Irregular changes in lake surface water temperature and ice cover in subalpine Lake Lunz, Austria

Abstract

Evidence is growing that the surface water temperature increases and the duration of ice cover has decreased in many lakes worldwide during the past few decades. Here, we present changes in surface water temperature and ice-cover duration of Lake Lunz from 1921 to 2015 and evaluate how fast these changes occur over time, in particular with respect to other lakes with similar long-term data series. Since 1921, the surface water temperature of this Austrian subalpine lake has increased by 0.8 °C, with the most intense increase recorded during the spring and summer months (~1–2 °C) and less during fall (~0.3 °C). The duration of full lake ice cover has decreased significantly since 1921. During the 1921–2015 study period, Lake Lunz was ice covered for 92 of 94 winters, with ice-free winters in 2006 and 2013. The interannual decrease of the lake ice cover corresponds to 0.36 days less ice cover per year from 1921 to 2015. Freeze dates now occur 17.4 d/100 yr later, and the ice breakup date change is similar to the freeze date, 17.6 d/100 yr earlier. Although the duration of interannual ice cover varied by only 8.3 days between 1921 and 1930, on 32 days between 2006 and 2015 this interannual variability increased by almost 4 times. Thus, the observed decrease of ice cover is accompanied by increasing interannual variability, during which the lake was fully ice covered. When comparing days of full ice cover to other lakes until 2005, Lake Lunz had a similar decline in ice-cover duration (1.85 d/decade), but a dramatic decrease in full ice cover during 2005 and 2015 indicates that the overall decline in lake ice-cover duration was sensitive to changes in lake ice cover during the past decade. This study emphasizes the importance of continuous and collaborative measurements in other (subalpine) lakes worldwide to test, and also manage, the sensitivity of lakes to ongoing effects of climate change.