Kernan, M; Allott, TEH; (1998) Spatial variability of nitrate concentration in lakes in Snowdonia, North Wales. (ECRC Research Paper 18 ). UCL Environmental Change Research Centre: London, UK.

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Abstract

At a regional scale, high nitrate (NO3") concentrations in upland surface waters generally occur in tandem with high nitrogen (N) deposition levels. However, significant differences in the patterns of surface water NO3· concentration have been observed within areas of similar N deposition yet relatively few studies have been undertaken which examine within-region variation of NO3. concentrations. A study of 76 lakes in Snowdonia, north Wales, an area of high deposition and sensitive catchments, was undertaken to assess variation in lake-water NO3. concentration across a 20 x 20 km grid square and to identify catchments vulnerable to NO3. leaching. Nitrate concentration varies considerably, particularly during winter when values range between 0. 7 and 70 μeq r'. Retention by vegetation and soil microbes in summer reduces the amount of NO3 reaching the lakes but nevertheless 37% of sites are characterised by NO1 concentrations greater than 4 μeq r'. The elevated concentrations occurring in summer suggests that N breakthrough has occurred. By examining the ratio of NO,· to total strong acid anions it is shown that No,· contributes significantly to freshwater acidity, particularly during the winter. Redundancy analysis shows that the highest No,· leaching level, both in winter and summer occur in catchments where soil and vegetation cover is limited. Nitrogen cycling in these catchments, generally at higher altitudes, may no longer be governed by seasonal biological controls. They are likely to be saturated with respect to nitrogen as a result of the elevated deposition levels in the area. The contribution of No,· to acidity is also greatest at these sites. In winter positive significant relationships are also evident between NO.1· concentration and soil pH and coniterous woodland. The study demonstrates the importance of catchment factors in modifying the relationship between N deposition and N leaching in upland catchments.

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