Substantial warming of Central European mountain rivers under climate change.

Water bodies around the world are currently warming with unprecedented rates since observations started, but warming occurs highly variable among ecoregions. So far, mountain rivers were expected to experience attenuated warming due to cold water input from snow or ice. However, air temperatures in mountain areas are increasing faster than the global average, and therefore warming effects are expected for cold riverine ecosystems. In decomposing multi-decadal water temperature data of two Central European mountain rivers with different discharge and water source regime, this work identified so far unreported (a) long-term warming trends (with river-size dependent rates between +0.24 and +0.44 °C decade-1); but also (b) seasonal shifts with both rivers warming not only during summer, but also in winter months (i.e., up to +0.52 °C decade-1 in November); (c) significantly increasing minimum and maximum temperatures (e.g., temperatures in a larger river no longer reach freezing point since 1996 and maximum temperatures increased at rates between +0.4 and +0.7 °C decade-1); and (d) an expanding of warm-water periods during recent decades in these ecosystems. Our results show a substantial warming effect of mountain rivers with significant month-specific warming rates not only during summer but also in winter, suggesting that mountain river phenology continues to change with ongoing atmospheric warming. Furthermore, this work demonstrates that apart from a general warming, also seasonal shifts, changes in extreme temperatures, and expanding warm periods will play a role for ecological components of mountain rivers and should be considered in climate change assessments and mitigation management. Supplementary Information: The online version contains supplementary material available at 10.1007/s10113-023-02037-y.

Diet composition and quality of a Salmo trutta (L.) population stocked in a high mountain lake since the Middle Ages.

The introduction of fish into mountain lakes typically leads to profound ecological changes within the food web, but its consequences depend on the dietary preferences of fish and on the resistance of prey organisms against predation. Here we used stable isotopes and fatty acid analyses in combination with the traditional stomach content analysis to examine the diet of an allopatric population of Salmo trutta, which has originally been stocked during the Middle Ages in an alpine lake, and to identify what components of the food web are more affected. The results from stable isotopes and fatty acids indicated that planktonic and benthic food sources, in particular chironomids larvae, were the most important prey items all year round. Airborne terrestrial insects made most of the stomach content during the ice-free period, but their stable isotope and fatty acids values did not match up with those in fish, suggesting a minor role for fish nutrition. Copepods were relevant as fish diet only during the ice-covered period. In contrast to the stable isotope values of the fish muscle tissue, those of the liver, which reflect potentially short-term changes in diet, were significantly different between the ice-covered and ice-free period. Fatty acid analysis revealed that polyunsaturated fatty acids contents of chironomids, copepods, and chydorids contributed similarly to fish diet. Overall, our results suggest that the introduction of this fish species has decreased the lake-to-land resource transfer by reducing the abundance of emerging midges and that the population is food-limited as indicated by its low condition factor. This field study eventually acts as a reference for possible future reintroduction efforts, as this population is one of few existing in Europe with pure Danubian origin.

A first attempt at a holistic analysis of various influencing factors on the fish fauna in the Eastern European Alps.

Fish are some of the most threatened vertebrates in the world due to their often-sensitive response to environmental changes. Major land-use changes in the European Alps have direct and indirect impacts on fish communities, and these impacts are expected to increase in the future. Therefore, the identification of factors that are associated with the distribution of fish communities is of great importance to develop guidelines for management, precautions and sustainable use of running waters. In this study, the relationship of various factors - landscape structure and land use, topography, morphology, hydrology, physical and chemical water characteristics, hormonally active substances, pesticides, food availability, fisheries and piscivores birds - with fish assemblages are analysed. Field data from 81 stream sections from 2001 metres above sea level (m.a.s.l.) down to 219 m.a.s.l. are used in the study. The results reveal that the number of fish species has a strong association with topographic characteristics in the catchment area as well as with landscape configuration. Fish abundance and biomass are associated mostly with land-use type, hydrology, morphology as well as topography. In addition, there are indirect connections between fish abundance and biomass through land-use type, topography, water properties and hydromorphology. The results clearly indicate that not a single factor, but a multitude of factors are associated with the fish communities in the Eastern European Alps.

Salinization of Alpine rivers during winter months.

Human-induced (i.e., secondary) salinization affects aquatic biodiversity and ecosystem functioning worldwide. While agriculture or resource extraction are the main drivers of secondary salinization in arid and semi-arid regions of the world, the application of deicing road salt in winter can be an important source of salts entering freshwaters in cold regions. Alpine rivers are probably affected by salinization, especially in highly populated mountain regions, although this remains to be explored. In this study, we analyzed multi-year conductance time series from four rivers in the European Alps and demonstrated that the application of deicing road salt is linked to peaking rivers' salinity levels during late winter/early spring. Especially in small catchments with more urban surfaces close to the rivers, conductance increased during constant low-flow periods in late winter and was less correlated with discharge than in summer. Thus, our results suggest that small rivers highly connected to urban infrastructures are prone to considerable salinity peaks during late winter/early spring. Given the low natural level of salinities in Alpine rivers, the aquatic biodiversity might be significantly affected by the recorded changes in conductance, with potential consequences on ecosystem functioning. Thereby, we urge the research community to assess the impact of secondary salinization in Alpine rivers and call for an implementation of management practices to prevent the degradation of these pristine and valuable ecosystems.

The interplay between total mercury, methylmercury and dissolved organic matter in fluvial systems: A latitudinal study across Europe.

Large-scale studies are needed to identify the drivers of total mercury (THg) and monomethyl-mercury (MeHg) concentrations in aquatic ecosystems. Studies attempting to link dissolved organic matter (DOM) to levels of THg or MeHg are few and geographically constrained. Additionally, stream and river systems have been understudied as compared to lakes. Hence, the aim of this study was to examine the influence of DOM concentration and composition, morphological descriptors, land uses and water chemistry on THg and MeHg concentrations and the percentage of THg as MeHg (%MeHg) in 29 streams across Europe spanning from 41°N to 64 °N. THg concentrations (0.06-2.78 ng L-1) were highest in streams characterized by DOM with a high terrestrial soil signature and low nutrient content. MeHg concentrations (7.8-159 pg L-1) varied non-systematically across systems. Relationships between DOM bulk characteristics and THg and MeHg suggest that while soil derived DOM inputs control THg concentrations, autochthonous DOM (aquatically produced) and the availability of electron acceptors for Hg methylating microorganisms (e.g. sulfate) drive %MeHg and potentially MeHg concentration. Overall, these results highlight the large spatial variability in THg and MeHg concentrations at the European scale, and underscore the importance of DOM composition on mercury cycling in fluvial systems.