Few would argue the importance of oxygen in our environment. Without a doubt, it is the most valuable component found in our atmosphere; a fundamental factor that sustains us with every breath we take. As vital as oxygen is to us land lovers, it is equally important to life found in our aquatic eco-systems. Though concentrations of oxygen are but a tiny fraction of what is contained in our land environment (less than 1% in water compared to roughly 21% in our atmosphere), just like us, fish and other aquatic life are dependent on it to live. However, though oxygen is a sure and steady part of our environment, it is an ever-changing variable for life found underwater.
Balancing oxygen levels in a water system is a precarious endeavor; and one that proves to be a challenging juggling act. Variations in light, temperature, water circulation, depth and content can alter the delicate balance of oxygen, affecting plant and wildlife throughout the entire eco-system.
Perhaps the best way to understand this balancing act is to understand the circumstances in which they are produced, maintained and depleted
The most significant way oxygen is introduced into the aquatic eco-system is through photosynthesis. Additional oxygen is absorbed through water movement which acts to “stir” the aquatic environment causing oxygen molecules in the air to dissolve into the water. The more turbulent the water, the more oxygen is introduced. Oxygen is depleted through wildlife respiration and the decomposition of organic material by bacteria and fungi. Together, this system of oxygen production and depletion traverse an unsteady course of maintaining the proper amount of oxygen throughout the entire ecosystem.
Photosynthesis, plants using the sun’s energy to convert carbon dioxide into sugar and oxygen, is the greatest source of H2O. Because photosynthesis requires sunlight, oxygen introduced through this method can only occur during the day. When the sun sets, the process of photosynthesis ceases while decomposition and wildlife respiration continue. As night wears on, the oxygen levels in the water are slowly depleted until dawn breaks and photosynthesis can resume. Large amounts of decomposing organic matter along with an overpopulation of wildlife can easily shatter the delicate balance of oxygen levels, literally, overnight.
Though the majority of lakes in Florida are relatively shallow, understanding the role size and depth plays in making up the anatomy of a lake is also essential. Oxygen is primarily produced in the top layer of a lake where sunlight penetrates the water, driving photosynthesis. Winds further increase oxygen absorption as they push their way across the water, mixing in oxygen as they create waves and eddies along the surface.
At the lake bottom, rotting organic matter collects and decomposes, using vast amounts of oxygen in the process. Sunlight and winds are unable to reach these murky depths, leaving this lower level dark and still. This lack of light and movement keeps the water cool and, even though cooler water has the ability to hold more oxygen than warmer water, it lacks the oxygen produced through photosynthesis and motion. Because of this, little oxygen is available to replenish that which is used in decomposition and respiration, leaving the lake bottom depleted. The lake becomes stratified, creating horizontal columns of water with varying oxygen levels - plenty of oxygen near the top but practically none near the bottom.
This is where water temperature, and therefore seasons, plays a key role in affecting the lake’s ability to regulate adequate oxygen concentrations. Because cooler water has the capability to hold more oxygen than warm water, as water temperature increases, it holds less and less dissolved oxygen. This increase in temperature usually isn’t a serious problem as long as adequate sunlight and winds can penetrate the surface and maintain the supply of oxygen. However, when high temperatures combine with little wind and high cloud cover, fish become trapped in a squeeze that often result in massive fish kills.
Cloud cover reduces the process of photosynthesis and lack of wind movement restricts oxygen penetration from the atmosphere. Water near the surface of the lake quickly becomes anoxic as oxygen is consumed from the bottom up. The warm temperatures of late spring and summer limit the water’s ability to “hold” the small amount of oxygen that is produced. As the sun sets, the process of photosynthesis ceases completely as decomposition and respiration continue, further taxing an already stressed lake. Fish become ensnared in an eco-system that can no longer maintain the levels of oxygen necessary to sustain life.
In an attempt to juggle the many varying factors at play, mechanical aeration becomes a valuable tool in helping maintain adequate oxygen levels. However, there seem to be as many different ways to aerate a body of water as there are companies who supply solutions. And determining which system works best can be confusing. So let’s break it down…
Perhaps the most common systems available include diffuser systems and fountains. 
Diffusers work by introducing oxygen into the water through bubbles that rise up through the water from the lake bottom. Unfortunately, according to Thomas Lawson, author of Fundamentals of Aquacultural Engineering, diffused aeration has not proven terribly effective in shallow lakes because the contact time of the air bubbles with the water is not great enough for sufficient oxygen transfer.1
Fountains introduce dissolved O2 by creating oxygen transfer when water from the fountain hits the lake surface. Though the most aesthetically attractive of the systems mentioned, fountain aeration is perhaps the least effective. This is because water moving through the fountain system is taken from the top, healthiest layer and falls back into the same top layer. Because the water is not redistributed, the depleted bottom layer is not affected.
New on the horizon is a system call the venturi. The venturi system causes a pressure 
differential that forms a vacuum, sucking air from the atmosphere into water captured from the lake bottom, mixed and pushed out at the surface level. According to research, this helps, not only aerate depleted water, but also helps circulate the stratified layers. The results indicate that venturi systems have a higher air and liquid injection efficiency compared to other aeration system. 2
Though nature has it own unique way of handling the distribution of oxygen within an aquatic system, sometime we find a little assistance is required when Mother Nature needs a helping hand.
|
Diffuser |
Fountain |
|
| Power Requirements |
6-12 amps |
6-14 amps |
3.3 amps |
| Oxygen Transfer Eff. 3 |
1.2-2.0 |
1.2 |
2.0-3.3 |
| Rated Depth |
8 feet min |
Surface |
1 to 20 feet |
| Coverage Area ** |
1/2 to 1 acre ft* |
1/2 to 1 acre ft* |
4 acre feet |
| Elec 24 hr .12 kwh |
$1.90 - $3.80 |
$1.90 - $4.44 |
$1.05 |
** The oxygen transfer efficiency (OTE) of a diffuser system is a function of its depth in the ponds. Typically, an OTE of about 1.6% per foot of depth is found for fine bubble diffusers in a pond setting. For a lagoon with ten feet of depth, a transfer efficiency of about 16% could be expected. This means that 16% of the air added at a depth of ten feet will actively be transferred into the water while 84% will be excess and will bubble to the surface.
*Based on depth, shape and size of water body. Estimated from Kasco Marine
1 acre foot = 1 acre pond 1 foot deep.
1. Lawson, Thomas B.. Fundamentals of Aquacultural Engineering. First Edition. New York: Chapman & Hall, Inc., 1997. pg. 283-284.
2. Baylar, Ahmet, Fahri Ozkan, Mualla Ozturk. "Experimental investigations of air and liquid injection by venturi tubes." Water and Environment Journal. v.20 no.3. (2006) pg. 114-22.
3. Colt and Orwicz (1991)
4.D.E.P. Maine
