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Nature of the problem Anthropogenic increase of nitrogen in water poses direct threats to human and aquatic ecosystems. High nitrate concentrations in drink-• ing water are dangerous for human health. In aquatic ecosystems the nitrogen enrichment produces eutrophication, which is responsible for toxic algal blooms, water anoxia, fi sh kills and habitat and biodiversity loss. Th e continuous nitrogen export to waters reduces the capacity of aquatic ecosystems to absorb, reorganise and adapt to external stress, • increasing their vulnerability to future unexpected natural or climate events.

Nature of the problem (science/management/policy) Freshwater ecosystems play a key role in the European nitrogen (N) cycle, both as a reactive agent that transfers, stores and processes • N loadings from the atmosphere and terrestrial ecosystems, and as a natural environment severely impacted by the increase of these loadings. Approaches Th is chapter is a review of major processes and factors controlling N transport and transformations for running waters, standing waters, • groundwaters and riparian wetlands.

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The rate of bitumen extraction in northeastern Alberta, Canada, is outpacing the state of ecological understanding of the region, so that the extent of potential disturbances caused by atmospheric deposition remains largely unknown. Atmospheric SO2 emissions from the Fort McMurray region of Alberta (∼300 t·day–1) constitute ∼5% of the Canadian total. Combined with an estimated NOx production of ∼300 t·day–1, these emissions have the potential to acidify surface waters. Diatom assemblages in dated sediment cores from eight acid-sensitive lakes were analyzed to assess the effects of acidifying emissions on boreal lake ecosystems. There is no evidence that these lakes have become acidified. Instead, many of the lakes show characteristic changes towards greater productivity and occasionally greater alkalinity. The absence of evidence for acidification does not imply that emissions from the Oil Sands are environmentally benign, but rather suggests that the biogeochemistry of these lakes differs fundamentally from well-studied acidified counterparts in northern Europe and eastern North America. Complex interactions involving in-lake alkalinity production, internal nutrient loading, and climate change appear to be driving these lakes towards the new ecological states reported.