Phosphorus control measures at two major (>10000 people equivalent, p.e.) sewage treatment works (STWs) were installed in the lowland calcareous basin of the River Wensum (England). In-stream phosphorus concentrations were monitored seasonally from subcatchments with different levels of phosphorus impacts, as well as before and after phosphorus control, above and below the two major STWs. Point source effluents raised in-stream soluble reactive phosphorus (SRP) concentrations from 9–15 μg L−1 (agricultural sub-catchments) to 580–3270 μg L−1. This was accompanied by an increase of the SRP relative to total phosphorus from 27% to 80–90%. The phosphorus content of the suspended sediment was high (0.2 to 7.7%). Molybdate unreactive phosphorus (1–29 μg L−1) was surprisingly not affected by point source effluents. The river bed sediment bioavailable phosphorus concentrations were higher (4–18 μg g−1 wet weight) downstream from the main effluents, compared to upstream (2–6 μg g−1 wet weight). Phosphorus control at the STWs in 1999 has allowed to reduce in-stream soluble reactive concentrations to 140–280 μg L−1 but has had no significant impact on bioavailable phosphorus in the sediment by 2001, suggesting that either net sediment desorption did not occur or that it is a much slower, longer term response. The relative contribution of the diffuse sources increased from 10% to 27% of the total phosphorus loads at Fakenham. The management of these rivers is therefore problematic
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