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

At a regional scale, high nitrate (NO3-) concentrations in upland surface w aters generally occur in tandem with high nitrogen (N) deposition levels. H owever, significant differences in the patterns of surface water NO3- conce ntration have been observed within areas of similar N deposition yet relati vely 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 surface water NO3- concentration across a 20 x 20 km grid squa re and to identify catchments vulnerable to NO3- leaching. Nitrate concentr ations vary considerably, particularly during winter when values range from 0.7 to 70 mu eq l(-1). Although retention by vegetation and soil microbes in summer reduces the amount of NO3- reaching the lakes, 37% of sites are c haracterised by NO3- concentrations greater than 4 mu eq l(-1). The elevate d concentrations occurring in summer suggests that N breakthrough has occur red. By examining the ratio of NO3- to total strong acid anions, it is show n that NO3- contributes significantly to freshwater acidity, particularly d uring the winter. Redundancy analysis shows that NO3- leaching is greatest, both in winter and summer, in catchments with high proportions of bare roc k where soil and vegetation cover is limited. Nitrogen cycling in these cat chments, generally at higher altitudes, may no longer be governed by season al biological controls. They are likely to be saturated with respect to nit rogen (i.e. incapable of further N retention) as a result of the elevated d eposition levels in the area. The contribution of NO3- to acidity is also g reatest at these sites. In winter, significant positive relationships are a lso evident between NO3- concentration and soil pH and coniferous woodland. The study demonstrates the importance of catchment factors in modifying th e relationship between N deposition and N leaching in upland catchments.