the top which supports the growth of phytoplankton. This process is called spring overturn. For a time in late spring, due to wind and wave conditions, the water will achieve a uniform density and temperature throughout.

As the summer approaches, the sun warms the upper layers of the water faster than the wind can mix it. By mid summer, the lake usually has three layers. The upper layer or epilimnion, contains warmer water that circulates freely. The water becomes too warm for cold-water fish to tolerate, but Algae and phytoplankton photosynthesize in the top layers, adding nutrients and oxygen to the water. The lower layer, or hypolimnion, contains colder water that does not circulate well. In between is a layer of transition from warm to colder water called the mesolimnion, or thermocline. Because of density differences, the hypolimnion is cut off from circulation, causing the oxygen remaining there to be used up. The lake will reach summer stagnation, when the bottom layer becomes unsuitable for life that requires large amounts of oxygen.

As the fall approaches, the top layer cools and gradually reaches deeper. At a point in the fall, the water at the surface will rapidly cool. The cooler, denser, more oxygenated water will sink, displacing nutrient-rich water on the bottom. This is called fall overturn. The fall winds are now able to form another mixing of water.

With the coming of winter, the upper part of the lake cools even more. This cooling increases the density of water. The cool, denser water sinks to the bottom, allowing the less dense ice to float on top. Three layers form; frozen water at the top to warmer water at the bottom. No additional oxygen will reach the bottom until spring turnover.

Because all these mixing process are not efficient enough, we observe for in many cases what is called lake stratification.

Lake stratification (the division of a lake into water layers of different temperatures) high nutrient conditions and low oxygen levels can lead to deteriorating water quality and fish kill. Recreational water use (swimming, boating and fishing) can be significantly impacted by deteriorated water quality.

As a stratified lake becomes anoxic or anaerobic in the bottom waters (hypolimnion) phosphorous and ammonia are solubolized and released from the bottom sediments into the water and stimulate an algal bloom. In summer the anoxic or anaerobic condition in the hypolimnion can force fish populations to the surface waters in search of oxygen. However, sometimes the water temperatures are too warm at the surface for fish to survive, this results in a summer fish kill. The cause of winter fish kill is a depletion of oxygen levels with ice and snow cover restricting photosynthesis and natural atmospheric oxygenation of the water.

Solution

An ADS aeration sytem supplies oxygenation at the bottom of a lake, increases mixing and oxygen transfert through the water column and provides year-round habitat for fish and aquatic life. The direct result is a complete destratification and prevention of thermoclines or stratification. Furthermore, anaerobic zones cannot develop.

Lake destratification/circulation with ADS aeration reduces algae growth by:

Physically mixing algal cells and algae-eating zooplankton
Prevent the formation of algae
Creating aerobic conditions at the sludge water interface, which minimizes the release of nutrients (nitrogen and phosphorous) into the water column. Minimizing available nutrients is essential for algae and plant control

Performance

The slowly rising air bubbles create a gentle laminar flow that prevents stratification and oxidizes organics, reversing the natural aging process of a lake. Algae growth is diminuished and algae bloom are eliminated. The water is cleaner, purer, more inviting for swimming, boating and fishing. Since fish thrive in both summer and winter, their numbers and size greatly increase.