Decreasing organic nitrogen concentrations in European water bodies - links to organic carbon trends and land cover

Sammanfattning: European rivers experience increasing concentrations of total organic carbon (TOC) from terrestrial sources due to factors involving changes in land use, climate and soil acidity. However, low interest is given to the evolution of total organic nitrogen (TON) concentrations in surface waters, the links with TOC concentrations and the influence of the surrounding land cover on nutrient inputs. Investigating TON concentrations in parallel with TOC concentrations on a wide spatiotemporal scale would help understanding the role and implications of TON on surface water quality and on the occurence of eutrophication episodes. Consequently, linking concentration trends to the surrounding land cover would help understanding the influence of human development on surface water quality in Europe. By filtering TOC and TON annual concentration means from the EU’s Waterbase aggregated database, we gathered data from monitoring sites distributed over Europe and spread from 1990 to 2012. For each site, a concentration anomaly was first calculated as the difference between a given year and a reference year, and then regressed as a linear fonction of time. Annually averaged concentration differences and rates of change were also calculated for each site to apprehend the intensity and visualize the distribution of nutrient inputs in Europe. TON concentrations were also regressed on TOC concentrations to study possible correlations between them. Using GIS spatial analysis tools and the Corine Land Cover 2012 dataset, monitoring sites were categorized into dominant land cover types (depending on the land cover found within a 1-km buffer). Trends and correlation analysis were then detailed and linked to those specific categories. We showed that TON and TOC concentrations observed in European surface waters are qualified high compared to ranges observed globally for rivers and estuaries in the Northern hemisphere. Nonetheless, these observations are to be considered with care as they do not relate to the water bodies’ morphologies. The highest TOC concentrations are observed for “Reference” sites (>95% natural land cover) and the highest TON concentrations observed for “Urban” and “Agricultural” sites. The TOC:TON ratios observed are consequently high, regardless of the surrounding land use, with a strong positive correlation observed in ratio ranging from 0.25 to 0.40 in favor of organic C. TOC concentrations showed an increasing trend overall from 1990 to 2012, with “Urban” and “Agricultural” sites showing slightly decreasing trends and “Natural” and “Reference” sites showing steeply increasing trends. TON concentrations showed the opposite behavior with decreasing trends for all land cover categories, with particularly strong correlation R2 coefficient for “Urban” and “Agricultural” sites. TON decreasing trends follow the observations made of decreasing N and P concentrations in European freshwaters and can be explained as the results of improved monitoring methodologies and policies applied by the EU. They suggest optimistic outcomes on the occurrence of eutrophication episodes and the improvement of water quality.