When it Gets Warmer, Your Pond Layers Up!

Lake stratification is when the water in a lake or pond forms three distinct horizontal layers caused by differences in temperature.  Temperature changes coincide with changes in density.  The large differences in temperature and density throughout the water column cause the layers or zones to form.  The water column is the entire vertical section of the water from the surface to the bottom.

The three layers can be seen below:

•         Epilimnion: This layer is the warmest layer that forms at the top of a body of water during spring and summer when the temperature in the water increases rapidly. Due to contact with the atmosphere as well as light penetration, this layer has a high oxygen content compared to the lower layers.
•         Thermocline: This middle layer is the smallest of the three, and more so functions as a barrier between the warm epilimnion and the much cooler Hypolimnion. The temperature gradient across the thermocline can often be several degrees or more across a depth of only a few inches.
  This layer contains some oxygen, but usually significantly less than the oxygen present in the Epilimnion present above.
•          Hypolimnion: This layer of cool water sits below the thermocline and appears when the cooler, more dense water separates from the warmer, less dense water present above.

Seasonality

Summer

In late spring and early summer, the lake surface is warmed rapidly and differentiates (Stratification) from the colder deeper water.  The three primary layers of stratification form during this period.  This is called summer stratification.  The uppermost layer, called the epilimnion, gets warmer and less dense.  This layer is the most well-mixed due to wind action on the surface of the water. 

Most commonly in lakes and ponds deeper than 10-12 feet deep, the temperature variations throughout the water column can create a physical barrier that can block mixing by wind. Despite this, all ponds are capable of stratifying, especially in the southeast where high temperatures can raise the water temperature quickly.  Layer two, the metalimnion, is where water temperature begins changing rapidly.  The thermocline is a horizontal plane within the metalimnion where the greatest temperature change occurs.  The third, and deepest layer, is the hypolimnion which is cold and has almost no mixing.

Fall

In fall, colder air temperatures cool the lakes surface increasing the density at the surface.  Surface water then sinks to the bottom of the lake which breaks the thermocline and allows the water column to mix completely. This event can often pose a large risk to the health of your lake or pond, causing oxygen concentrations throughout the water column to plummet. This event can create an anoxic environment not capable of supporting aquatic life.

To protect a lake against this risk, we recommend at least one, but preferably both of the following aeration systems:

• Surface Aeration: These systems chop up the water and can introduce additional oxygen directly into the surface water. This will not increase the oxygen in the water below the thermocline, but can improve the conditions present in the Epilimnion.
• Bottom Diffused Aeration: These systems do not directly introduce much oxygen into a system, but will mix the water column by physically disrupting the thermocline. These systems can improve conditions by distributing surface oxygen throughout the pond.
  
  

Winter

In most cases, the water column will stay completely destratified in the winter due to low temperatures throughout the water column. In some very cold climates, the surface layer can freeze, inducing a “reverse stratification” if the water temperature drops below around 4°C, the lake will become inversely stratified.  Inverse stratification is when the water temperature at the surface is colder than the lower layers.  When the surface of the water forms a sheet of ice it blocks wind action, resulting in very little mixing thought the lake.   In Georgia and other parts of the southeast lakes generally do not get colder than 4°C, so they remain in a mixed uniform state throughout the water column until spring. 

Water Quality

During summer stratification, the hypolimnion is rich in dissolved oxygen caused by the mixing in spring.  By mid-summer, the lake becomes fully stratified.  The metalimnion then becomes a barrier that cuts off atmospheric oxygen from mixing to the lower hypolimnion.  On top of limited oxygen, the hypolimnion is typically too dark for aquatic plants and algae to photosynthesize and produce more oxygen. 

Eutrophic Environments

In nutrient rich lakes, also referred to as eutrophic, the hypolimnion can become a dead zone without oxygen.  In these anoxic conditions at the bottom of the lake, nutrients such as nitrogen and phosphorus become more easily dissolved.  These nutrients will disperse into the hypolimnion from the soil.  After a heavy rain event or a cold front in summer, which can allow for temporary lake mixing and a breaking of the thermocline, these nutrients can seep into the upper layers of the lake and stimulate an algal bloom.  Algal blooms, once formed at the surface, will block out sunlight for the lower layers of the lake. 

Most fish will avoid these areas as certain algae can hinder the fish’s ability to breathe through their gills.  Some of these blooms can also contain cyanobacteria, which can be toxic to fish and other animals depending on the exact species of cyanobacteria in question.  When a cyanobacteria bloom occurs, we consider it a harmful algal bloom, or HAB. 

Aeration

Aeration systems, as mentioned above, come in two different styles which each have their own benefits for a body of water. At AES, we highly recommend all lakes employ both types of aeration to see the greatest benefit and reduction of risk. The insurance these systems provide against fish kills alone can save pond owners thousands of dollars.

Bottom Diffused Aeration

Bottom diffused aeration can prevent the hypolimnion dead zone from forming in the first place.  This system involves pumping fresh air to the bottom of the lake which then gets broken up into small bubbles via a diffuser head. It breaks the fresh air into small bubbles with a mesh screen or filter.  The small bubbles are more easily dissolved into the water column than large bubbles which would simply rise to the surface and mostly release into the atmosphere rather than into the lake.

The bubbles mix the stratification layers by forcibly breaking the thermocline as they rise to the surface.  This type of aeration does not pump pure oxygen into the lake, it only pumps atmospheric air.  The air we breathe is a mixture of oxygen and other atmospheric gases.  It does circulate enough oxygen for fish and other aquatic organisms to survive in all three stratification layers throughout the summer months.  By preventing the dead zone at the bottom of the lake from forming, it becomes more difficult for nutrients that can cause algal blooms to escape from the soil and dissolve into the water column. 

Surface Aeration & Aerating Fountains

Surface aeration does not have much of an effect on lake stratification.  An aerating fountain or surface aerator will only mix the upper layer, the epilimnion, by the constant movement of the surface water.  Only in a small pond with a large surface aerator can the movement of water have enough force to mix all three layers of the water column in summer.  However, by moving the surface water it increases the surface area for gas exchange.  This means that certain nutrients which might promote algal blooms can escape into the atmosphere in gas form more easily and efficiently. 

In most lakes and ponds, it is advantageous to have a combination of bottom diffused and a type of surface aerator.  The bottom diffused aeration will mix the lake allowing gases and nutrients to circulate from the bottom of the lake to the surface.  The surface aerator or fountain will create ripples on the surface of the lake which will allow these gases to more easily escape.

Don’t wait!
Water temperatures are getting warmer, and now is the perfect chance to get a surface or bottom diffused aeration system to both protect your pond from the risk of fish kills, but also to ensure that your pond has the best chance to thrive this summer. Any pond can experience stratification, and it's crucial to know how this could affect your fish.

If you're interested in one of these systems, give us a call and we can work with you to find the best aeration solutions for your pond or lake!