The Art Of Juggling Oxygen

Shelly Steck

The most significant way dissolved 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 dissolved 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 dissolved 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 dissolved 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 dissolved 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 dissolved 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 dissolved 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 dissolved oxygen levels - plenty of dissolved 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 dissolved oxygen concentrations.  Because cooler water has the capability to hold more dissolved 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 dissolved 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 dissolved 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 dissolved 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 dissolved oxygen necessary to sustain life.      

In an attempt to juggle the many varying factors at play, mechanical aeration (pond aeration) becomes a valuable tool in helping maintain adequate dissolved 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 fine bubble diffuser systems and pond 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 

Pond Fountains introduce dissolved oxygen by creating oxygen transfer when water from the pond 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 pond 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 called the venturi aerator. The venturi aerator 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 dissolved oxygen 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.

** 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

Art Of Juggling Oxygen© - Shelly Steck

WATER QUALITY EVALUATION OF THE IMPACTS OF AERATION ON DEEP AND
SHALLOW WET DETENTION PONDS IN SOUTHWEST FLORIDA

Florida Department of Environmental Protection - FULL TEXT

Since the beginning of Florida’s stormwater treatment program in the early 1980s, a fundamental
principle has been to limit the depth of wet detention systems so that anaerobic conditions do not occur in either the water column or the pond sediments. In Lee County, Florida, the site of this study, the County’s Land Development Code requires ponds greater than twelve feet deep to have aerators to prevent low dissolved oxygen conditions or stratification of the water column.
This study compared dissolved oxygen levels and other water quality data in aerated and non-aerated wet detention ponds of various depths. Four ponds were selected from a series of wet detention ponds within The Brooks residential development in Bonita Springs, Florida. In the fall of 2004, a variety of water quality data was collected from the ponds using submersible data sondes, portable multi-parameter meters and traditional grab sampling for laboratory analysis.

The study was conducted in two 15-day phases. In each phase, aerators in two of the four ponds were turned off and the other two were left on as usual. During the two phases, water quality data were continuously recorded by the data sondes, which were suspended two feet above the bottom of each pond. A portable multi-parameter meter was used to record measurements at one-foot intervals throughout the water column to determine the dissolved oxygen levels at various depths and the presence of stratification. In addition to the monitored data, grab samples were collected from each of the ponds at the same depth as the data sondes. The grab samples were laboratory analyzed for pH, specific conductance, turbidity, ammonia nitrogen, total Kjeldahl nitrogen, nitrate + nitrite, orthophosphorus, total phosphorus and chlorophyll-a.

Aeration Systems Influence

Department of Biology, University of South Florida, 33620 Tampa, FL, USA - FULL TEXT

Abstract To determine the influence of a multiple inversion aeration system upon the limnology of a small sinkhole lake, we monitored physical-chemical and biological parameters for 15 months prior to starting aeration and for 24 months thereafter. Aeration eliminated thermal stratification and dissolved oxygen concentrations of bottom waters increased significantly. Secchi disk transparency
increased during aeration while turbidity, pH, alkalinity, total nitrogen, hydrogen sulfide and iron concentrations decreased significantly. Primary production and mean chlorophyll a did not change significantly during aeration but total phytoplankton cell volume decreased 2-fold. This decrease was caused by a marked reduction in blue-green algae which appears to be attributable to rapid
mixing of the lake and to decreases in the pH. Cell volumes of green algae remained constant but numbers of taxa increased 70%.

Densities of crustacean zooplankton were reduced markedly by aeration while densities of rotifers increased significantly during the first year but then returned to pre-aeration levels during the second year. Large-bodied cladocerans were replaced by mallbodied
forms during aeration, and copepod populations became dominated by nauplii (97%). Densities of benthic macroinvertebrates declined 2-fold during aeration due to to a marked reduction (10-fold) in the Chaoborus population which correlated strongly with decreases in crustacean zooplankton abundance. The total number of taxa collected on individual sample dates increased throughout the two year aeration period (from 12 to 25) and chironomids became the predominant group (70%). The multiple inversion aeration system successfully eliminated many of the undesirable features of eutrophication (e.g., oxygen depletion, blue-green algal blooms, low benthic diversity), but it did not change the trophic state. Aeration of hypereutrophic lakes for multiple years may be necessary to maintain desired conditions.

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