Lizzie Greenway, Durham University / West Country River Trust

One of the major challenges to achieving good ecological status under the Water Framework Directive (WFD) is the large volume of pollutants reaching the river system from diffuse sources. Each biological element within the WFD has differing sensitivities to pollution and diatoms, due to their sensitivity to water quality, are viewed as one of the biological elements most affected by diffuse pollution.

An investigation set out to determine the extent to which diatoms are being adversely affected by diffuse pollution in the Exe catchment, Devon. The methodology incorporates the SCIMAP Fitted approach, which allows the model to be tailored to the characteristics of each unique catchment and to the sensitivities of specific parameters, therefore more accurately reproducing critical source areas for targeted management.

While other studies have successfully employed the SCIMAP Fitted approach to reflect instream data, the poor performance of the model in this study has partially been explained by the complexity of the spatial distribution of ecological data in response to interacting, often antagonistic controls, operating over multiple scales. These influences cannot simply be explained by nutrients from diffuse pollution but are also strongly influenced by the distinct transitions from the upper to the lower Exe catchment in terms of climate, geology, land use, pH, dissolved oxygen and dissolved solids, as well as local scale influences. The research highlights opportunities for further refinement of the model in order to take advantage of the model’s potential for targeting catchment management and maximising improvements to aquatic ecosystems.

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