Ponds are dynamic; that is, they are in a state of constant change throughout their existence. This may not seem evident to most as the pond you watch over may appear to look the same today as it did yesterday. But in actuality, your pond is undergoing yearly, monthly, weekly, and even daily changes that can have large impacts on how they function and are managed. If a pond is going to be managed properly, then a knowledge of these changes is an important component to consider. Today we will focus on a seasonal change (at least in the Northern portion of the United States) known as stratification.
Stratification is the process by which layers of water of different densities are created through a pond or lake’s water column. More times than not, this is a factor of temperature as colder water sinks toward the bottom and warmer water rises toward the surface. In the North this characteristic of water will promote seasonal changes in waterbodies since we have a cold winter, warm summer, and shifting periods in the fall and spring. In the summer, the warmer temperature above your pond or lake will increase the temperature of your surface waters while leaving the bottom depths colder. This creates two distinct layers: a warmer upper layer called a epilimnion and the colder layer on the bottom called the hypolimnion. In between these two layers is the thermocline where the temperature change is the most rapid. Once fall comes and temperatures begin to drop, the warmer epilimnion begins to cool which causes it to become denser and sink. This eventually causes a mixing event where the top and bottom layers of water will blend together and become uniform. Winter will once again create layers of water. This time however, the coldest water will be on the surface while warmer water will be on the bottom. This is due to the fact that water is its most dense at 4°C (about 39°F) and temperatures in the winter are commonly well below this temperature (also helps explain why ice floats). Then once spring arrives, mixing will occur again until summer comes and the process starts over.
So, seasonal temperature changes in a pond or lake can impact stratification but what does this mean for pond management? As it turns out it can do a lot! In the summer, the hypolimnion and epilimnion may not interact. Since oxygen is lost from the bottom-up and there is little interaction with the oxygen-rich surface waters, the hypolimnion may lose all of its oxygen. This will change the chemistry of the bottom sediments and cause phosphorus to be released internally which will, in turn, support nuisance algae and plant growth. Additionally, oxygen poor waters can negatively impact gilled organisms and reduce their habitat range in your pond or lake. If this is a concern in your waterbody, we would suggest consider adding aeration to discourage stratification from ever even occurring.
By: Edward Kwietniewski
AQUA DOC Lake & Pond Management