Climate Change and the European Water Dimension - Agri ...

Most of the processes in nutrient cycles are biologically mediated and thus
temperature-dependent.
Nitrate concentration (de-trended)
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400
0
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1960 1970 1980 1990 2000
Year
Figure IV.B.4. De-trended winter nitrate data for Blelham Tarn
Catchment processes
Melting of glaciers and loss of soil permafrost in catchments of high alpine and sub
arctic lakes as a result of climate warming causes dramatic changes in chemistry and
biota of these lakes. For example, a temperature increase by two degrees that
melted the permafrost and left Lake Schwarzensee, a remote high alpine lake in
Austria, ice-free for two months in summer, changed totally the regime of the lake.
Conductivity and silica doubled, the lake became warmer and hence more
productive, and pH dramatically increased (Psenner and Schmidt, 1992).
One of the simplest measures of this catchment effect is the concentration of
nutrients measured in the lakes during the winter when the biological uptake rate is
very low (Sutcliffe et al, 1982). Year-to-year variation in winter air temperature has
been found to influence the nitrate concentration in lakes in the English Lake District,
as warm winters lead to a reduced nitrate concentration in the water (George et al.,
2004). (Figure IV.B.4) shows the result of de-trending the winter nitrate data for
Blelham Tarn, a small lake in the Windermere catchment. Once the long-term
‘anthropogenic’ trend has been removed, the residual variation is clearly related to
the short-term variations in the air temperature.
Very similar interannual variations in the winter concentration of dissolved reactive
phosphorus have also been detected in de-trended time-series from the English
lakes. The most pronounced effects are detected in lakes with relatively short
residence times. The most likely explanation for this trend is the effect that heavy rain
has on the routing of drainage in the catchment. The first step in the transfer of
dissolved reactive P (DRP) in the drainage system is the dissolution of phosphorus
from the superficial layers of the soil. If the rainfall is light, much of the dissolved
phosphorus is adsorbed as the water passes through the soil (Sharpley and Sayers,
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0
2
4
Air temperature ( o C)