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Title: Assessment of the Impact of Nutrient Removal on Eutrophic Rivers
Author: B O L Demars
Author: D M Harper
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1061, Representation ID: 322, Object ID: 2334
The river Wensum, a calcareous lowland river in north Norfolk, is a site of Special Scientific Interest and a candidate Special Area of Conservation under the EU Habitats Directive as a representative of the calcareous Ranunculus lowland stream habitat. The two main urban sewage works discharging into the Wensum, from Fakenham and East Dereham, had phosphorus stripped from their effluents by sedimentation with iron salts, from autumn 1999. This project was initiated in order to understand the consequences it would have for the river's physical and ecological condition. The project commenced one year before the phosphorus-stripping, to enable one year's ”before” data to be collected. The spatial pattern of total phosphorus in the Wensum showed that the effluents had clearly increased the water column total phosphorus. The mills weirs caused total phosphorus to accumulate in the sediment behind them. Bioavailable phosphorus (BAP) was related to sediment type. Sediment total phosphorus was probably bound to iron. Background levels of soluble phosphorus from sub-catchments not known to be impacted by any effluents were 9-15 ~kg/L (of PO4-P) and total phosphorus 39 - 77 ~kg/L. These represent the runoff from diffuse agricultural sources; no examples could be found of 'natural' sites. There were significant in-stream decreases of SRP and TP downstream from the two major STWs after phosphorus removal. The impact on sediment BAP was less convincing but this may be due to the high variability of BAP due to sediment heterogeneity. The phosphorus control at Dereham and Fakenham led to a decrease of 64% and 76% respectively of the loads of total phosphorus (TP) in the River Wensum. Aquatic plants were not found to be an indicator of the level of nutrients in the lowland rivers investigated, this over a wide gradient of soluble reactive phosphorus and ammonium in a nitrate-rich environment. This is contrary to general opinion. There were only marginal differences in species richness between rivers in the wider survey (Wensum, Nar, Bure) that could be attributed to phosphorus enrichment. The spatial species turnover along rivers was high, resulting from both natural and anthropogenic factors. Fifteen recommendations for conservation, river management and future water management arising from this study are made.
Publisher: Environment Agency
Subject Keywords: RiversSedimentsNutrientsPhosphorusAquatic plantsEffluentsMonitoringMacrophytes
Geographic Keywords: NorfolkWensum
Extent: 119
Total file downloads: 21

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