| Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Water Pollution > Effects of Nitrogen Cycling On Aquatic Ecosystems > Nitrogen and Acidification of Lakes |
Nitric acid is playing an increasing role in the acidification of lakes and streams for two major reasons. One is that most efforts to control acid deposition — which includes acid rain, snow, fog, mist, and dry deposits — have focused on cutting emissions of sulfur dioxide to limit the formation of sulfuric acid in the atmosphere. In many areas, these efforts have succeeded in reducing inputs of sulfuric acid to soils and water while emissions of nitrogen oxides, the precursors of nitric acid, have gone largely unchecked. The second reason is that many watersheds in areas of moderate to high nitrogen deposition appear to be approaching nitrogen saturation, and the increasingly acidified soils have little capacity to buffer acid rain before it enters streams. An additional factor in many areas is that nitric acid predominates among the pollutants that accumulate in the winter snowpack. Much of this nitric acid is flushed out with the first batch of spring meltwater, often washing a sudden, concentrated “acid pulse” into vulnerable lakes. Adding inorganic nitrogen to freshwater ecosystems that are also rich in phosphorus can eutrophy as well as acidify the waters. Both eutrophication and acidification generally lead to decreased diversity of both plant and animal species. Fish populations, in particular, have been reduced or eliminated in many acidified lakes across Scandinavia, Canada, and northeastern United States. Because the extent of nitrogen-saturated ecosystems continues to grow, along with human-caused nitrogen deposition, controls on sulfur dioxide emissions alone clearly will not be sufficient to decrease acid rain or prevent its detrimental effects on streams and lakes. European governments already have recognized the importance of nitrogen in acidifying soils and waters, and intergovernmental efforts are underway there to reduce emissions and deposition of nitrogen on a regional basis. |
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