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Most mountain lakes and their catchments are, due to their remoteness, less impacted by human actions than lakes in lowland regions. They are, therefore, often considered pristine systems. Nevertheless, even remote, uninhabited areas are polluted via atmospheric deposition of aerosols that transport acid rain, heavy metals, organic compounds, and nutrients.

Remote mountain lakes, whether found at high altitudes or high latitudes, usually appear to be in pristine condition. In particular, those lakes that are situated above or beyond the tree-line are rarely disturbed by agricultural or forestry practices and few if any people inhabit their catchments. However, recent research indicates that even the most remote lakes are impacted by atmospherically transported pollutants, and that greenhouse-gas forced climate change is beginning to have a significant influence on ecosystem functioning. UV-B radiation is also increasing and, in interaction with global warming, may already be changing biogeochemical cycles in many mountain lakes (Vinebrooke and Leavitt, this volume). All sites are subject to multiple stresses, and studies of the ecological response of mountain lakes to such combined stress need to consider interactions between all factors, both natural and anthropogenic. In this chapter, we consider acid deposition, toxic substances and climate change as the three main drivers of ecosystem change in high mountain lakes.

Many anthropogenic pollutants emitted to the atmosphere can be transported over large distances and affect ecosystems and human health thousands of kilometres from their source. In recent years, concern has grown over the increased contamination of remote areas, particularly the Arctic and mountain regions, and the unprecedented levels of pollutants observed in areas previously considered to be pristine. Atmospheric transport is one of the most efficient and rapid means by which toxic pollutants, including trace metals and persistent organic pollutants (POPs), can be transferred to remote areas. Understanding the pathways and mechanisms from source to sink is thus vitally important. Atmospheric transport models predict that sources of pollutants to remote areas are widespread and diverse, such that there are contributions from “local” and regional sources, as well as transboundary and even global inputs (e.g. Hanisch 1998).

The steep environmental gradients of mountain ecosystems over short distances reflect large gradients of several climatic parameters and hence provide excellent possibilities for ecological research on the effects of environmental change. To gain a better understanding of the dynamics of abiotic and biotic parameters of mountain ecosystems, long-term records are required since permanent plots in mountain regions cover in the best case about 50–70 years. In order to extend investigations of ecological dynamics beyond these temporal limitations of permanent plots, paleoecological approaches can be used if the sampling resolution can be adapted to ecological research questions, e.g. a sample every 10 years. Paleoecological studies in mountain ecosystems can provide new ecological insights through the combination of different spatial and temporal scales. If we thus improve our understanding of processes across both steep environmental gradients and different time scales, we may be able to better estimate ecosystem responses to current and future environmental change (Ammann et al. 1993; Lotter et al. 1997).

Treeline and high elevation sites in the central and southern Chilean Andes (32°39′ to 55°S) have shown to be an excellent source of paleoenvironmental records because their physical and biological systems are highly sensitive to climatic and environmental variations. In addition, most of these sites have been less disturbed by logging and other human induced disturbances, which enhances the climatic signals present in the proxy records (Luckman 1990; Villalba et al. 1997).

Long-term trends of temperature variations across the Southern Andes (37–55°S) have been recently examined using a combination of instrumental and proxy records. Tree-ring based reconstructions indicate that the annual temperatures during the 20 century have been anomalously warm across the Southern Andes in the context of the past four centuries. The mean annual temperatures for northern and southern Patagonia during the interval 1900–1990 are 0.53°C and 0.86°C above the AD 1640–1899 means, respectively. Increased temperatures are seriously impacting the physical and biological systems across the Southern Andes.

Fluctuations of glaciers and ice caps in cold mountain areas have been systematically observed for more than a century in various parts of the world and are considered to be highly reliable indications of worldwide warming trends (cf. Fig. 2.39a in IPCC 2001). Mountain glaciers and ice caps are, therefore, key variables for early-detection strategies in global climate-related observations. Advanced monitoring strategies integrate detailed observations of mass and energy balance at selected reference glaciers with more widely distributed determinations of changes in area, volume and length; repeated compilation of glacier inventories enables global representativity to be reached (IAHS(ICSI)/UNEP/UNESCO 1989; 1998; 2001; cf. Haeberli et al. 2000; 2002).

Mountain glaciers are a product of climate and are important environmental components of local, regional and global water cycles. Glaciers are sources of beauty in the mountain landscapes and, in many cases, have been among the primary agents responsible for forming these landscapes. Glacier mass balance data have received increasing attention in recent years because of their usefulness in detecting climate change and explaining rising sea level (Meier 1984; Church et al. 2001). Understanding changes in glacier volume is important for regional water supply and power generation. In addition, observations made by the scientific community, tourists and climbers have shown that alpine glaciers are disappearing from mountain ranges around the globe. These changes have profound implications for sources of fresh-water on land, cause sea-level rise and make mountains less attractive, and more difficult and less appealing to climb (Bowen 2001; Meier and Wahr 2002; Meier et al. 2003).

Greenhouse gases in the atmosphere trap energy and, if their concentrations increase, e.g. from anthropogenic sources, the aggregate energy of the earth system increases as well. As a consequence, intensities of fluid dynamic processes (atmosphere and oceans), phase changing processes, biochemical processes, and the thermal status of the system will change in a complex and highly interactive manner. Manifold changes in local, regional and global climate are therefore to be expected, but are anything but easy to detect because: Firstly, climate itself is characterised by multi-scale dynamic variability of interacting processes and states. Thus, trends, fluctuations or changes can only be analysed for selected parameters and must be extracted from noise. Secondly, instrumental records, which concentrate on isolated parameters, are limited in time, and proxy-indicators, although covering longer time scales, show complex dependencies on climate, which can be difficult to interpret unequivocally. This paper emphasizes the role of low-latitude glaciers as i) climate proxies and ii) climate-dependent freshwater sources.

Over the last decade, mass balance has been monitored on several glaciers of the tropical Andes by the Institute of Research for Development (IRD, France) in collaboration with South American partners. This network includes glaciers in the Cordillera Real of Bolivia, Zongo and Chacaltaya (16°S), glaciers in the Cordillera Blanca of Peru, Yanamarey and Artezonraju (9°S), and glaciers in the eastern and western cordilleras of Ecuador, Antizana (0°28’S) and Carihuayrazo (1°S) (Fig. 1). Some of these have been listed as benchmark glaciers by the Word Glacier Monitoring Service (WGMS 2001), and the data are accessible to the scientific community. This network is designed to capture the effects of climate change, and especially ENSO variability, both in the outer (Bolivia, Peru) and the inner (Ecuador) tropical Andes. Glaciers have been selected to be representative of the regional glacierization. Each monitoring programme includes two glaciers, a large one (1 km or more) with a substantial accumulation zone, and a small one that is more directly sensitive to ablation processes. Information about the long-term evolution of some of these glaciers has been extracted from aerial photographs, available for the last five decades (Francou et al. 2000; Ramirez et al. 2001). The particular nature of climate in the Tropics allows ablation to occur at anytime throughout the year in the lowest part of glaciers. Thus, the ablation zone has been surveyed in monthly intervals at several sites, providing interesting details about the seasonal response of tropical glaciers (Francou et al. 2003).