Nutrient variations in the Sava River Basin

The first comprehensive evaluation and trend analysis of long-term nitrogen (N) and phosphorus (P) data was performed for the Sava River Basin (SRB) at locations in Slovenia, Croatia, and Serbia. Data on long-term water quality at eight selected locations were evaluated statistically based on the period of reporting, availability of flow data, and the experience from our previous investigations. Trends in NO3 (-) and PO4 (3-) concentrations at locations were identified using simple linear regressions on log-transformed data. Analysis indicated spatial and seasonal variability between locations. The highest long-term annual mean NO3 (-) and PO4 (3-) concentrations were observed in Croatia and Serbia at locations that are susceptible to anthropogenic nutrient loading arising from high human population densities and intensive agricultural land use. Temporal variations indicate that river flow plays an important role in the distribution of NO3 (-) and PO4 (3-) concentrations. Low flow during the summer months promotes reductive conditions leading to a minimum in annual mean NO3 (-) concentrations and a maximum in annual mean PO4 (3-) concentrations resulting from the release of inorganic P from sediments. The highest annual mean NO3 (-) concentrations were observed in winter/autumn under higher flow conditions that leach NO3 (-) from soil and banks into the river. The results of trend analysis show a reduction of nutrients at the majority of locations ranging from 8 to 58 %. Although trends in NO3 (-) and PO4 (3-) concentrations indicate a decrease at the majority of selected locations, it is still necessary to apply appropriate removal technologies in order to reduce the nutrient loads into the Sava River Basin and thereby improve water quality.