BIOLOGICAL SIGNIFICANCE OF UK CRITICAL LOAD EXCEEDANCE ESTIMATES FOR FLOWING WATERS - ASSESSMENTS OF DIPPER CINCLUS-CINCLUS POPULATIONS IN SCOTLAND

1. This study assessed the ability of critical load exceedance estimat es to predict the abundance of an indicator species, the dipper Cinclu s cinclus. 2. Twenty-seven 10-km grid squares were selected from three areas; north-west, west-central and south-west Scotland, with nine 's quares' being surveyed in each region. Within each 'square' two river sections, 4 km in length, were censused for dippers during October and early November 1993. 3. Published data were used to derive two predic tive models of dipper density based on altitude and gradient. Comparis ons of predicted with observed densities revealed no significant relat ionships, suggesting that, alone, these measures of stream topography were insufficient to describe the variation in density observed. 4. A regression model including stream pH and riffle area explained 75% of the variation in dipper density, supporting the use of dippers as indi cators of stream acidity. Considering only river sections, a positive relationship with bankside deciduous cover and a negative relationship with altitude were also identified. 5. A significant correlation betw een predicted critical load exceedance values, derived from both the ' Henriksen' and 'diatom' models, and dipper density was found. 6. It is suggested that measures of water chemistry simulated from exceedance classes may be biologically relevant and that standardized dipper coun ts could allow the general biotic and chemical status of flowing water s to be assessed at the river or catchment level. 7. Although critical load (CL) models do not capture all the detail of ecosystem effects w ithin catchments, they provide a means of predicting the current biolo gical status of surface waters at coarser resolutions, such as the 10- km grid square, and of anticipating some consequences of any future ch anges in acid deposition.