Four more threats to English chalk streams, following on from the previous blog post in this series: catchment-scale land-use change; allochthonous nutrient inputs; urbanisation; and neglect.
Catchment-scale land-use change
As well as exploitation for amenity, chalk rivers are under increasing pressure due to catchment-scale land-use changes. In 2000, 49% of land within chalk river catchments was used for arable farming, with only 27% as grassland and 13% as woodland. Agricultural land contributes both dissolved (e.g. phosphates) and particulate pollutants (e.g. fine sediment) from diffuse sources (e.g. on the River Wensum in Norfolk). In their study of the River Kennet catchment, Collins et al. (2012) suggest that unmetalled farm track surfaces contribute the greatest amount of sediment from agricultural land (55%), whilst agriculturual topsoils contribute the lowest (4%).
Sufficient nitrates and phosphates from fertilizer can lead to algal blooms and subsequent eutrophication, particularly in slow flowing rivers…
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