River basin perspective on long-term changes in the transport of nitrogen and phosphorus

Abstract

Large-scale transports of plant nutrients and long-term changes of such transports have been investigated in a river basin in southern Sweden. By means of statistical methods that optimally combined information from frequently recorded water-flow data with information from somewhat less frequently recorded water-quality data, the transport of nitrogen and phosphorus along different segments of a river was reconstructed for a period of about two decades. The results thus obtained challenged prevailing opinions of large-scale transports of nutrients in at least two respects: (i) the transport of phosphorus decreased markedly during the late 1960s, but this decrease coincided neither in space nor in time with the introduction of improved phosphorus removal at municipal wastewater treatment plants; (ii) the transport of nitrogen along the river was much lower than could be expected when considering results from hectare-scale field experiments on agricultural drainage. In general, the present case study demonstrates the need for an extended space and time perspective in the evaluation of nutrient transports.