Recent trends in nutrient concentrations in Swedish agricultural rivers

In five out of twelve Swedish agricultural rivers examined during the period 1993–2004, significant trends for decreasing concentrations of reactive inorganic nitrogen (RIN) were indicated after flow normalisation. These decreases were constant (equal to 2–4% per year), most apparent in the Scania region, and weakly correlated to reductions in livestock density (Pearson correlation coefficient −0.825). The number of grazing cattle livestock units per unit area of arable land decreased on average by 14% and that of non-grazing cattle by 17% during 1985–2003. Based on estimates of root-zone leaching, increased area of set-aside and recent EU subsidisation of catch crops (with/without spring tillage) were suggested to be additional substantial causes but these changes were only rapid very recently (years 2000–2001). A significant and constant decrease in reactive phosphorus (RP) (3% per year) was observed in one river, mainly during the season of low flow, with reduced load from point sources suggested to be the main reason. Significant and constant reductions equal to 3–8% per year in concentrations of non-reactive phosphorus (NRP) were calculated for five rivers. These improvements were weakly correlated to the length of grassed buffer zones along the watercourses in arable parts of the river basin (Pearson correlation coefficient −0.845). Establishment of such zones also took place more recently, and together with constructed wetlands represent on average 0.5% of the agricultural area.