The role of fatty acids in changing alpine stream ecosystems

Alpine streams are crucial for clear water supply in lowland areas, which requires functioning freshwater communities. Current environmental changes in mountain regions, however, such as rising air and water temperature or the consequent glacier retreat, have drastic effects on organisms living in these aquatic ecosystems. While species composition of different groups of organisms are known to change significantly, functional effects on the interaction of these groups in food webs cannot be estimated. For example, different producers (green algae, cyanobacteria or diatoms) generally provide qualitatively different food for consumers in aquatic food webs (such as insect larvae), but projected environmental changes will cause shifts within these producer communities and thereby affect the quality of invertebrate food and thus the entire food web in alpine streams. In this project we therefore examine to what extent climate-induced environmental changes affect the dietary quality of food sources in terms of lipid and essential fatty acid content, and subsequently impact higher levels in alpine stream food webs. This multidisciplinary work aims at a first quantification of dietary energy transfer in changing alpine stream food webs. By scrutinizing producers and consumers regarding structural and molecular composition across a continuum of catchments with varying degree of glaciation, we are testing the hypothesis that the already known decline in diatoms and golden algae due to decreasing glacial meltwater contribution causes a reduced availability of essential fatty acids in the consumers diet. In addition, we examine whether different availabilities of lipids and essential fatty acids is critical to the composition and characteristics (e.g., body mass) of invertebrate consumers in these waters. In addition, experiments will allow quantifying how glacier retreat and concomitant changes of in-stream conditions affect the production of biomass and essential fatty acids in mountain streams. This first consideration of biochemical dietary quality in alpine stream ecology in cooperation with national partners, and the link of molecular and biochemical methods with trait analysis, will significantly improve our understanding of animal survival mechanisms in harsh habitats. This study will also reveal how climate-related environmental changes affect the nutritional quality of mountain stream food webs. And since lowland rivers in and around mountain ranges are mostly fed by mountain streams, the findings of this project will be of uppermost interest for the estimation and mitigation of climate impacts on biomass production in fish habitats.

How glacier retreat changes food production and survival in alpine streams Alpine glacier-fed streams are among extreme freshwater ecosystems in Europe and are currently undergoing rapid change. This project shows that glacier retreat does not only alter these systems physically, but also fundamentally affects their ecological functioning by changing the availability of food resources and the transfer from primary producers to consumers. The study was conducted in high-altitude catchments of the Central Alps (Ötztal region and Hohe Tauern National Park), where streams with varying glacier influence provide a natural laboratory for anticipating future alpine stream conditions with less or no glacial input. Central was the functional link between streambed food producers (algae and bacterial biofilms) and their invertebrate consumers, mainly aquatic insect larvae. Rather than quantifying biomass alone, analyses targeted food quality using essential omega-3 and omega-6 fatty acids, which are synthesized at the base of the food web, transferred across trophic levels, and are critical for growth, fitness and reproduction. Results reveal a partly opposing response to declining glacier influence. Reduced turbidity and sediment loads promote biofilm growth, but biofilm composition shifts towards a higher contribution of cyanobacteria. Although they add biomass, cyanobacteria provide few nutritionally valuable fatty acids and are poorly utilized by consumers, weakening the energetic basis of the food web despite higher apparent productivity. At the consumer level, unexpected adaptive strategies were identified. Highly specialized insect larvae, particularly dominant midges of the genus Diamesa, appear capable of synthesizing certain long-chain fatty acids, partially compensating for reduced dietary quality. This challenges assumptions from lowland streams and provides new insights into survival in extreme alpine environments. In addition, the project examined broader environmental change in mountain streams, revealing strong increases in water temperatures especially over the past ten years. Further, Europe-wide trait-based comparisons revealed a disproportionately high vulnerability of aquatic invertebrates to climate change consequences in the European Alps. These results allowed identifying potential winners, losers and indicator species of expected changes and estimating of community turnovers. The project leader received the City of Innsbruck Prize for Scientific Research in 2023 for these two publications. The project was conducted at the University of Innsbruck in cooperation with the research group led by Martin J. Kainz at WasserCluster Lunz, specializing in fatty-acid analyses, and was complemented by a review paper identifying key open questions on the role of food quality in mountain streams. Overall, the results show that glacier retreat alters alpine streams not only visibly but also in how they function, which is of interest since food quality shifts in headwaters propagate along food webs into larger lowland rivers. Further, it contributes to a better understanding of the ecological impacts of climate change in the Alpine region.