Spatial variability of nitrate concentration in lakes in Snowdonia, North Wales, UK

摘要

At a regional scale, high nitrate (NO₃‾) concentrations in upland surfacewaters 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 yetrelatively few studies have been undertaken which examine within-regionvariation of NO₃‾ concentrations. A study of 76 lakes in Snowdonia, north Wales,an area of high deposition and sensitive catchments, was undertaken to assessvariation in surface water NO₃‾ concentration across a 20 x 20 km grid squareand to identify catchments vulnerable to NO₃‾ leaching. Nitrate concentrationsvary considerably, particularly during winter when values range from 0.7 to 70 μeq l-1. Although retention by vegetation and soil microbes in summer reducesthe amount of NO₃‾ reaching the lakes, 37 % of sites are characterised by NO₃‾concentrations greater then 4 μeq l-1. The elevated concentrations occurring insummer suggests that N breakthrough has occurred. By examining the ratio of NO₃‾to total strong acid anions, it is shown that NO₃‾ contributes significantly tofreshwater acidity, particularly during the winter. Redundancy analysis showsthat NO₃‾ leaching is greatest, both in winter and summer, in catchments withhigh proportions of bare rock where soil and vegetation cover is limited.Nitrogen cycling in these catchments, generally at higher altitudes, may nolonger be governed by seasonal biological controls. They are likely to besaturated with respect to nitrogen (i.e. incapable of further N retention)as a result of the elevated deposition levels in the area. The contribution ofNO₃‾ to acidity is also greatest at these sites. In winter, significant positiverelationships are also evident between NO₃‾ concentration and soil pH andconiferous woodland. The study demonstrates the importance of catchment factorsin modifying the relationship between N deposition and N leaching in uplandcatchments.