Conditions in Agricultural Headwater Streams

11 Whilst the processes involved in the cycling of dissolved phosphorus (P) in rivers have been 12 extensively studied, less is known about the mechanisms controlling particulate P concentrations 13 during small and large flows. This deficiency is addressed through an analysis of large numbers of 14 suspended particulate matter (SPM) samples collected under baseflow (n = 222) and storm event (n = 15 721) conditions over a 23-month period across three agricultural headwater catchments of the River 16 Wensum, UK. Relationships between clay mineral and metal oxyhydroxide associated elements were 17 assessed and multiple linear regression models for the prediction of SPM P concentration under 18 baseflow and storm event conditions were formulated. These models, which explained 71-96% of the 19 variation in SPM P concentration, revealed a pronounced shift in P association from iron (Fe) 20 dominated during baseflow conditions to particulate organic carbon (POC) dominated during storm 21 events. It is hypothesised this pronounced transition in P control mechanism, which is consistent 22 across the three study catchments, is driven by changes in SPM source area under differing 23 hydrological conditions. In particular, changes in SPM Fe-P ratios between small and large flows 24 suggest there are three distinct sources of SPM Fe – surface soils, subsurface sediments and 25 streambed iron sulphide. Further examination of weekly baseflow data also revealed seasonality in the 26 Fe-P and aluminium oxalate-dithionate (Alox-Aldi) ratios of SPM, indicating temporal variability in 27