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

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Case study: Lake Võrtsjärv, Estonia Latitude 58º05’-58º25’N Longitude 25º55’-26º10’E Altitude 34 m Area 270 km 2 Mean depth 2.8 m Maximum depth 6 m TP 50 mg m 3 TN 2000 mg m 3 Retention time 1 year Ice cover duration 135 days Stratification non-stratified Figure VI.A.9 General view, location and general characteristics of Lake Võrtsjärv ► Large shallow ecosystem, which chemical and biological parameters strongly depend on the fluctuating water level determined by the amount of precipitation. VI.A.4. Limnolofy of Lake Võrtsjärv Lake Võrtsjärv (Figure VI.A.9) is a large shallow non-stratified eutrophic lake is located in Central Estonia. The mean annual amplitude of water level fluctuations in this lake is 1.4 m, and the maximum range is 3.2 m. The latter corresponds to 1.4times difference in the lake area, 2.4-times difference in the mean depth and threetimes difference in lake volume (Nõges and Nõges 1999). Thus, changing water level is considered to be the leading factor controlling the ecosystem dynamics of Lake Võrtsjärv, first of all through phytoplankton (Nõges et al., 2003). 162
Data availability and investigations Climatic and hydrological data series for L. Võrtsjärv basin reach back up to more than 100 years. Air temperature has been measured since 1894, precipitations since 1866, water level since1923, ice-on and ice-off dates since 1924, and water temperature since 1947 by Estonian Institute of Hydrology and Meteorology. Data on water chemistry and biota have been collected since 1960s at Võrtsjärv Limnological Station, the main centre of freshwater research in Estonia. North Atlantic Oscillation and lake water level In Estonia the western airflow from the Atlantic during positive NAO remarkably increases air temperature and the amount of precipitation in winter (Tomingas and Jaagus 1999). In high-NAO years the ice cover on Lake Võrtsjärv has a shorter duration Mean air temperature Ice cover, days y -1 8r = 0.529, p = 0.000000004 7 6 5 4 3 2 -3 -2 -1 0 1 2 3 4 NAOw 180 160 140 120 100 80 60 40 r = -0.2805, p = 0.0148 -3 -2 -1 0 1 2 3 4 NAOw while the yearly average air temperature and the amount of precipitations in the vicinity of the lake are higher (Figure VI.A.10). Because of flat lanscape and restricted outflow, the increased amount of precipitation is directly reflected in lake’s water level that remains high for several months after the flood. In this way the water level in spring determines the water level throughout the whole year. The effect of changing water level on the ecosystem Phytoplankton biomass is higher in the springs after high-NAO winters but during the other seasons the relation is rather opposite. In summer, and autumn phytoplankton biomass is inversely related with the depth (FigureVI.A.11). This phenomenon has been explained by the reverse relationship between average light intensity and water depth in polymictic water column bringing about light limitation and worse growth conditions to phytoplankton (Nõges and Nõges 1999). Weaker resuspension in deeper water releases less phosphorus from the bottom sediments while lower denitrification rate keeps nitrogen concentration high (FigureVI.A.12). Consequently, in a warmer world the N/P ratio in Lake Võrtsjärv would probably be higher and potentially toxic N2-fixing cyanobacteria (blue-green algae) will have less chance to develop (Figure VI.A.13), thus the risk of toxic water blooms will be reduced. 163 Precipitations, mm y -1 1000 900 800 700 600 500 400 300 r = 0.2075, p = 0.0157 -3 -2 -1 0 1 2 3 4 NAOw Figure VI.A.10. Correlation of the yearly average amount of precipitations in Estonia, air temperature in central Estonia and duration of ice cover on L. Võrtsjärv with North Atlantic Oscillation Index in winter (NAOw)
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Climate Change and the European Wat
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European Commission- Joint Research
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At the 2003 Rome Informal Meeting o
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Changes in the diurnal variation of
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Fugure I.2. (a) Maximum grid covera
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century. This corresponds to a sea
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Other causes Other causes of climat
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tropospheric ozone are photochemica
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Figure. I.4. Radiative forcings and
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There are two indirect effects of a
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The projected warming is not unifor
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water towards the west Pacific caus
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Aerosols and climate change in the
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precipitation intensities have been
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Chapter III. The Hydrologic Cycle I
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Figure III.2. Long-term average ann
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Figure III.4. Projected change in s
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eduction in the rate of evaporation
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characteristics may or may not be u
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temperatures potentially lead to hi
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Chapter IV.A. Proxy indicators of C
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Mean Air Temperature (Dec.-March) [
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correlation coefficient -0.4 -0.3 -
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Chapter IV. B. The Impact of Climat
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numbers refer to water bodies bigge
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Table IV.B.2. Number of large dams*
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quality degradation and meet the in
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sufficient resolution and accuracy
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Windermere in the English Lake Dist
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their vertical structure. In partic
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have recently described a method th
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The available evidence suggests tha
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1979). In contrast, heavy rain incr
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The projected increase in the atmos
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Aulacoseira spp. favour the changed
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vertical bars are a simple measure
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values, with an upper limit of 30°
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the processes controlling, regional
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Sea level rise is partially due to
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Box # 1: The North Sea The North Se
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and organic material more rapidly t
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suggesting that climate rather than
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CO2 2- ). In turn, the alkalinity i
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looms are often associated with tox
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events of dense water through the D
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(1) Are we already observing a resp
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eastern and western side of the Nor
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system, their physiology and intern
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each coastal element be studied and
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IV.D.1. Introduction Coastal lagoon
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In the last decade, a general model
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and ice have quite a low capacity t
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Table VI.F.1. Priority substances i
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22. Monteith, J.L., 1965. Evaporati
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31. Rodionov, S.N. 1994. Global and
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28. Dokulil, M.T., 2003. Algae as e
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63. Hejzlar, J., Dubrovský, M. Buc
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99. Melack, J.M., J. Dozier, C.R. G
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136. Straile, D. & Adrian, R. 2000.
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14. Blaas, M., Kerkhoven, D., and d
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60. McCleery, R. H. and Perrins, C.
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105. UNESCO 2003. The integrated st
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ecosystems and the landscape and de
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2. Carter, T.R., Saarikko, R.A., an
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24. Hydrographisches Zentralbüro 1
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European Coastal Lagoons: The Influ
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4. Bouma MJ, Sondrop HE, van der Ka
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variable trophic status: a paired l
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France GIANMARCO GIORDANI Departmen
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Chapter VI.E. Climate Change and Wa
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