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Perennially frozen and glacierized high mountain areas react very sensitively to changes in atmospheric temperature (Haeberli & Beniston, 1998). As a consequence, since the middle of 19th century, i.e. since the end of the Little Ice Age, the glaciers in the European Alps have lost about 30 to 40% of surface area and ca. 50% of their original volume (Haeberli & Hölzle, 1995). Since 1980, an additional loss of this remaining ice volume was ca. 25% (Haeberli, 2005). Less visible but also very significant are changes in Alpine permafrost. During 20th century century, it has warmed by about 0.5 to 0.8Â°C in the upper tens of meters (Harris et al., 2003; Harris & Haeberli, 2003). The most direct information about the climate-related changes in mountain permafrost derives from borehole temperature measurements. The longest, high-resolution time series of borehole temperatures in the European Alps is available from permafrost in the active rock glacier Murtèl, Grisons Alps, Switzerland (Haeberli & Gruber, 2009). The general trend observed there since 1987 is permafrost warming by ca. 0.4Â°C per decade at 10 m depth, and roughly twice as much for the summer temperatures in the active layer (Haeberli & Gruber, 2009). Since the end of the Little Ice Age, the lower permafrost limit in the European Alps is estimated to have risen vertically by about 1 m per year (Frauenfelder, 2005).
Permafrost, or permanently frozen ground, consists of lithospheric material (soil, sediment, or rock) that remains at or below 0Â°C for at least two years (van Everdingen, 2005). By definition, glaciers are not permafrost. In the European Alps, a mean annual air temperature below -3Â°C can be used for first-order classification of altitudinal belts that have significant amounts of permafrost (Gruber & Haeberli, 2009). Permafrost exists in different forms, such as steep bedrock, rock glaciers, and debris deposited by glaciers, and can contain over 50% ice (French, 2007). Most permafrost areas experience seasonal thaw, during which surface temperatures rise above the melting point and a certain volume of material directly beneath the surface ('active layer') thaws. A thickness of active layer is usually in the range of 0.5-8 m (Haeberli et al., 2006). Discharge measurements suggest that meltwater from areas underlain by permafrost represents an important share (up to ca. 30%) of the river discharge during summer (Schrott, 1996).
Permafrost is invisible because it is a thermal phenomenon. It usually lies beneath an active layer and its reliable detection requires temperature measurements at greater depths using core drilling into permafrost. The difficulty in detecting permafrost and expensive access hamper the progress in permafrost research in mountain areas. As a consequence, the warming-related changes of perennially frozen mountain slopes have been studied for little more than a decade only (Haeberli & Gruber, 2009).
Alpine rock glaciers as a form of mountain permafrost
Rock glaciers and other creep phenomena often visually indicate the presence of permafrost in mountain areas: they form distinct landforms caused by the slow deformation of cohesive, ice-rich sediments (Haeberli et al., 2006). Rock glaciers can be categorized into three types depending on activity and ice content: (1) active, (2) inactive, and (3) relict (Barsch, 1977). Active rock glaciers show actual movement due to interstitial ice and/or core ice. Inactive rock glaciers show no movement but still contain ice. Relict rock glaciers are free of ice; they sometimes show collapse structures but no evidence of movement. In many catchments of high mountain regions where permafrost occurs, rock glaciers cover larger areas than that of glacier ice. Rock glaciers are one of the most widespread forms of permafrost in the European Alps. For example, results of a preliminary inventory showed that there are 1 594 rock glaciers in the Italian Alps (Guglielmin & Smiraglia, 1998). Later, a comprehensive inventory of rock glaciers in South Tyrol resulted in 1 778 only in the eastern sector of the Italian Alps (Monreal & Stötter, 2010).
Hydrological significance of mountain glacial and periglacial environments
Mountains play a central role in collecting and storing the most vital element - fresh water. The streams and rivers that flow from mountain slopes are living bonds connecting mountain and lowland communities. More than half the world's population relies on the fresh water that flows from mountains. The European Alps form the watershed of the Mediterranean Sea, the North Sea, and the Black Sea and are often called the "water towers" of Europe (Schwaiger, 2007). The Alps provide not only enormous quantities of water but also water of excellent quality. Glaciers play an important role in the hydrologic system of the Alps by storing water during cold and wet periods and releasing water in hot and dry phases. The hydrological contribution of rock glacier ice melt is considerably lower, because ice loss in a rock glacier is orders of magnitude slower than from a glacier (Arenson & Jakob, 2010). Rock glacier ice can be old (hundreds to several thousands of years), in contrast to glacier ice, where a continuous mass exchange occurs at much shorter temporal scales (French, 2007). In the current period of marked glacier recession and permafrost degradation, in spite of the differences in hydrological contribution, meltwater discharge from both glaciers and rock glaciers can sometimes cause serious poisoning of freshwater ecosystems by inorganic and organic pollutants.
