All bodies of water are complex ecosystems. From a small retention pond to a large glaciated lake, there are many factors and forces contributing to the health and appearance of the water. While the study of aquatic resources can be a lifelong pursuit, we’ve summarized some of the more basic concepts of water bodies. There are two different time-frames to consider when looking at ponds: short term and long term. The short term cycle of pond is defined by stratification. And the long term cycle of a pond is defined by eutriphication. We’ll briefly discuss each below.
Keep in mind that, while learning about these cycles is the first step in understanding aquatic resources, there are quite a few other complexities we do not discuss on this page. Food chains, water chemistry, geological characteristics, and many other areas of studies all contribute to a more complete understanding of water bodies.
The annual cycle of a pond is defined by thermal stratification, which is the layering of water based on temperature. Depending on the season, ponds cycle through different stratifications. Starting in the spring the pond begins to warm, and what is known as an overturn occurs. The ice melts and wind and waves create a uniform water temperature of around 4°C. As wind becomes less prevalent and the weather warms, the upper water layer warms and the colder, denser water sinks to the bottom. In the summer months clear stratification occurs with the following three layers:
Epilimnion is the upper layer of warmer water. It typically has a higher pH and dissolved oxygen concentration than the rest of the water. Being at the surface, it typically more turbid as the result of wind-mixing. The exchange of dissolved gases (ie O2 and CO2) with the atmosphere also occurs here. Because this layer receives the most light it contains the most phytoplankton. As they grow and reproduce they absorb nutrients from the water, and when they die they sink into the lower layer.
Hypolimnion is the colder bottom layer. Being at depth, it is isolated from surface wind-mixing during summer, and usually receives insufficient light for photosynthesis to occur. In deep, temperate lakes, the bottom-most waters of the hypolimnion are typically close to 4°C throughout the year. In the summer this layer is often lacking oxygen and promotes anaerobic activity. This can lead to the release of nutrients from the sediment into the water column.
Thermocline, sometimes called the metalimnion is the layer in between the epilimnion and the hypolimnion. It characterized by the rapid change in temperature when compared to the other layers. In essence the thermocline acts as a barrier between the two layers, and prevents uniform mixing.
In the fall as the weather heads toward winter, the upper water begins to cool and sink to the bottom. With the help of wind mixing the pond once again turns over. In the winter the water once again stratifies, but this time the cooler water is near the layer of ice on the surface, and the warmer water remains near the bottom. In the spring, the cycle begins again when the ice melts.
Fish kills can result from this natural pond cycle. If a pond turns over to quickly, fish are often stressed by the quick change in temperature. Fish also struggle In the winter due to lack of oxygen. Heavy snow and ice prevent any photosynthesis and can cause a dangerous drop in DO.
The aging of a pond is the result of a natural succession, called Eutrophication. Defined as the increase in the concentration of chemical nutrients in the water. This process is, in it of itself, a natural cycle. Ponds begin as clear, nutrient deprived, bodies of water, and as they mature they will accumulate nutrients. If left alone all natural bodies of water would ultimately fill in and become wetland-like. Unfortunately, human activities greatly accelerate this process. Lakes would naturally take hundreds of year to become eutrophic. We are now seeing an abundance of eutrophic ponds, regardless of age.
Non-point source pollution such as stormwater runoff and atmospheric deposition as well as source pollution all contribute to accelerated eutrophication. Excessive nutrients can have many harmful effects, but most relevantly lead to algal blooms. Limiting sources of nutrients in water, not only slows this process, but can also help limit unwanted aquatic growth.