Have you ever been swimming in a pond or lake in late summer and noticed the top few feet of water is like bath water and below that, the water is cool and refreshing? This cooler water, although very refreshing to dive into, can be very dangerous to your pond and its inhabitants.

Terms & Definitions

Stratification is the layering of water by temperature and/or chemical properties and density. This layering of water can vary in depth, but is often very distinct within a pond or lake.

Thermocline is the name of the separation between the pond layers. It is often the transition from the top, warm water and the cold bottom water.

Euphotic Zone is the area of the water column that receives sunlight, or the top layer of the water.

Epilimnion is the term for the warm water layer at the top of the pond during the summer months.

Pond Turnover or simply Turnover is the process of a layered or stratified pond mixing once the density barrier is no longer present. This typically occurs in the fall, but can also happen in the spring. It is often very abrupt and can be catastrophic to a pond environment.

Uniqueness of Water

Water sustains life on Earth. Some of its unique properties help ensure life continues even in the winter months. Most compounds on Earth are densest in their solid state. Water is not, that is why ice floats on a pond and in your drink glass. Since ice floats, ponds and lakes can sustain life in the winter months. If ice was denser than liquid water, it would sink and allow the entire pond or lake to freeze.

The density of liquid water changes with temperature as well. Generally speaking, the warmer the water, the less dense or lighter the water. That means 80F water will “float” on top of 55F water. However, at roughly 39F or 4C, water becomes less dense until it hits the freezing point and turns to a solid. This key density change is critical as it ensures warm water at the bottom of the pond during the winter months and a place for fish to find more consistent temperatures.

Seasonal Progression of Water

We’ll start our seasonal progression in winter. The water temperature is fairly consistent throughout the entire water column. As stated earlier, warmer water is less dense than cold water and will float to the top. However, as water cools and gets gradually heavier, it is reversed at about 4 degrees Celsius and continues until the water is frozen (this is why ice floats). If you are in a northern climate that gets ice, there will be warmer water near the bottom of the pond that is warmed by the Earth and will not float to the top because the surface water under the ice is cold enough to be lighter.

As the ice melts in late winter or early spring, that 4 degree Celsius less dense water warms to above 4 degrees. When this happens it is denser or heavier than the warm bottom water. What then occurs is called Turnover. The heavier water is on top, but when the density barrier is eliminated the entire water column will be uniform density. This mixes up the entire water column and the pond will typically look dirty with suspended debris. Spring turnover is much less severe than fall turnover because there are not as much oxygen problems in spring as in late summer.

As spring progresses into summer, the top layer or epilimnion warms by the sun and increased air temperature. The warmer the weather and more intense the sun, the more distinct the temperature layering and greater density difference from top to bottom of the pond. The volume of the epilimnion can vary in pond to pond and from year to year, but is often very small in size compared to the colder, bottom water.

With fall comes cooler temperatures. The pond water will begin to cool throughout fall. The cooler temperatures will continue to cool the surface water until the water above the thermocline is the same temperature and density as the bottom water. Now you have a situation where turnover could occur. The quicker turnover occurs, the more severe the effects. A cool fall rain or cold front with lots of wind can quickly cool the surface water and cause a rapid turnover. Once the cooler surface water becomes the same density as the bottom water, the pond volume will mix and displace the water volumes. Again, the water column will be mixed and look dirty with suspended debris.

Fall Turnover is much more severe than Spring Turnover. During turnover, the closer the thermocline is to the surface, the worse the effects will be on the pond. If there is a large volume of water below the thermocline that has very little or no oxygen when turnover occurs, the low oxygen water gets mixed with the rest of the pond, thus decreasing the dissolved oxygen throughout the entire water column. Also, the chances of turnover are greater in fall due to common fall rains and cold fronts. This decrease in DO can be drastic enough to cause massive fish kills. Turnover will also mix bottom debris into the water column and can spur more bacteria activity, further lowering DO levels.

Once the water column is homogeneous again, the water will continue to cool at the surface as fall progresses into winter. Once the surface water hits 39F or 4C, it will float on the warmer, denser water until it cools enough to turn into a solid. At this time, your pond will freeze over and remain until the spring sun and warmer temperatures thaw the ice and start the process over again.