Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 51

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Figure 3.10 Mean water temperature of rivers and streams measured at 25 stations in
Switzerland (black line) and mean air temperature at Basle and Zurich (thin blue curve).
The data illustrated are annual running means. (Modified from Hari et al . 2006.)

found that such a climate-related decrease in brown trout population had already
occurred in alpine rivers and streams in Switzerland, and was accelerated by an
increasing incidence of temperature-dependent Proliferative Kidney Disease.

Ice-cover
The seasonal ice-cover of lakes and rivers plays an important role in freshwater
systems, and changes in the thickness and duration of the ice layer are of ecological
importance and have consequences for human activities. According to IPCC
(2007), freeze-up is defined conceptually as the time at which a continuous and
immobile ice-cover forms, while break-up is generally the time when open water
becomes extensive in a lake or when the ice-cover starts to move downstream in a
river. Major variables affecting duration and thickness of lake and river ice are air
temperature, wind, snow depth, heat content of the water body and rate and
temperature of potential inflows. Dates of freeze-up and ice break-up have proved
to be good indicators of climate variability at local to regional scales, and as a
response to large-scale atmospheric forcing (e.g. Walsh 1995; Livingstone 1999,
2000; Yoo & D'Odorico 2002; Blenckner et al . 2004). Thus, as climate changes,
and air temperatures, particularly in the winter, tend to increase, these shifts should
be reflected in ice-cover. Moreover, several authors have used the correlation of
ice-cover dates and air temperature to translate shifts in freeze-up and break-up
into estimated changes in air temperature (e.g. Palecki & Barry 1986; Robertson
et al . 1992; Assel & Robertson 1995; Magnuson et al . 2000). A typical value for
lakes at mid-latitudes is a 4-5-day shift in mean freeze-up or break-up dates for
each degree Celsius change in mean autumn or spring temperatures. Relationships
tend to be stronger for freezing dates and in colder climates (Walsh 1995).
Generally, time series of ice phenology data from lakes and rivers in Eurasia and
North America provide evidence of later freezing and earlier break-up of the
seasonal ice-cover. For instance, long-term ice-cover records of 26 selected lakes
and rivers in the northern hemisphere revealed that from 1884 to 1995, the average