Rationale

Justification for indicator selection

Since water temperature is mainly determined by heat exchange with the atmosphere, higher air temperatures lead to higher water temperatures. Higher water temperatures, particularly in standing waters and low-flow situations in rivers, will bring about changes in the physico-chemical condition of water bodies with subsequent impacts on biological conditions. This may have severe consequences for ecosystem structure and function as well as for water use and ecosystem services.
Impacts of increased water temperatures may also include more stable vertical stratification of deep lakes and increased oxygen depletion in lake bottoms (stratification in other lakes may become less stable), more frequent harmful algal blooms, reduced habitats for cold-water aquatic species, and increased incidence of temperature-dependent diseases.
Human intervention can only help freshwater ecosystems to adapt to increasing water temperature in a limited way, for example by reducing the pressures from other human activities such as pollution by nutrients and hazardous substances and pressure from hydromorphological modifications. Such actions may make the water bodies less vulnerable to stress resulting from higher water temperature. Additional pollution load reduction measures may be needed in river basin management plans to obtain good ecological status, as required by the Water Framework Directive.

Scientific references

• References Ambrosetti, W. and Barbanti, L., 1999. Deep water warming in lakes: an indicator of climate change. Journal of Limnology 58: 1-9. Anneville, O.; Gammeter, S. and Straile, D., 2005. Phosphorus decrease and climate variability: mediators of synchrony in phytoplankton changes among European peri-alpine lakes. Freshwater Biology 50: 1731- 1746. BUWAL, BWG, MeteoSchweiz, 2004. Auswirkungen des Hitzesommers 2003 auf die Gewässer.  Schriftenreihe Umwelt Nr. 369. Bern-Ittigen: Bundesamt für Umwelt, Wald und Landschaft, 174 p. Dabrowski, M.; Marszelewski, W.; and Skowron, R., 2004. The trends and dependencies between air and water temperatures in lakes in northern Poland from 1961-2000. Hydrology and Earth System Sciences 8: 79-87. Dokulil, M. T.; Jagsch, A.; George, G. D.; Anneville, O.; Jankowski, T.; Wahl, B.; Lenhart, B.; Blenckner, T. and Teubner, K., 2006. Twenty years of spatially coherent deepwater warming in lakes across Europe related to the North Atlantic Oscillation. Limnology and Oceanography 51: 2787-2793. Estonian Meteorological and Hydrological Institute, water temperature measurements, Võrtsjärv period 1947-2006 (non published). See also Nõges and Järvet, 2005. George G.; Hurley M. and Hewitt D., 2007. The impact of climate change on the physical characteristics of the larger lakes in the English Lake District. Freshwater Biology 52: 1647-1666. George, D. G. and Hurley, M. A., 2004. The influence of sampling frequency on the detection of long-term change in three lakes in the English Lake District. Aquatic Ecosystem Health and Management 7: 1-14. Hari, R. E.; Livingstone, D. M.; Siber, R.; Burkhardt-Holm, P. and Guttinger, H., 2006. Consequences of climatic change for water temperature and brown trout populations in Alpine rivers and streams. Global Change Biology 12: 10-26. Hohensinner, S.; Haidvogel, G.; Jungwirth, M., 2006. Natural landscape dynamics and human interferences: the Danube river landscape in the Austrian Machland 1715-1991. Rivers Run Through  Them. Landscapes in Environmental History. Annual Meeting of the American      Society for Environmental History, 29.3.-1.4.2006, St. Paul, Minnesota, USA. Livingstone, D. M., 1993. Lake oxygenation: Application of a one-box model with ice cover. Internationale Revue der Gesamten Hydrobiologie 78: 465-480. Malmaeus, J. M.; Blenckner, T.; Markensten, H. and Persson,  I., 2006. Lake phosphorus dynamics and climate warming: A mechanistic model approach. Ecological Modelling 190: 1-14. MNP, 2006. The effects of climate change in the Netherlands. (Bresser et al. (eds) Report from MNP available at http://www.mnp.nl/en/publications/2006/TheeffectsofclimatechangeintheNetherlands.html . PernaraviČiŪtĖ, B., 2004. The impact of climate change on thermal regime of Lithuanian lakes. Ekologija 2: 58-63. Rijkswaterstaat, measurements Rhine River at Lobith period 1908-2006 (unpublished). See also MNP, 2006.