High-alpine lakes: glacial meltwater input and ecotoxicological risk
Continued, if not accelerated, warming causes enhanced water exchange of high-alpine lakes due to a strong meltwater influx from glaciers and permafrost. In a situation like this, glaciers and glacial meltwater accordingly may represent a secondary source of airborne anthropogenic pollutants deposited to glaciers in earlier time (Schwikowski & Eichler, 2010). For instance, persistent organic pollutants (POPs) are known to accumulate in cold environments because of progressive volatilization from warm source regions and condensation in colder regions (Kallenborn, 2006; Westgate & Wania, 2010). Palaeoecological studies in the Swiss Alps have shown that melting glaciers may represent a secondary source of POPs that were previously deposited to and incorporated into glaciers and are now discharged into high-alpine lakes due to the accelerated melting of glaciers (Bogdal et al., 2009, 2010; Schmid et al., 2011).
In contrast to these examples of organic pollutants of definitely anthropogenic origin, a study in the Italian Alps has shown that pronounced changes in water chemistry of high-alpine lakes may also be caused by meltwater discharge from active rock glaciers into the lakes (Thies et al., 2007). In, Rasass See, a high-alpine lake with an active rock glacier in the catchment, concentrations of the most abundant ions magnesium, sulfate and calcium have reached the 68-fold, 26- and 13-fold values, respectively, during the last two decades. In addition, unexpected high nickel concentrations exceeding the limit for drinking water by more than one order of magnitude have been found in this lake recently. Nickel and other heavy metals are amongst the toxic contaminants that may concentrate through the food chains at its top; high concentrations of heavy metals in water and/or sediments can lead to a number of disorders in aquatic ecosystems (Moore & Ramamoorthy, 1983). Since the adjacent pond, not affected by the rock glacier, has negligible metal concentrations, the current high value of nickel in Rasass See cannot be attributed to catchment geology but rather to meltwaters from an active rock glacier (Thies et al., 2007). These high-alpine water bodies, Rasass See and the adjacent pond, are the study sites of the present project and will be discussed in detail below.
Climate modellers predict that under different global warming scenarios, by 2100 the near-surface permafrost area will shrink by ca. 90% in the Northern Hemisphere (Lawrence & Slater, 2005). Despite the extent and speed of these changes, very little is known about how melting permafrost will affect global geochemical cycles and freshwater ecosystems. Zones of high ion concentration in areas of ice-rich permafrost are a reservoir of chemicals that can potentially be transferred to fresh waters during thawing. A recent study of arctic lakes (Kokelj et al., 2005, 2009; Mesquita et al., 2008, 2010) has shown that lakes disturbed by retrogressive permafrost thaw slumps have sediments richer in calcium, magnesium and strontium, and greater transparency of the water column than undisturbed lakes. Interestingly, a massive increase calcium and magnesium concentrations has also been observed in two alpine lakes in Italy (Rasass See) and Austria (Schwarzsee ob Sölden) under the influence of melting rock glaciers (Thies et al., 2007). In addition, the results suggest that retrogressive permafrost slumping can significantly affect food webs in lakes through an increase in lake's water clarity and a subsequent increase in biomass of submerged macrophytes (especially aquatic mosses) and benthic invertebrates (Mesquita et al., 2008, 2010).
1.2 TRACE METALS: THEIR FATES AND EFFECTS IN FRESHWATER ENVIRONMENT
Trace metals (Ni, Zn, Mn, etc) play an important role in various biological processes as essential cofactors. However, when their concentration exceeds metabolic requirements, they become harmful. Weathering of minerals, industrial effluents, atmospheric precipitation and nonpoint discharges are important sources of high concentrations of trace elements in aquatic ecosystems. In aquatic environments, sediments have the capacity to accumulate and integrate low concentrations of trace elements in water, and can store toxicants after the original sources of contamination are eliminated. Further, metals can enter the food chain and increase in concentration from the environment to the first consumer (bioaccumulation). Some metals become more concentrated in successive trophic levels of a food web (biomagnification). Benthic macroinvertebrates feeding on sediments, algae, macrophyte tissues, and other invertebrates show great bioaccumulation and biomagnification rates (Markert & Friese, 2000).
In Europe, nickel is listed on the European Commission List II (Dangerous Substances Directive) and regulated through the Council of European Communities because of its toxicity, persistence, affinity for bioaccumulation, and potential for biomagnification. The World Health Organization classifies nickel compounds in Group 1 (human carcinogens) (Eisler, 2008). Nickel is also the metal that causes most frequent allergic reactions in humans. Some metals have been reported to produce synergistic and antagonistic interactions whenever in a mixture (Sprague, 1985). Zinc can interact with numerous chemicals, sometimes producing altered patterns of accumulation, metabolism, and toxicity. For example, nickel-zinc mixtures were additive in toxicity to marine copepods (Verriopoulos & Dimas, 1988). Besides, zinc bioavailability and toxicity to aquatic organisms are highest under conditions of low pH, low alkalinity, low dissolved oxygen, and elevated temperatures (Eisler, 2008).