An investigation into the effects of catchment processes on the water quality of southern chalk rivers
The physical, chemical and biological characteristics of five southern English chalk streams in neighbouring catchments were investigated seasonally over two years.
At the catchment scale, the five chalk rivers had very similar physio-chemical properties. Differences between years and between seasons were much greater than those between rivers or within-river longitudinal differences. Elevated inorganic nutrient concentrations, relative to reportedly 'pristine' systems, indicated some degree of catchment water quality deterioration in all five rivers.
The effect of physical habitat degradation (channel overwidening and sedimentation) on river biota was investigated at a within-river, reach scale. The increased proportion of fine sediment within the coarse gravel substratum had a marked effect on invertebrate communities. Overwidening, however, was found to have little impact over the two years. Communities showed a high degree of stability between years, and between individual catchments, reflecting the high overall biological stability of these aquifer-fed river systems.
Habitat manipulation experiments at the within-reach scale demonstrated the importance of substratum characteristics to macroinvertebrate communities. Invertebrate colonisation of newly exposed substrata over time was quantitatively and qualitatively different for coarse and fine particles - a function of shifting resource utilisation and microhabitat preferences. The relatively long time taken to fully colonise new substrata demonstrated the weak ability of chalk stream invertebrate communities to respond rapidly to changes in habitat, reflecting the stable physical nature of natural chalk streams and the vulnerability of their communities to anthropogenic disturbance.