Humans have dramatically disturbed the earth's N cycle to the point that this alteration is considered one of the most serious threats to the global ecosystem. Part of the excess N circulates through the atmosphere and is deposited with precipitation, reaching areas that would otherwise be free of direct human impact. Atmospherically deposited N has an acidifying effect on aquatic ecosystems, and causes also changes in the pattern of ecological nutrient limitation in lakes. Pyrenean catchments are saturated of N of human origin and streams are leaching that N excess.