Direct impact | Global change

Direct human impact

Alpine lakes have been influenced by humans since a long time ago. Sheep and cattle farming have a long tradition in many alpine areas. Animals grazing in the catchment area of lakes and streams increase the nutrient supply of these otherwise often nutrient poor aquatic habitats.

Fish were introduced to many alpine lakes in medieval times, with quite strong effects on the invertebrate community. A significant reduction of algal grazers by fish has obviously led to higher algal biomass. This is indicated by changing carbon, nitrogen and pigment contents in the sediment of those lakes.

Since the last century, many alpine streams and rivers have been intensively used for electric power generation, and most lotic systems have been corrected and dammed for flood protection.
Ecological consequences of stream corrections and building reservoirs:

• River morphology and particle transport are changed. Nutrients and particles are trapped by reservoirs.
• Reservoirs disrupt the stream ecosystem and lead to shifts in community composition.
• Water temperature may be changed, as water sent through the turbines originates from the bottom of large reservoirs and often has an even temperature of 4-5 °C irrespective of the season.
• Seasonal discharge patterns are changed. Organisms adapted to more constant environmental conditions are favoured with a concomitant decrease of organisms adapted to natural flow patterns.

Impact of global change

In addition to direct human impact, consequences of global change also affect alpine aquatic systems, even those, not directly affected by humans.

Temperature is a key factor for alpine aquatic systems. In addition, most mountain lakes and streams are naturally nutrient poor and often have a low buffering capacity. Therefore, they are particularly threatened by increased deposition of nutrients and acids, toxic air pollutants and by climatic change.

Complex direct and indirect consequences

There are complex interactions between climate, soil, vegetation, and catchment processes and physical, chemical and biological lake responses. For example, in the last twenty years, SO₄^2- concentrations show a decreasing trend in many high mountain lakes. Yet, in some lakes SO₄^2- concentrations increased. This is probably due to enhanced weathering of pyrite in the catchment and decreased SO₄^2- reduction in the lake, which is a result of direct and indirect effects of increasing air temperatures, e.g. better 0₂ saturation due to longer ice free periods.

Consequences even outside the alpine environment

Alpine streams are of great importance for many lowland regions, providing drinking and irrigation water. Pollution of alpine streams will reduce the water quality also outside the alpine environment. Also, other probable consequences of climate change like more dynamic discharge patterns, larger flood events during the winter, lower discharge volumes during the summer, and increased evapotranspiration, will impact the entire course of the streams.

29 August 2011
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