Contract EVK1-1999-00087 - RECOVER:2010

Part of the 'Sustainable Management and Quality of Water'

Ecosystem Functioning

Directorate General Research

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Recovery from acidification in European surface waters

C.D.Evans1, J.M.Cullen1, C.Alewell2, K.Jopacek3, A.Marchetto4, F.Moldan5, A.Prechtel2, M.Rogora3, J.Vesely6 and R.F.Wright7
1Centre for Ecology and Hydrology, Wallingford, Oxon OX10 8BB,UK
2Institute for Terrestrial Ecosystem Research (BITÖK)
3Hydrobiological Institute AS CR, Na sádkách 7, 370 05 Èeské Budejovice, Czech Republic
4Consiglio Nazionale delle Richerche- Istituto Italiano di Idrobiologia, 28922 Verbania Pallanza, Italy
5Swedish Environmental Research Institute (IVL), Dagjämningsgatan 1, Box 47086, S-402 58 Göteborg, Sweden
6Czech Geological Survey, Geologická 6, 152 00 Praha 5,Czech Republic
7Norwegian Institute for Water Research (NIVA), Box 173, Kjelsas, N-0411 Oslo, Norway

Full Reference

Evans, C.D., Cullen, J.M., Alewell, C., Kopácek, K., Marchetto, A., Moldan, F., Prechtel, A., Rogora, M., Veselý, J. and Wright, R.F. (2001). Recovery from acidification in European surface waters. Hydrology and Earth System Sciences Vol. 5, No. 3, 283-297.

Summary of Research

Water quality data for 56 long-term monitoring sites in eight European countries are used to assess freshwater responses to reductions in acid deposition at a large spatial scale. In a consistent analysis of trends from 1980 onwards the majority of surface waters (38 of 56) showed significant (p < 0.05) decreasing trends in pollution-derived sulphate. Only two sites showed a significant increase. Nitrate, on the other hand, had a much weaker and more varied pattern, with no significant trend at 35 of 56 sites, decreases at some sites in Scandinavia and Central Europe, and increases at some sites in Italy and the UK. The general reduction in surface water acid anion concentrations, has led to increases in acid neutralising capacity (significant at 27 of 56 sites) but has also been offset in part by decreases in base cations, particularly calcium (significant at 26 of 56 sites) indicating that much of the improvement in runoff quality to date has been the result of decreasing ionic strength. Increases in acid neutralising capacity, have been accompanied by increases in pH and decreases in aluminium, although fewer trends were significant (pH 19 of 56, aluminium 13 of 53). Increases in pH appear to have been limited in some areas by rising concentrations of organic acids. Within a general trend towards recovery, some inter-regional variation is evident, with recovery strongest in the Czech Republic and Slovakia, moderate in Scandinavia and the United Kingdom, and apparently weakest in Germany.

Appendix 1 Site used in European Trend analysis

Site Description

The analysis of trends for RECOVER:2010 draws together data for a total of 56 lakes and streams from eight European countries (Figure. 1). These sites cover a wide range of acid-sensitive landscapes, from high-altitude alpine meadows in the Italian Alps and Tatra Mountains of Slovakia through Central European and Scandinavian forests to peaty moorlands in the UK (Table 1). A full list of the sites used is given in Appendix 1.

Since these data have been collated from a large number of different monitoring programmes, there is inevitably some heterogeneity in dataset characteristics, with start dates ranging from 1971 to 1988, sampling frequencies from weekly to monthly at streams, and monthly to yearly at lakes, and varying sets of measured determinands. Here, a set of six key variables are considered: acid neutralising capacity (ANC), pH, non-marine sulphate (xSO4), calcium (Ca), nitrate (NO3), and soluble aluminium (Al). These variables were measured at all study sites with the exception of Al, for which data were unavailable for three of the five Italian sites. ANC was calculated as the difference in the equivalent sum of base cations (Ca, magnesium (Mg), sodium (Na), potassium (K)) and acid anions (chloride (Cl), SO4, NO3) in all regions except the UK, where it was calculated by adjusting measured alkalinity for inorganic aluminium and organic acids (Evans and Monteith, 2001).

Fig.1.Sites used in European trend assessment.

Fig 2a.Spatial distribution of observed trends.

