23 Interactions between solar ultraviolet radiation and climatic warming in alpine lakes

Abstract

Abstract Solar ultraviolet (UV) radiation has been a crucial environmental factor since the origin of alpine lakes (i.e., lakes above treeline) ca. 10,000 years ago, because those ecosystems receive high solar UV fluxes due to a thinner ozone column and usually lower aerosol scattering. Another important change that takes place with increasing elevation is the reduction of in-lake colored dissolved organic carbon (cDOC) concentrations, which reflect the dominance of small and sparsely vegetated watersheds at high altitude. Concomitantly with the reduction in cDOC concentrations, there is a significant decrease in the UV absorption capacity of this carbon pool (i.e., reduced content in humic substances). The consequence of these qualitative and quantitative changes of cDOC is that most high-mountain lakes (with the exception of those fed by glacier streams) rank among the most UV-transparent aquatic systems, with UVB (290–320 nm wavelength) penetration depths of up to ca. 30 m. Climatic warming has been particularly pronounced in mountain regions. For example, in the northern Alps, the mean air temperature has increased by 1°C since 1985. Several environmental changes associated with climatic warming, such as glacier retreat and the timing, extent and duration of ice and snow cover, are already taking place in several mountainous regions. Alpine lakes are particularly sensitive to climatic variability because several crucial ecosystem processes are directly affected by those changes. Based on observational analysis, possible changes in alpine lakes are presented. On a short timescale, the decrease in the snow- and ice-cover duration will lead to an increase in UV stress on the ecosystem. However, on a longer timescale, the increase in mean air temperature could favor the development of terrestrial vegetation in the catchment, and result in an increase of cDOC export to alpine lakes, particularly to those located near the treeline. As a consequence, underwater UV transparency and UV stress on these ecosystems will be reduced. This scenario is supported by the finding that the alpine-nival flora in some mountainous regions, such as in the western Austrian and eastern Swiss Alps has moved upward in recent years. On the other hand, glacier retreat may cause turbid lakes to become more UV transparent. The magnitude of these changes will be more important for underwater UVB exposure levels than those expected from ozone reduction at mid-latitudes.