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This report presents a suggestion for ecological quality criteria for nitrogen in lakes and streams. Nitrogen was included in the previous quality criteria from 1999, but not in the latest developed in 2007. The reason for this is that eutrophication of freshwaters is most often caused by phosphorus, thus it was the only parameter included. Authorities in water management have since expressed a demand for ecological quality criteria for nitrogen because this nutrient is a larger problem in the sea and criteria are needed as a tool to make measures recommendations. Further, the protection of nitrogen lim- ited lakes in northern Sweden is another motivation to develop additional ecological quality criteria. The proposed criteria herein follow the outline for the Swedish ecological quality criteria for phosphorus. Reference values were developed using re- gression models and data from lakes and streams not directly affected by pollution sources (e.g. agriculture and urban areas). In runoff from agricul- tural land, the reference value was defined as half the calculated leaching below the root zone based on leaching region and soil type. In the recommendation for ecological quality criteria, reference values for lakes are calculated by the formula: logTNref = 1,597 + 0,614 * logTOC + 0,147 * LogNDep2008 where TOC = the concentration of total organic carbon (mg/l) and NDep2008 = nitrogen deposition for the year 2008. Reference values calculated in this manner represents the expected value with present land use and deposition. An alternative method is to set NDep to the average value for year 1860, resulting in the following formula: logTNref =1,774+0,614*LogTOC Reference values for streams are calculated with the corresponding formula: logTNref = 1,702 + 0,533 * Log TOC + 0,105 * LogNDep If more than 10 % of the land cover is agricultural land, the reference value for a stream is calculated instead using: TNref,jo = (TNjo * Ajo*f + TNref*(100-Ajo))/100 where: TNref,jo = weighted reference value for TN (μg/l) in the agricultural land- scape. Njo = reference value (TN μg/l) for runoff from for agricultural land Ajo = fraction of agricultural land in the catchment area (%) Nref = reference value outside the agricultural area (but in the same region) according to the formula above. f = 0,5 5 Lakes are generally considered nitrogen limited when the ration of dissolved organic nitrogen (DIN) to total phosphorus (TP) concentration is < 1.5 dur- ing spring circulation.) For these lakes, the concentration of DIN is not al- lowed to increase so that DIN/TP exceeds 3.4, as this is the limit where a lake becomes phosphorus limited. The boundaries between classes of ecologic status were then set to >0.7 (high), 0.5-0.7 (good), 0.3-0.5 (moderate), 0.2-0.3 (unsatisfactory), and 0.2 (bad) for the ratio between reference concentration and measured concentra- tion. These are the same boundaries as for phosphorus. This means that the threshold between good and moderate status is the double reference value. While developing ecologic quality criteria for nitrogen, some ambiguity in the implementation of the Water Framework Directive (WFD) on how ref- erence values should be set was detected. According to the WFD, the refer- ence condition should correspond to a state minimally disturbed by humans. In a guideline document from 2003, reference values for agricultural land- scapes should inherently include leaching from preindustrial (or pre- modern) agriculture. In the Swedish implementation, this was not specified to a defined year or to a certain agricultural practice type. Instead, the refer- ence value for phosphorus in streams in agricultural areas was defined as half of the modelled phosphorus concentration in leachate from the root zoon of unfertilized fallow. Further, it has not been defined how to handle the large-scale changes in the agricultural landscape that occurred contem- porary to the industrialisation, e.g. draining of wetlands and lowering of lakes, which had a large impact on leaching and retention of nutrients. In- creased nitrogen deposition was also not considered when defining refer- ence conditions, but lakes were considered phosphorus limited for primary production. Finally, there is also a lack of harmonisation of the present eco- logical quality criteria for chemical and biological parameters, and between fresh water and coastal waters. In this study we recommend calculating the reference concentration for TN in waters outside the agricultural landscape using the present-day nitrogen deposition, which will protect lakes that are currently nitrogen limited from becoming phosphorus limited. This is motivated by the fact that it is nearly impossible to restore surface waters to a preindustrial nitrogen limitation state, and that the biological losses when a lake switches from nitrogen to phosphorus limitation are far less important (in a general perspective) com- pared to those caused by e.g. acidification and construction of dams for hy- droelectric power.