In upland areas of the UK located away from direct human disturbance through agriculture,
industrial activities and urban pollution, atmospheric pollution poses one of the major threats
to the chemical and biological quality of lakes and streams. One of the most important groups
of pollutants is nitrogen (N) compounds, including oxidised forms of N called NOx, generated
mainly by fossil fuel combustion especially in motor vehicles, and reduced forms of N
(ammonia gas or dissolved ammonium compounds) generated mainly from agricultural
activities and livestock. These nitrogen compounds may dissolve in rain or soilwater to form
acids, or may be taken up as nutrients by plants and soil microbes in upland catchments, and
then subsequently released in acid form associated with nitrate leaching at a later date. It is
well established that nitrate leaching contributes to acidification of upland waters, with
damage to aquatic ecosystems including plants, invertebrates and fish. However it has
recently been suggested that nitrate leaching may also be associated with nutrient enrichment
of upland waters that contain biological communities adapted to very low nutrient levels