Årstidsvariationer i sjöar vid klimatförändringar / Seasonal variations in lakes in northern Sweden in a changing climate

Lakes are suitable sentinels of climate change because they constitute a well defined ecosystem and they respond quickly to changes in climate. Many lakes in the northern hemisphere are covered by ice during winter and they are dimictic, which means that the water circulate two times each year. The ice not only prevents gas exchanges or wind mixing, the ice, together with snow, are also a large obstacles that shortens the growing period for all the producers in the lake by reducing light penetration. Spring bloom of phytoplankton along with onset of stratification might occur earlier in the season if the ice breaks up earlier. Climate warming will lead to higher temperatures and it will alter the mixing conditions as well as affect thermal stratification in lakes. All the lakes included in this study are situated in the southern parts of the region Norrland in Sweden. The average ice break-up between 2001 – 2010 occurs about 12 days earlier compared to the average ice break-up for the period 1916 – 1925 and 11 days earlier compared to 1976 – 1985. The mean annual air temperature in Junsele shows a trend in which the air temperature is getting warmer with time. Higher air temperature shows a correlation with changes in ice phenology, where the lakes experience a shortened ice cover duration.