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the drinking water limit. The high concentrations can only be explained by an
increase in the mobilization and release of solutes from active rock glaciers in the
lake catchments entering the lakes via melt water, related to the observed increase
in average air temperature in the region over recent decades (Auer et al . 2007).
The findings are supported by studies on active rock glaciers and meltwater
streams in the Austrian Alps by Krainer & Mostler (2002, 2006). Increasing
conductivity values of a similar magnitude have also been recorded in a high
elevation stream draining from a rock glacier in the US Rocky Mountains
(Williams et al . 2006).
An important question is why have Rasass See and Schwarzsee reacted so
differently in respect to the solute increase, although their catchments are situated
only 45 km apart and are characterized by the same geology. The probable explanation
is that the catchments differ in the size of the active rock glaciers in relation to the
size and volume of the lakes. At Rasass See, rock glaciers occupy c. 20% of the
catchment equivalent to c. 200% of the lake surface, whereas at Schwarzee ob
Sölden, rock glaciers occupy only c. 5% of the catchment, which is equivalent to
c. 30% of the lake surface, and the volume of Rasass See is four to five times smaller
than Schwarzee. In addition, Rasass See is situated at an altitude 100 m lower than
Schwarzee. Although these factors probably explain the differences between the
lakes, the specific sources and pathways of solutes and heavy metals released from
the melting rock glaciers into adjacent surface waters are still unknown.
Conclusions
Changing climate is already having an impact on the physical, chemical and
biological characteristics of freshwater ecosystems, both directly through changes
in air temperature and precipitation and indirectly through interaction with other
stressors. In future, non-climatic impacts should be reduced if pollutant loadings
decrease and surface-water ecosystems are progressively restored. But global
warming is very likely to continue, even if greenhouse gases and aerosols are kept
constant at year 2000 levels, giving rise to a minimum projected average further
increase in air temperature by 0.6°C by the end of this century (IPCC 2007).
Changes in the characteristics of freshwater ecosystems as illustrated here are
likely to continue and will become much more pronounced as greenhouse gas
emissions rise and ecosystems cross critical thresholds that cause abrupt non-
linear system shifts to occur.
References
Ågren, A., Buffam, I., Jansson, M. & Laudon, H. (2007) Importance of seasonality and small streams for
the landscape regulation of DOC export. Journal of Geophysical Research-Biogeosciences , 112 ,
G03003, doi:10.1029/2006 JG000381.
Ambrosetti, W. & Barbanti, L. (1999) Deep water warming in lakes: An indicator of climatic change.
Journal of Limnology , 58 , 1-9.
Assel, R. & Robertson, D.M. (1995) Changes in winter air temperature near Lake Michigan, 1851-1993,
as determined from regional lake-ice records. Limnology and Oceanography , 40 , 165-176.
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