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

1979). In contrast, heavy rain increases the proportion of water reaching the lake as
overland flow, which contains higher concentrations of DRP (McDiffet et al., 1989).
In Lake Erken (Sweden) the hypolimnetic phosphorus concentration increased during
the 1990s. It was concluded that the increase in water residence time was the main
factor causing strong internal eutrophication, illustrating the potential sensitivity to
climate change of lakes with a long water residence time.
Weather-related effects on the flux of nutrients are more difficult to detect in summer
since the concentrations reaching the lakes are rapidly depleted by the growth of
phytoplankton.
Changes in the water richness predicted for the Mediterranean Region adversely
affect water quality in summer and winter (Haas, 2002). More frequent high river
flows in winter may improve water quality because of dilution effect. Still the
intensified runoff increases pollution loading from catchments. More intense
precipitation and frequent torrential downpours would be primarily responsible for soil
erosion, leaching of agricultural chemicals, and runoff of urban and livestock wastes
and nutrients into water bodies that will accelerate eutrophication of lakes and
reservoirs.
Lower flows (either from climate change or increased abstraction, or both) coupled
with increasing volumes of effluent discharges into watercourses from growing
populations, agriculture activity (pesticides and fertilizers) and industrial activities will
result in increased concentrations of pollutants including nutrients in water courses.
In-lake processes
In thermally stratified lakes, the internal recycling of nutrients is strongly influenced by
the entrainment of nutrients across the seasonal thermocline. This process is
particularly important in more productive lakes where large concentrations of DRP
accumulate in the anoxic hypolimnion. In a warmer world, most lakes in the Atlantic
Region will stratify earlier in the year and will tend to accumulate higher
concentrations of decaying matter in the hypolimnion. Oxygen concentrations at
depth are therefore likely to fall, which can enhance the nutrient release from the
sediment.
In terms of phosphorus, an earlier timing of ice break-up, as found in a Swedish lake
(Lake Erken), prolonged the P-limitation period for phytoplankton during the mixing
period, but increased the nutrient availability in summer due to a probably enhanced
bacterial activity at warmer water temperatures in combination with the prolonged
mineralization period (Blenckner et al., 2002). In general, the nutrient turnover might
be enhanced in a warmer climate (Hamilton et al., 2001), leading to enhanced
internal eutrophication.
In contrast, earlier onset and longer duration of thermal stratification in small sharply
stratified lakes may result in strong epilimnetic phosphorus depletion during summer.
Phosphorus taken up by algae during the spring bloom will settle out when the bloom
ceases. Further the established thermocline isolates the nutrient-rich hypolimnion
from the nutrient-depleted epilimnion. During summer, metalimnetic maxima of bluegreen
algae (Konopka, 1981) and photosynthesizing bacteria often occur in this type
of lake while the transparent epilimnion allows sufficient light penetration to this
depth. Strong algal blooms may reoccur during autumn when the thermocline is
eroded and deeper mixing returns hypolimnetic phosphorus back to the surface
layer.
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