Organic nitrogen and carbon in Swedish rivers : increasing trends from 1987-2017

Abstract: In the past decades, increased organic carbon concentrations have been reported from freshwaters across the Northern Hemisphere. However, there is little knowledge regarding whether or not these changes are linked to increasing concentrations of organic nitrogen. This poor understanding of nitrogen biogeochemistry is a problem in face of the potential eutrophication effects of nitrogen in receiving coastal ecosystems. The biogeochemical cause as well as the subsequent ecosystem impacts are well known when it comes to increased dissolved organic carbon, but trends in organic nitrogen concentration in rivers remain poorly explored. Recent studies show that organic nitrogen should be included in when discussing eutrophication impacted by runoff from land, which leads to the question of how much the Baltic Sea has been affected by organic nitrogen. Have the changes been proportional to the increase of organic carbon or are they not significant? Eutrophication is a known problem in the Baltic Sea and the impact of dead zones is a cause of the ecosystem changing. The aim of this report was to investigate the trends of organic nitrogen, organic carbon and water color in 30 streams across Sweden from 1987-2017. The correlation between total organic nitrogen and total organic carbon showed positive significant results in nine out of thirty rivers and ten rivers had positive correlations between total organic nitrogen and water color. The correlations were, however, only significant in the area of the Baltic Proper and on the west coast, which shows regional differences that require further research to explain the underlying reason. It is a relevant issue for the presence and the future, and for the interest of the environment along Sweden’s coastline. More research is needed to investigate the source of the increase and its consequences thoroughly.