Recovery of acidified waters in the UK

WP 4: Metal deposition and cycling at Lochnagar

At end of 2003, up to 7 years of high quality data had been collected for trace metals in a range of ecological compartments at Lochnagar, including bulk deposition, lake waters, terrestrial and aquatic plants, aquatic invertebrate fauna and sediment traps. Few of the potential trends identified at the end of the previous CLAM project have been shown to continue whilst most have been shown to be short-term phenomena emphasising the need for long-term monitoring data prior to any attempt at statistical interpretation. With this in mind, recent lake water and deposition data lead to an ambiguity in trends. In particular, 2003 is seen to be unusually high for most metals, except Mercury (Hg). Although these high fluxes are not due to unusually high rainfall, it is likely that this is an anomaly. The declining trend in Hg is observed over a period of years and is more likely to be "real". Similarly, lead (Pb) in both lake water and bulk deposition appears to have been increasing over a number of years, although this is in contrast to most biota Pb data which appear to show declines over the monitoring period.

The lack of clear trends is probably a result of two factors; first, the short lifetime of the monitoring period and second, current low levels of metal emission and deposition following decades of considerable decline. Long-term monitoring will resolve both of these issues.

Intra-annual methyl Hg data are beginning to show intriguing temporal trends with elevated levels in winter deposition. The reason for this is unclear and this pattern is not known to be observed elsewhere. Nickel levels have been below detection limit for a number of parameters over the period of the project, although they have been seen to increase slightly over the last year in deposition and lake water. However, Ni levels remain detectable in sediment trap data. This situation will remain under review as long periods of below detection limit values are of limited use.

Whilst the Lochnagar data are valuable and continuing this monitoring remains a priority, their usefulness and our confidence in observed trends would be increased by the introduction of monitoring at other sites. It is hoped that the move of the trace metal monitoring at Lochnagar to the UK Acid Waters Monitoring Network (AWMN) will allow this expansion to occur.