Control of nutrient fluxes in large river basins

Abstract: Nitrogen and phosphorus transported with rivers from land to sea have been reported to be the major sources contributing to the eutrophication of the Baltic Sea. In this thesis, I describe how the sources and waterborne fluxes of nutrients in large river basins can be systematically assessed. Information about point and nonpoint sources, measured as emissions (input), is combined with data on riverine transport (output) into procedures that permit quantification of processes that control nutrient fluxes in a river system, so called riverine metabolism, and the impact of different sources on the transport of nutrients at the mouths of the rivers. Case studies undertaken in the basins of the Vistula and Oder Rivers in Poland provided the first comprehensive inventory of nutrient emissions in this region; nonpoint sources amounted to about 50% and 30% of all emissions of nitrogen and phosphorus, respectively. Analysis of riverine transport data in relation to discharge and temperature showed that at least 70% of the nitrogen transported at the mouths of the rivers was derived from nonpoint sources. Combination of input and output data revealed that riverine metabolism was more efficient with regard to nitrogen originating from point sources than for nitrogen from nonpoint sources. Closer examination of emissions from different nonpoint sources revealed that unit area loads of nitrogen from agricultural land in Poland were remarkably low in a European perspective. Moreover, despite an unprecedented decrease in the agricultural inputs in Poland since 1989, not until 1995 was there unequivocal evidence of downward trends in the transport of nitrogen in the Vistula and Oder Rivers. Methodological improvements of the proposed approach include: (i) an analytical expression for the variance of a widely used estimator of nutrient transport in large rivers; (ii) a general procedure to use river monitoring data to quantify the respective roles of point and nonpoint sources in the total riverine transport of nutrients.