Soluble nitrogen and phosphorus in the River Ythan system, U.K.: Annual and seasonal trends

Annual and seasonal trends in soluble nitrogen and phosphorus concentrations in the River Ythan were examined using routine monitoring data collected between 1980 and 1992. NO3---N concentrations increased significantly over the 13 year period, with the greatest seasonal increase evidence in the spring (P < 0.001). Although this trend coincided with changes in cropping practice within the catchment, problems associated with assessing trends and causal relationships over relatively short time scales have been highlighted. PO4---P concentrations were highly variable and showed no significant trend. Output from a sewage treatment works at Ellon, serving a population of 8670, resulted in an increase in PO4---P concentration to more than double that upstream, the impact on NO3---N concentration was negligible. Total mean annual loss of NO3---N and PO4---P from the catchment was 1305 tonnes and 18 tonnes, equivalent to losses of 19 kg N/ha and 0.26 kg P/ha respectively.