Interpretation of trends in acidic deposition and surface water chemistry in Scotland during the past three decades

R. Harriman, A.W.Watt, A.E.G.Christie, P. Collen, D.W. Moore, A.G. McCartney, E.M. Taylor and J.Watson
FRS - Freshwater Laboratory, Pitlochry, Perthshire, Scotland PH16 5LB

Full Reference

Harriman, R., Watt, A.W., Christie, A.E.G., Collen, P., Moore, D.W., McCartney, A.G., Taylor, E.M., and Watson, J. (2001). Interpretation of trends in acidic deposition and surface water chemistry in Scotland during the past three decades. Hydrology and Earth System Sciences Vol. 5, No. 3, 407-420.

Summary of Research

Trends in major ionic components of bulk precipitation were analysed for two sites, Faskally and Loch Ard forest in Scotland, for the period 1972-2000. The pattern of change was not linear. Large reductions in sulphur deposition occurred in the early 1980s and, to a lesser extent, during 1995-2000, with a period of relative stability between 1988-95. pH increased significantly at both sites but nitrate and ammonia only increased significantly at Loch Ard forest. Long -term chemical data from a total of 37 streams and lochs in four selected regions of Scotland were analysed over three time periods (all available data (mostly 1978-2000), 1988-98 and 1995-2000) to match the deposition patterns.

Fig.1.Location of scottish study sites
Sites were categorised into four geographic areas. Area A (North West); Area B (East Central); Area C (South Central) and Area D ( South West) which also gave the general coverage of the full range of non-marine S deposition in Scotland (Figure 1).

Most of the sites are located in the two most acidified areas, C and D, where the most severe biological problems have been reported.

Essentially , the majority of the streams and lochs drain catchments dominated by slow weathering granitic, schist or gneiss-type rocks. The main exception is the Kirkton catchment in Area C which is underlain with narrow limestone bands, therefore providing a contrasting assessment of chemical trends in more alkaline waters

Catchment vegetation is typically moorland or moorland with variable proportions of different age conifer plantations. Sites which remained as moorland or were planted during the study period were classified as M/M or M/F respectively, while sites which were forested, or forested then clearfelled, were classified as F/F or F/FX respectively

Fig.2.Comparison of mean SO4 concentrations from LAR rain and loch
The temporal pattern of concentration change in acidity and SO₄* at Loch Ard forest (LAR) and Faskally (FR) was very similar with the highest concentrations in the 1970s, coinciding with peak emissions of sulphur dioxide (SO₂).

Thereafter a sharp decline in the early 1980s was followed by a period of relative stability (1985-1995), then a further downward trend during the last 5 years of the century (Figure 2).

For the whole study period a significant decline in non-marine sulphate was found at all sites (Figure 3) while the most consistent increases in pH and alkalinity were recorded at all the high elevation loch sites in the Galloway area. Significant reductions in toxic forms of aluminium were also recorded, mostly at sites where pH had increased. Nitrate trends were equivocal except for catchments with clear-felling operations (Figure 3). For these sites, negative trends were found where felling occurred in the 1980s, while positive trends were found at sites with felling in the 1990s.With the exception of one site, dissolved organic carbon concentrations increased significantly with moorland sites showing smaller increases than forested sites. Associated with this change was a significant increase in complexed forms of aluminium. Trends for the 1988-98 period were much smaller than those for the whole study period and in many cases were insignificant. This contrasts with the 1995-2000 period when large and significant reductions in sulphate and nitrate were recorded along with increases in marine salts, probably as a result of climatically related events.

Fig.3.Numbers of brown trout at Loch Riecawr

Figure 3. Median annual trends in (a) SO₄*, (b) ALK, (C) NO₃and (d) Cl at the 37 study sites

Qualitative, experimental and monitoring data from lochs in the Galloway area revealed evidence of recovery of fish populations. Interpretation of chemical and biological trends was clearly influenced by the choice of the time series, especially in relation to deposition and climatic changes. Nevertheless, all the fresh waters included in this study are currently in the best ecological condition since the 1970s in the context of recovery from acidification (Figure 4).

PREDICTING RECOVERY IN ACIDIFIED FRESHWATERS BY THE YEAR 2010, AND BEYOND

The James Hutton Institute

THE REGIONS

RESULTS

Interpretation of trends in acidic deposition and surface water chemistry in Scotland during the past three decades