Consequences of glacier retreat for the structure and function of alpine lakes (BACK-ALP)

The rapid current retreat of glaciers constitutes one of the most prominent signs of climate change. Glacier retreat enlarges existing lakes and at the same time is creating new ones at the glacier terminus. A remarkable characteristic of glacier-fed lakes is their high content of suspended minerogenic particles, so-called "glacial flour". The high minerogenic turbidity constitutes a real challenge for filtering planktonic groups such as cladocerans and leads to unfavorable conditions for phytoplankton primary production. The current rapid glacier retreat is expected to change lake transparency and ecosystem productivity. For example, transparency could be further reduced during rapid melt events/runoff, whereas in lakes where the glacier terminus loose connectivity with the basin, transparency will increase. Overall, little is known on the ecology of glacier-fed lakes even if they are at the origin of most lakes on earth. The overarching objective of this proposal is to understand the consequences of glacier retreat for the structure and function of the biota of alpine lakes and to understand the governing ecological conditions in glacier-fed lakes, particularly of those recently created. In particular, we will concentrate on the analysis of the structure and function of microbial food web and on how this relates to the expected change in prokaryotic diversity along a turbidity gradient. We will use a combination of experimental and field work using state-of-the-art methods such as next generation sequencing to analyse prokaryotic diversity or ultrahigh-resolution Fourier-transform ion cyclotron resonance mass spectrometry to assess the molecular diversity of dissolved organic matter. For this study, we will take advantage of the existence of a series of lakes in the Tyrolean Alps that originated from the same glacier and range from highly turbid (e.g., recently created pro-glacial lake) to lakes that became already transparent. In addition, work will be done in Chilean Andes where glacier retreat represents a major socio-economical problem. In this deep lake, glacial-flour enters as surface runoff causing a localized horizontal turbidity gradient. Through a multidisciplinary approach, our project will yield critical data on how glacier retreat is affecting biodiversity, biogeochemical cycles, and ecosystem function. We envisage to obtain an exciting perspective of the ecology of lakes at their origin (i.e., in geological time scale) and of their microbial diversity, as well as of its main drivers.

The current melting of glaciers and ice sheets is a consequence of climatic change and their turbid meltwaters are creating and enlarging many new proglacial and ice-contact lakes around the world. Paradoxically, very little was known on the ecology of turbid glacier-fed fed lakes even though they are analogs of the most common type of lakes on Earth. In this project, the overarching objective was to investigate the ecology of those lakes in the Alps and in west Greenland mainly focusing on planktonic organisms. Glacier-fed turbid mountain lakes were very different in physical and chemical characteristics from those clear ones, which have been very well studied. For instance, instead of having the typical two mixing periods during the year, glacial turbid lakes mixed several times during the ice-free season. Another remarkable characteristic of lakes receiving the discharge of glacial meltwaters is their high concentration of mineral particles, so-called glacial or rock flour that poses a challenge for filter-feeding planktonic organisms that were absent in the most turbid systems. Thus, at the origin of the lakes, the food-web structure in those highly turbid meltwater lakes is truncated and mainly a microbial world. Unexpectedly, we found a higher diversity of microbes in glacier-fed lakes than in those that lost connectivity to the glacier and became transparent. Also unexpected was the high abundance and richness of protists (algae, ciliates) in the glacier-fed lakes of intermediate turbidity. Likely, those lakes offer more suitable environmental conditions and resource niches for protists and prokaryotes than clear ones, where for example the potential for negative effects of solar UV radiation is higher. Glacial meltwaters turned also to be a source of inorganic and organic nutrients that seems to improve growth conditions in those otherwise oligotrophic lakes. Another finding was the existence of large shifts in bacterial community structure between glacier-fed turbid and clear lakes. Overall, the results obtained in this project contributed to increase awareness on the direct effects of climate change and on the relative analogy of the newly created lakes to those at the origin of most lakes on Earth. At the same time, provided information on changes in biodiversity in lakes under the near future scenario of mountains without permanent ice cover.