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, where biotic interactions can be altered and, in extreme cases, deoxygenation can create dead zones. Fine sediment inputs from agricultural activity (e.g. ploughing) can hinder macrophyte and macroinvertebrate growth, and smother fish eggs. Organic inputs from defecating livestock can also deoxygenate river waters (bacterial decomposers use dissolved oxygen for respiration) and contribute nutrients that encourage photosynthetic autotroph growth (See video).
Urbanisation has also impacted chalk river ecosystems, with phosphorous inputs from sewage point sources posing a particular risk of eutrophication. Increased surface runoff from impermeable urban surface can occur in chalk river catchments, although the effects are manifest in increased sediment inputs from urban sources rather than flooding; the underlying, porous chalk geology and groundwater flows often alleviate terrestrial flooding pressures. Consequently, chalk river managers are usually more concerned about changes in water quality resulting from urban runoff than the threat posed to property and land through flooding.
The final major threat to chalk river sustainability is neglect. With a long history of management by humans, many existing chalk river environments require regular intervention to maintain their characteristics. In-stream weed cutting, sward height maintenance and coppicing may all be necessary; in their comprehensive review of England’s wildlife sites and ecological network, Lawton et al. (2010) note how the removal of cattle from formerly flower-rich chalk grassland environments has led to habitat deterioration and scrub invasion. As such, even chalk river environments of good ecological status need to be managed to maintain their qualities.
And that’s it. There are of course other threats. I haven’t touched specifically upon the effects of cattle, for example (although as explained in my previous post, cattle aren’t evil). However, there’s still enough information here to help us prevent the degradation of English chalk streams.
I hope you’ve enjoyed this blog series! This is the final post, but if you want to catch up on any of the other entries then visit my posts page!