Did you know that water temperature in a lake can vary across its cross-section and change seasonally? Continuing the descriptions of various parameters characterizing water, it is necessary to highlight the temperature. Depending on it, a number of chemical indicators change, as well as temperature changes affect organisms living in water.
At our latitude water temperature has a characteristic annual cycle. In summer, the upper layer of water warms up and becomes lighter due to the density properties of water. Water at depth is cold, thus denser and heavier. In the middle there is a layer of water in which the temperature changes drastically, from warmer to colder. And such a layer is called a jump layer or thermocline.
Due to stratification, mixing of the lower water layer with other layers is problematic in summer. The heat capacity of water is high, which means that when the air changes temperature, the water temperature will not change rapidly. The larger the lake or pond, the smaller the changes in water temperature will be relative to changes in the surrounding environment. In autumn, as the water cools, it becomes denser and mixing of the entire water column takes place. As a result, various substances, including oxygen, diffuse into all layers.
Ice formation is also associated with temperature changes. Water is densest when it reaches a temperature of 4°C, but as it continues to cool, the water becomes lighter again until it freezes. These properties ensure that ice forms on the surface of the water and does not sink. In spring, when the ice melts and the water temperature rises, the water layer mixes again, so the temperature is almost the same at different depths of the water body. This gives the opportunity for different substances to mix evenly among the water layers again.
The solubility of substances also changes along with the temperature. For example, the colder the water, the better various gases dissolve in it (including oxygen). In Latvia, the water layers of most lakes and ponds mix twice a year – in spring and autumn, such lakes are called dimictic lakes.
Water temperature also affects fish, since fish being cold-blooded organisms depend on temperature changes. As the water temperature rises, the metabolic rate of fish also rises, which affects the fish behavior, feeding, growth and reproduction.
The first year of temperature measurements at the pilot site of the “TRUST ALUM” project – lake Velnezers – has ended. We can observe that the equalization of the water temperature in the lake can be observed precisely in the autumn and spring months. During these months, as the water temperature stabilizes, the temperature difference between the upper and lower water layer is 0.4°C. On the other hand, in winter and summer, the differences between the upper and deeper water layers increase rapidly, that is, in July, the temperature of the upper water layer is 24.8 °C, and the lower one is 7.9 °C. In winter, this phenomenon manifests itself in the opposite way – the upper layer has cooled down to 0.9 °C, and the lower one to 4.4 °C.
Photo: Latvian Institute of Aquatic Ecology