The lake during the year
The distribution of substances (nutrients, oxygen concentration, toxins) in the lake depends to a large extent on what happens in the lake itself: do consumers and producers maintain a balance? How many nutrients are deprived of the lake by the growth of phytoplankton? Is there enough oxygen to bind nitrates and phosphates so that they do not turn into toxins?
All these internal processes are subject to a great external process: the change of seasons.
Spring circulation / autumn circulation
Frьhjahrszirkulation: The wind in the spring ensures a full circulation in the lake and thus a dissolution of the stratification. As a result, there is an even distribution of oxygen and nutrients in the lake. The temperature is about 4 ° C.
Fall circulation: During the autumn circulation the process of circulation repeats itself. The only difference is the now higher temperature in the lake. After one summer the temperature during circulation in the lake, of course, is higher, than at after one winter.
summer stagnation: Due to the high light intensity in the surface water, aquatic plants and phytoplankton can produce an enormous amount of oxygen during photosynthesis. Accordingly, low in nutrient salts (necessary aquatic plants and phytoplankton for growth) becomes epilimnion.
In Hypolimnion it is the other way round: It can be operated because of lack of light, no photosynthesis, so that hardly any dissolved oxygen is present. Because biomass producers are completely lacking here, no nutrients are consumed. The result is a high nutrient salt content in the deep layer.
Because of the density anomaly of the water it comes to a stable stratification over the entire summer. Strong sunlight ensures temperatures of over 20 ° C in the epilimnion. The metalimnion (thermocline) separates the warm epilimnion from the 4 ° C cold water. H2O molecules have the highest density at 4 ° C and thus sink to the bottom of the lake.
winter stagnationWith the winter, there is a significant cooling in the lake. In epilimnion temperatures are now just above freezing point. In the end, an ice cap closes the lake in addition, thus ensuring an oxygen termination. The oxygen continues to decrease during the winter because it is needed by the fish and other organisms to breathe. New oxygen is due to the cold (most Plankton dies in cold temperatures or then operates little / no photosynthesis) the lake no longer supplied.
The density anomaly of the water again ensures a constant temperature of exactly 4 ° C in the hypolimnion.