| Popis: |
Water quality data for three rainwater sites and thirteen streams were collected during the period February 1981–January 1983. Although rainfall in the upper Tywi is, on average, amongst the least acidic in the United Kingdom, episodic depositions of acidity do occur. ‘Excess’ sulphate concentrations in streams were elevated in comparison with those in rainfall, possibly as a result of the collection of ‘dry’ and ‘occult’ deposition by vegetation, evapo-transpiration and the oxidation of sulphur in drained soils. Stream acidity and aluminium concentrations were, therefore, determined by the combination of stream buffering capacity and vegetation within the catchment. Consequently, the study streams could be classified on the basis of water hardness and land use. Stream surveys and survival studies using caged fish showed that unafforested streams with an average hardness of >8 mg litre−1 can support trout populations and a reasonably diverse invertebrate fauna. Unafforested streams with mean hardness of |
