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

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VI.A.3. Limnology of Lake Erken Lake Erken is a mesotrophic lake in eastern Sweden 10 km north of Norrtälje and 70 km northeast of Stockholm. Granites and gneisses dominate the bedrock of the area, while the overlaying soils are rich in calcareous material. The lake is stratified during summer and winter (dimictic) and has a relatively small inflow of humic matter from the catchment. The mean Secchi depth is 4.5 m and the theoretical lake water retention time 7.4 years. The catchment area (140 km 2 ) consists of 70% forest, 10% agricultural land and 20% lakes. The lake is always ice-covered in winter. The ice break-up occurs between March and early May Dominant phytoplankton species are: Stephanodiscus hantschii var. pusillus, Asterionella formosa, Chrysochromulina parva, Rhodomonas lacustris, and Gloeotrichia echinulata. The latter causes water blooms in the lake in July and August (Pettersson et al., 1993). VI.A.2. Data availability and investigations The ice break-up has been registered since 1954. Also water samples have been taken since 1954 with more intensive sampling both in the 1970s and from 1993 onwards. Thus, at least 20 years of data from Lake Erken are available. Climate impacts on Erken have previously been analysed by Weyhenmeyer et al. (1999), Blenckner et al. (2002) and Pettersson and Grust (2002). These analysis indicate that this mesoeutrophic lake is a very good ecosystem to analyse climatic impacts on lake processes, as the land use in the small catchment has not been change over time. This unique study environment offers therefore the possibility to analyse direct and indirect effects on climatic change, as the observed changes are not due to changes in the catchment. Observed changes The studies so far performed document a one month earlier ice break-up during the later period, compared with the 1960s, and a decrease of springtime snow cover. These changes, in turn, can explain a shift in the timing of the spring phytoplankton bloom, which now occurs one month earlier than in the 1960s and 1970s. Additionally, warmer winters seem to shift the phytoplankton species composition towards diatom dominance. A longer ice-free period at present has led to an increase in water temperature, especially in May, which may stimulate bacterial activity (Goedkoop and Törnblom, 1996). The resulting enhanced remineralization of nutrients combined with the longer ice-free period has enhanced the availability of nutrients to algae. Indeed, a significantly higher summer phytoplankton biomass has occurred during the late 1990s (Blenckner et al., 2002) (Figure VI.A.7). Chlorophyll a (µg/l) 20 16 12 8 4 0 1 3 5 7 9 11 13 15 17 19 21 Week of the year 160 70s 90s Figure VI.A.7 The comparison of phytoplankton biomass (Chl a, lines) and the ice cover periods (bars) between the 70s (blue) and 90s (brown) in Lake Erken.
TP (µg l -1 ) Modelling To analyse this process further, a physical lake model and a mechanistic phosphorus model were combined with two emission scenarios generated by a regional climate model (RCM) in three sites in central Sweden – Lake Erken and two basins of Lake Mälaren (Galten and Ekoln). In the phosphorus model water mixing, mineralization, diffusion and biological uptake are temperature dependent. 80 70 60 50 40 30 20 10 E rken, control, sc enario control A2 B2 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month In the simulations, Lake Erken was much more sensitive to climate warming than the two basins of Lake Mälaren (Figure VI.A.8), and the reason was shown to be the much longer water residence time in Lake Erken (seven years), stressing the importance of internal processes. In Galten and Ekoln the water residence times are less than one year, and the effects of water temperature changes are small. In Lake Erken the concentration of epilimnetic, dissolved phosphorus almost doubles in spring and autumn in the scenarios. Long-term datasets from Lake Erken from the 1970’s and 1980’s showed that earlier ice-breaks and higher temperatures in May led to increased phosphorus concentration and phytoplankton biomass in summer (Blenckner et al., 2002). Since the lake is mostly phosphorus-limited, this means that the phytoplankton production is almost doubled in the future scenarios. The implication would be that in Lake Erken, and in other eutrophic lakes with long water residence times, eutrophication problems might become serious in a warmer future climate. Water managers may need to take action today in order to maintain good water quality in these lakes. 161 Lake Mälaren (Ekoln) j f m a m j j a s o n d FigureVI.A.8. Mean, maximum and minimum values of predicted total phosphorus (TP) concentration (µg/l) in Lake Erken and in the Ekoln basin of Lake Mälaren for the three climate emission scenarios (control 1960-1990; A2 - 2070-2100; B2, - 2070- 2100). 80 70 60 50 40 30 20 10 Month
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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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POPs exist in the atmosphere in the
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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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