Long-term temporal dynamics and trends of particle-bound phosphorus and nitrate in agricultural stream waters

Abstract

One problem in evaluating efforts to reduce phosphorus (P) and nitrogen (N) losses to waters is that variations in weather conditions cause nutrient concentrations and waterflow to vary. Analyses of biweekly stream water samples collected manually from two small, neighbouring Swedish agricultural catchments with clay soil (E23 and E24) demonstrated unpredictability in P and N concentrations. However, particulate P (PP) concentrations in the two separate catchments, usually sampled within 2–3 hours on the same day, were clearly correlated to each other (Spearman correlation coefficient r=0.70). Corresponding nitrate–nitrogen (NO3–N) concentrations were also correlated to each other (r=0.79). Particulate P concentrations could reasonably be predicted from suspended solid (SS) concentrations above base flow (BF) in both catchments (regression coefficient R 2=0.84 and 0.86, respectively). In the period 1993–2009, before eutrophication control programmes were introduced in catchment E23, there was no general trend in PP or SS in either catchment. Mean PP (0.13 mg L−1) predicted (R 2=0.88) from high-resolution (15 minute) turbidity concentrations was significantly higher than flow-weighted mean PP concentration estimated from discrete samples (0.10 mg L−1) collected manually at the catchment E23 outlet. Mean PP concentration estimated directly from flow-proportional sampling was also higher. High synoptic concentrations of PP (up to 0.65 mg L−1) were recorded along the open reach of the stream in the ascending limb of high-flow pulses. Using high-resolution monitoring at the catchment outlet, episodes with a clear clockwise hysteresis effect for PP concentration (seen as turbidity) were frequently observed. By contrast, the NO3–N peak appeared 4–7 hours after the flow peak and anticlockwise hysteresis was observed. Significant erosion along stream banks may take place, and the degree of erosion was estimated based both on farmers’ observations and on results from a distributed erosion model (USPED). Monitoring and erosion mapping are currently being used in practical remedial work.