WW-I-5: Water temperature of standing waters – case study

The picture directs the view from the shore of a lake between large trees out onto the surface of the water. In the background, the sun is just above the horizon.Click to enlarge
Rising water temperatures in lakes have fundamental impacts on water ecosystems.
Source: Photograph: © Maurice Tricatelle/stock.adobe.com

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents


WW-I-5: Water temperature of standing waters – case study

Water temperatures rose significantly in the course of the observation periods. This is true for both the annual mean temperature and the seasonal mean from March to October. The temperature increase applies to both, the lakes in the Alps and foothills (Lake Constance), the upland lakes (Saidenbachtal Reservoir) and different types of lakes in the North German Plain (Großer Müggelsee, Dahme and Lake Stechlin).

The line graph shows the development of the mean monthly temperatures of the season from March to October from 1971 for Lake Constance, the Saidenbachtalsperre, the Großer Müggelsee, the Stechlinsee and the Dahme. All time series show a significantly increasing trend with clear fluctuations between the years.
WW-I-5: Water temperature of standing waters – case study

The line graph shows the development of the mean monthly temperatures of the season from March to October from 1971 for Lake Constance, the Saidenbachtalsperre, the Großer Müggelsee, the Stechlinsee and the Dahme. All time series show a significantly increasing trend with clear fluctuations between the years.

Source: LUBW/ISF (Lake Constance); TU Dresden/Ökolog. Station Neuzehnhain; LTV (Saidenbach Reservoir); IGB (Großer Müggelsee; Lake Stechlin); SenUVK Berlin (Dahme)

Clear trend towards higher water temperatures in lakes

Water temperature is one of the pivotal cause variables for processes occurring in lakes, which also makes it an important factor which determines their utilisation and the framework conditions regarding their ecosystems and for managing their water regime. In turn, water temperature is directly dependent on air temperature and its daily and seasonal progression. Consequently it is plausible to assume that climate change has direct impacts on water temperature and ecosystems in standing water bodies.

Many creatures occurring in water bodies are adapted to specific temperature conditions. Therefore, even minor changes can entail shifts in the species composition in standing water bodies. This may result in the displacement of originally occurring organisms by other species. The incoming species may very well be non-indigenous species which benefit from higher temperatures. Furthermore, these circumstances can also lead to changes in the cycle of the seasonal development of creatures which belong to lake ecosystems.

The temperature and thermal regime of a water body control fundamental physical, biological and chemical processes. This explains why the response rate of numerous chemical and bio-chemical processes increases with rising temperatures; substances such as naturally occurring minerals dissolve more readily in warmer water whereas gases such as oxygen dissolve less readily. Some organisms can cope with oxygen content decreasing or mineral concentrations increasing whereas others are dependent on excellent water conditions. Likewise, material changes triggered by increases in temperature have a substantial effect on plants and animals in these water bodies.

It is difficult to attribute any noticeable changes in the species composition to climate change, as several impact factors interact, especially those related to the utilisation of water bodies and their peripheral areas. As things stand at present, we only have the changing temperature regime for deriving probable impacts on the composition of species in lakes. Uncertainties also exist in terms of climate change impacts on the types of utilisation feasible in lakes and on the challenges evolving in terms of water management. In this context, other impact factors also play an important role.

The situation prevailing in highland or lowland areas changes the characteristics of water bodies/water courses. In particular, there are changes in lime content as well as in the water passage and frequently also in the depth of water. Consequently, water temperature is also very dependent on the geographical location of the lake in question. An essential variable which allows the illustration of climate-change dependent changes is used for illustrating the development of water temperature in terms of lakes representative of various habitats.

Lake Constance is a lake typical of biomes found in the Alps and their foothills. It receives its water largely from its alpine catchment area via inflows from the alpine Rhine and the Bregenzer Aach. Besides, summer water levels of alpine rivers are essential in controlling the lake’s temperature. Lake Constance is the third-largest lake in Central Europe; consequently, it is characterised by a very distinct layering of temperatures.

The Saidenbach Reservoir is representative for the group of layered, calcium-rich upland lakes which have a relatively extensive catchment area. Owing to the prevailing regular operation of this reservoir, it is possible to exclude any relevant anthropogenic, i.e. operational effects on its surface temperature. The Saidenbach Reservoir can therefore be used in principle to illustrate climate-change dependent temperature changes.

The lakes of the North German Plain are characterised by warmer and calcium-rich inflows. There are some very shallow river lakes such as the Dahme, a tributary of the Spree, and numerous polymictic lakes such as the Große Müggelsee, which owing to their relatively low depth do not show any prolonged thermal layering phases. On the other hand, there are lakes shaped by glacial periods which are distinctly deeper. Lake Stechlin, with a depth of 70 metres, is the deepest lake in Brandenburg. This is why stable layering takes place in the course of the year. When illustrating the temperature progress for Lake Stechlin, it was taken into account that until 1990 the lake was to a great extent affected by the cooling cycles of the Rheinsberg Nuclear Energy Plant. The temperature chart was adjusted accordingly.

The water temperatures of all lakes analysed here show significant increases during the observation periods, for both the annual mean and the seasonal mean between March and October. In contrast with the 2015 Monitoring Report in which the indicator was calculated on the basis of the mean of the two warmest months, the mean temperatures here indicate a very clear trend. In respect of Lake Constance the seasonal increase between 1971 and 2017 for instance amounted to two degrees, while in the Saidenbach Reservoir an increase by as high as three degrees was calculated.



WW-I-6: Start of the spring algal bloom in standingwaters

FI-I-2: Occurrence of thermophilic species in inland waters



Management objectives for surface waters: good ecological and chemical condition / good ecological and chemical potential (WHG, §27).