Relationship between catchment characteristics and nutrient concentrations in an agricultural river system

We examined the effects of catchment characteristics and riverine processes on the concentrations of nutrients and total suspended solids (TSS) using data from the outlets of 12 tributaries (9–139 km2) and 22 main channel sites of a river draining an intensely cropped area of 1088 km2 in SW Finland. For the tributaries, the flow-weighted mean concentrations of TSS and total phosphorus (TP), which both reflect erosion, were best explained by field percentage and the mean slope of the fields of the catchment. The best model describing the concentration of dissolved reactive P (DRP) included field percentage and catchment area. Total nitrogen (TN) and nitrate+nitrite-nitrogen (NOx-N) correlated only with field percentage. Except for DRP, the regression models derived from the tributary data rather accurately predicted the concentrations in the main river channel. Our results suggest that, with regard to lakeless catchments of 10–1000 km2, the export of TP, TSS and N per unit area remains constant. Furthermore, although some P in eroded soil particles may be released into dissolved, algal-available form during the river transport, erosion control appears not to efficiently reduce dissolved P.