In temperate lakes, seasonal changes occur in temperature and density. The area includes most areas in North America, Europe and Asia along with the very tips of South America, Africa and Australia. These areas experience milder seasonal changes in temperature. Temperate lakes have three layers: the epilimnion, the hypolimnion and the thermocline. Seasonal changes come from the temperature controlled by the winds. These occur during the spring and fall.
Ice forms during the winter. Thaws occur in the spring. Summer often causes the surface of the water to become polluted and algae to form. Winter and summer lower the amount of oxygen circulation. The warmest lake temperatures occur in the summer. If the lake has enough circulation, the water remains healthy. Otherwise, they stagnant and become unusable.
Spring brings the melting and recirculation of the waters. The warm waters at the surface are the epilimnion. The hypolimnion is the area of deep, cool water. The waters move again with the winds and produce currents. The temperature becomes equal in density. During the spring and fall, the density changes the most. In the spring, the ice changes to water. In the autumn, the water changes to ice. The circulation affects not only density but the temperature of the water.
Summer is the most dangerous time for people and animals to swim in lakes because they have alga. If people or animals accidentally swallow any of that water, they can become seriously ill and in some cases die. Warmth of water makes the algae to bloom. In the summer, the epilimnion reaches at a certain depth. The thermocline is the area that separates the warm water from the cool. The warm water of the epilimnion cannot mix with the cool water so stagnation occurs. Any of the dead animals or bacteria adds to the summer stagnation of a lake. Summer is the only time when mixing of the cool, bottom water with the warm water at the surface stops.
Depending on how fast the water cools, the lake can remain dangerous for swimmers. It is not until the blooms die that the lake water returns to a safe source. Autumn winds bring the recirculation of the water that removes the stagnancy from the waters.
In conclusion, seasonal changes come from winds. In an area where winds prevail, ice will not form as quickly as in calmer areas. The whole cycle either speeds up or slows down, as a result of how strong, the winds are.
