Climate

Climate influences physical, chemical, and biological processes in and around lakes. Although much of Europe lies in the northern latitudes, the relatively warm seas that border the continent give most of Central and Western Europe a moderate climate, with cool winters and mild summers. From approximately central Poland eastward, the maritime effects are reduced, and cooler, drier, and more continental conditions prevail (Figure 2). Temperature differences between summer and winter are low, about 15 °C, in the coastal northwest regions of Europe but they are high in the northeast regions, especially north of the Caspian Sea. Here the temperature difference between summer and winter can be as high as 70 ° C. Because of the pronounced seasonal cycles across Europe, many lakes are dimictic, i.e., they undergo two periods of mixing, one in spring and one in autumn, and are thermally stratified in summer and winter. However, large lakes at the northern and southern slopes of the Alps, such as Lake Garda, Lake Leman, and Lake Constance, are usually warm monomictic, i.e., they mix only once a year during winter. During very cold winters these large lakes can switch to a dimictic regime, e.g., Lake Constance became dimictic in February 1963 during a very cold North Atlantic Oscillation period - the only occasion in the twentieth century.

| Northern Europe

■ Western Europe

■ Eastern Europe

| Southern Europe

| Northern Europe

■ Western Europe

■ Eastern Europe

| Southern Europe

Figure 1 Regions of Europe as delineated by the United Nations. The map has been taken from http://en.wikipedia.org/wiki/Image: Location-Europe-UNsubregions.png.

During winter, lakes especially in the north and the northeast and in the alpine regions are covered by ice. Ice cover and frozen ground in the drainage area diminishes the renewal of water from the catchment, decreases the underwater light conditions, and hampers water mixing. This influences the nutrient recycling and all biological processes in lakes. Toward the south of Europe an ice cover is less common, and lakes usually mix during winter. As soon as surface waters heat more rapidly than the heat is distributed by mixing, lakes become thermally stratified. Thermal stratification is one of the most important processes affecting the chemistry and biology of lake waters. Deep waters stay cool during summer, and are warmer than surface water in winter, with important implications for temperature-dependent biological processes. Even more importantly, the thermal stratification results in limited exchange of substances between deep (hypolimnion) and surface (epilimnion) waters. This frequently results in anoxic water in the hypolimnion, and depletion of nutrients in the epilim-nion, both strongly influencing biological processes. One of the consequences of thermal stratification in eutrophied waters is the occurrence of harmful algal blooms. The period of thermal stratification ends when water movements, usually induced by wind, cause a complete water column mixing.

The strong seasonal pattern of temperature does usually not apply to precipitation. Especially in the western part of Europe the prevailing westerly winds, warmed in part by passing over the North Atlantic Drift ocean current, bring precipitation throughout most of the year. Large parts of Europe receive between 500 and 1500 mm of precipitation per year (Figure 2) and most areas of Europe are classified as either mild and humid or cold and humid. In the Mediterranean region, almost all rainfall occurs in winter and the summer months are usually hot and dry. This is in contrast with the situation in the northern part of the Alps, where most of the precipitation

Figure 2 Present annual mean air temperature (a) and annual mean precipitation (b) in Europe. The map has been created using the European Geo-Portal at http://eu-geoportal.jrc.it.

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Figure 2 Present annual mean air temperature (a) and annual mean precipitation (b) in Europe. The map has been created using the European Geo-Portal at http://eu-geoportal.jrc.it.

falls in the summer months while winters are usually dry. The difference in precipitation, evaporation, and most importantly, discharge is very pronounced. In the Alps, the average discharge is 980 mm m~2, while it is only 270 mm m~2 in Europe without the Alps.

The amount and intensity of precipitation influences the flow of water that transports nutrients and other substances into lake waters. This flow is essential for chemical and biological processes both in the catchment area and in the lakes. The availability of water determines to a large extent the vegetation in the catchment area, which subsequently influences the nutrient flow into the lakes. Human-induced climate change will alter temperature and precipitation patterns, and thereby lake ecosystems. Increasing risks for floods, droughts, and heat waves have been identified as major threats of a changing climate.

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