Contract EVK1-1999-00087 - RECOVER:2010

Part of the 'Sustainable Management and Quality of Water'

Ecosystem Functioning

Directorate General Research

Macaulay Land Use Research Institute logo CEH Logo BIOTEK Logo NIVA Logo CNR Logo Czech Logo IIASA Logo University of Virginia IVL Logo



Chemical trends at lakes and streams in the UK Acid Waters Monitoring Network, 1988-2000: Evidence for recent recovery at a national scale

C.D Evans1 and D.T. Monteith2
1Centre for Ecology and Hydrology, Wallingford, Oxon OX10 8BB,UK
2Environmental Change Research Centre, University College London, 26 Bedford Way, London, WC1H 0AP, UK

Full Reference

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. . Hydrology and Earth System Sciences Vol. 5, No. 3, 351-366.

Summary of Reseach

A detailed trend analysis of 12 years of data (1988-2000) for 22 surface waters in acid-sensitive regions of the United Kingdom, in which individual site data have been combined to identify national-scale trends, has shown strong common patterns of temporal variation. Results suggest a widespread reduction in sulphate concentrations, hydrogen ion and inorganic aluminium species, and increases in acid neutralising capacity. Many chemical changes have not been linear. However, the first five years were characterised by high concentrations of marine ions, and relatively stable pollutant sulphate concentrations and the remaining period by lower concentrations of marine ions and declining sulphate. Genuine 'recovery',in terms of declining acidity in response to reduced anthropogenic sulphur deposition is only apparent, therefore, for the latter part of the monitoring period. Reductions in calcium concentrations appear to have partially offset the influence of sulphate reductions on acidity , as have increases in organic acidity associated with strong and widespread rising trends in dissolved organic carbon. Fluctuations in a number of climatic factors over the monitoring period have led to significant inter-annual variability in nitrate, which exhibits little long-term trend, marine ions and acidity, emphasising the need for long monitoring periods if underlying trends are to be correctly identified.

Fig1. Location of sites

The AWMN consists of 11 lakes and 11 streams in Scotland, England, Wales and Northern Ireland. The majority are located in western and northern areas, reflecting the distribution of acid-sensitive geology within the UK (Figure 1). Streams have been sampled monthly, and lakes quarterly, for a range of chemical determinands over a 12 year period. Site characteristics vary widely, with minimum altitudes ranging from 10 m to 785 m, catchment areas from 50 to 1300 ha, and soils from thin alpine podzols to deep blanket peats (Table 1). Land-use at most sites is restricted to low-intensity moorland grazing with five catchments containing significant areas of commercial coniferous forestry.

Table 1 Catchment characteristics

Table 2.Direction and significance of trendsTrend analyses of median standardised concentrations (Table 2), allow a number of general patterns at the 11 lakes to be identified. Despite strong seasonality, ANC has risen significantly during the 12-year period and that H and labile Al concentrations have both fallen. Long-term changes in SO4and xSO4are also strongly consistent among sites but xSO4 in particular does not exhibit a linear decrease.

Fig 2 Comparison of key trends for lakes and streams

Although short-term variability to some extent masks long-term trends at stream sites, the underlying patterns observed remain consistent with the lakes with recovery over the last 12 years. A direct comparison between median standardised concentrations for lakes and streams (Figure 2) shows clear similarities between the two surface water types in terms of both long-term trends and inter-annual variations. This suggests: (i) that in-lake processes have not greatly influenced temporal patterns at lake sites; and (ii) that differences in the geographical distribution of lakes and streams within the AWMN have not led to major systematic differences in chemical trends between the two surface water types.

Fig2 .Comparison of median standardised concentrations