Non-marine sulphate

Clear decreases in surface water xSO4 concentrations have occurred throughout the areas of Europe included in this study, with significant (p < 0.05) declining trends at 38 sites and weak (p <0.20) downward trends at a further 10. The largest median decreases have occurred in Slovakia and the Czech Republic, with large and widespread decreases also observed in Scandinavia and Germany (with the exception of one site, Lange Bramke). In all four of these regions, median xSO4 reductions have exceeded -2.5 meq l-1 yr-1. Significant downward trends are observed at all five Italian sites, but are smaller in magnitude (median -1.0 meq l-1 yr-1). In the UK, although most sites have shown an overall decrease in xSO4, a period of relatively stable concentrations between 1988 and 1995 (Evans and Monteith, 2001) resulted in fewer sites showing significant downward trends. European trends in SO4 are discussed in further detail by Prechtel et al. (2001).

Fig.2b.Spatial distribution of observed trends.


There is a clear tendency towards rising ANC in lakes and streams throughout the study regions; of the 56 sites, 27 exhibit significant rising trends, and weak increases are observed at a further nine. The majority of sites show increasing ANC in all regions except Germany, where only one weak rising trend is recorded (Table 2). In no region do more than two sites show an ANC decline and the only site at which ANC has decreased significantly is Markungsgraben in Germany, due to the NO3 pulse described above. The greatest median ANC increase (4.5 meq l-1 yr-1) has been observed in the Czech Republic and the smallest (0.7 meq l-1 yr-1) in Italy; overall the median increase has been 1.3 meq l-1 yr-1.

Fig 2c. Spatial distribution of observed trends.


Nineteen of the 56 sites exhibit significant rising trends in pH and another six show weak increases. Median slope estimates are positive for five regions, but narrowly negative in Germany. As for ANC, the results indicate widespread recovery, but fewer sites show increasing pH than show increasing ANC. The reasons for this apparent discrepancy are discussed below.

Fig.2d. Spatial distribution of observed trends.


Nitrate trends are highly variable among the study sites, with no evidence of consistent Europe-wide changes in concentrations since the 1980s. A substantial number of sites, mostly in low-deposition areas of the UK and Scandinavia, have near-zero NO3 trends as a result of continuously low surface water concentrations. The only area with clear evidence of rising concentrations is Italy, where significant increases have occurred at four sites and may be indicative of decreasing catchment N retention. A small number of sites in higher-deposition areas of the UK have also shown significant increases, but most trends here appear to result from a climate-related NO3 peak in 1996 (Evans and Monteith, 2001). The largest increase in NO3 occurred at Markungsgraben, following severe forest damage caused by a bark beetle infestation starting in 1993 (Alewell et al., 2001). Many sites in Central Europe (Germany, Slovakia and the Czech Republic), however, recorded decreases in NO3, probably reflecting reductions in N deposition during the 1990s. Smaller but significant downward trends were observed at four sites in Scandinavia. European trends in NO3 are discussed in further detail by Wright et al. (2001).

Table 1.Summary of trends and median estimated concentration changes by region.


Alewell, C., Armbruster, M., Bittersohl, J., Evans, C.D., Meesenburg, H., Moritz, K. and Prechtel, A., 2001. Are there signs of aquatic recovery after two decades of reduced acid deposition in the low mountain ranges of Germany? Hydrol. Earth Syst. Sci., 367-378.

Evans, C.D. and Monteith, D.T., 2001. Chemical trends at lakes and streams in the UK Acid Waters Monitoring Network, 1988-2000. Evidence for recent recovery at a national scale. Hydrol. Earth Syst. Sci., 283-297.

Prechtel, A., Alewell, C., Armbruster, M., Bittersohl, J., Cullen, J.M., Evans, C.D., Helliwell, R., Kopácek, J., Marchetto, A., Matzner, E., Meesenburg, H., Moldan, F., Moritz, K., Veselý, J. and Wright, R.F., 2001. Response of sulphur dynamics in European catchments to decreasing sulphate deposition. Hydrol. Earth Syst. Sci., 311-325.

Wright, R.F., Alewell, C., Cullen, J.M., Evans, C.D., Marchetto, A., Moldan, F., Prechtel, A. and Rogora, M., 2001. Trends in nitrogen deposition and leaching in acid-sensitive streams in Europe. Hydrol. Earth Syst. Sci., 299-310.