Effects of nitrogen on nutrient-limitation in oligotrophic northern surface waters

Nitrogen deposition has a potentially eutrophying impact on oligotrophic freshwaters. Here we present a data analysis and a literature review with a joint focus on nitrogen (N) limitation in oligotrophic lakes and rivers. The data analysis consists of a Nordic lake dataset where relations between algal biomass (chlorophyll a), water chemistry and N deposition are investigated, and an analysis of element stoichiometry (nitrogen : phosphorus ratios) in Norwegian natural rivers. The literature review includes international publications on N limitation from a wide variation of monitoring and experimental studies. The data analysis and literature both support that N limitation exists in surface waters that receive low N deposition and that elevated N deposition can shift an ecosystem from N to P limitation. The results are used to review and revise ranges of critical loads of nitrogen for surface waters, as a contribution to the ongoing review and revision of empirical critical loads under the Convention on Long-range Transboundary Air Pollution.