WW-I-10: Water temperature of watercourses | Umweltbundesamt

WW-I-10: Water temperature of watercourses

In recent decades the temperatures in watercourses have risen significantly in all fish regions. The greyling region is the only one for which, so far, there is no clear trend discernible. Anyway, this time series is too short to make any statements regarding long-term trends. The extremely high temperatures in 2018 and the associated lack of oxygen led to fish dying in many German water bodies, including the Upper Rhine basin.

Rivers are getting warmer

As in the case of lakes, watercourses too are indicating increases in water temperatures owing to higher air temperatures. There are indeed some direct anthropogenic influences such as infeeds of cooling or waste water, also the removal of trees growing on riverbanks and cases of increasing urbanisation. However, targeted investigations involving the river Rhine have revealed that water temperature increases despite reduced infeeds of warm water⁶⁴. Rising water temperatures are also found to be connected with the water volume present in water bodies. At low water levels (cf. WW-I-6) the risk of excessive warming resulting from a diminished water volume is increased even further.

The temperature of the water body controls and synchronises many processes involving plant life and other organisms in the water body, which makes temperature particularly important for the ecological functions concerned. Water temperature is of direct importance to the living conditions of aquatic organisms the majority of which are poikilothermic. Besides, water temperature is part of a complex effects relationship in terms of the chemistry of water bodies. Apart from other negative effects, the reduced solubility of oxygen in water worsens respiration conditions.

Individual parts of the course of streams or rivers are differentiated by individual zones which differ according to the occurrence of characteristic species of fish. These are therefore sometimes termed fish regions. The typically cool trout region is located in the upper regions of a river where the current is usually very strong and the water passes through gravel and where larger stones are overturned by the current and enriched with oxygen. This is followed by the greyling region with similar conditions albeit more intensive vegetation, followed in turn by the region characterised by cyprinids. In the barbel region the watercourse has become wider, with the current becoming weaker. The (lower) bream region is the most species-rich fish region, in addition characterised by luscious vegetation. The chub-flounder region is already classed as the area of brackish water, and this is the last of the fish regions. Owing to its location in the river delta close to the sea, this region is already subject to the tides.

As in the case of different types of lakes, there is no point in calculating the mean for all watercourses and fish regions. Consequently, the indicator illustrates the evaluation of individual fish regions separately.

The indicator is based on temperature measuring data of 43 gauging stations for measuring water-quality and levels distributed across Germany’s fish regions. Important criteria in the selection of gauging stations included sufficiently long, gap-free time series based on daily sampling. Furthermore, the gauging stations were chosen well away from any direct influence of dams and anthropogenic infeeds of warmth, such as the discharge of cooling water, process water and waste water.

As in the case of indicators for the water level of lakes (cf. Indicator WW-I-7) and the water temperature of lakes (cf. Indicator WW-I-8) this indicator, too, is illustrated by means of indexed values. It makes no sense to calculate an absolute mean temperature value, as the individual gauging stations indicate different temperature levels, even within individual fish regions according to the specific local water morphology, natural environments and climatic conditions. In the process of indexing, a mean value is calculated for each measuring point, using the deviations from the index year 2017, for which the temperature value is set at ‘0’. The values of these deviations are used to calculate the mean value across all gauging stations and fish regions respectively. 2017 is the earliest year for which data are available from all the gauging stations considered.

The water temperatures have been rising significantly in all fish regions. The greyling region is the only region for which there is no clear trend discernible. However, this may be due to the relatively short time series so far.

The comparatively low values recorded in 2010 and 1996 can be attributed to the relatively cool weather pattern prevailing in those years. For example, the annual mean temperature in 2010 of 7.8 °C was distinctly below 2011 with 9.6 °C. 1996 with an annual mean temperature of 7.2 °C was the coldest year in the decade of 1991–2000. The high values in 2018 and 2019 were due in part to high summer temperatures and low water levels.

So far the indicator, in its present design, does not permit any statements on the exceedance of critical temperatures (for instance in terms of tolerability to fish). The surface water regulations – differentiated by fish regions – provide benchmarks for maximum watercourse temperatures, with the objective to achieve a good and very good ecological condition. In order to maintain a very good ecological condition 18 °C is considered the maximum summer temperature for the trout and greyling regions, while in the regions characterised by cyprinids and barbels it is 20 °C, and in the bream and chub-flounder regions it is 25 °C.⁶⁵ In 2018 these critical temperatures were exceeded in many locations. For instance, in August 2018, the Rhine became overheated to 28 °C in places. In many of Germany’s watercourses, such as the Upper Rhine, fish died from lack of oxygen. In the case of massive losses of fish in the river Oder from the end of July until August 2022, the suddenly intensified concentration of salt, irradiation by the sun as well as high temperatures contributed to a growth burst in the populations of the poisonous brackish-water alga Prymnesium parvum. This example demonstrates the fatal chain reactions that can occur in ecosystems.

The situation is particularly critical in cases where the increase in water temperatures of water bodies is exacerbated by distinctly anthropogenic activities and when the watercourses lack structure and shading. In natural or near-natural water bodies fish can seek sanctuary in deeper and more shaded layers of water. Besides, in water bodies with natural watercourse dynamics, more oxygen is able to penetrate thanks to currents and turbulences. At high concentrations of nutrients and pollutants, thermophilic and potentially toxic algae are able to reproduce thus affecting aquatic organisms.

^{64 - IKSR – Internationale Kommission zum Schutz des Rheins 2014: Abschätzungen der Folgen des Klimawandels auf die Entwicklung zukünftiger Rheinwassertemperaturen auf Basis von Klimaszenarien – Kurzbericht. IKSR Bericht Nr. 213, 6 pp. https://www.iksr.org/fileadmin/user_upload/DKDM/Dokumente/Fachberichte/DE/rp_De_0213.pdf}

^{65 - OGewVO: Oberflächengewässerverordnung vom 20. Juni 2016 (BGBl. I S. 1373), die zuletzt durch Artikel 2 Absatz 4 des Gesetzes vom 9. Dezember 2020 (BGBl. I S. 2873) geändert worden ist.}