Biological Significance of UK Critical Load Exceedance Estimates for Flowing Waters: Assessments of Dipper Cinclus cinclus Populations in Scotland

Abstract

1. This study assessed the ability of critical load exceedance estimates to predict the abundance of an indicator species, the dipper Cinclus cinclus. 2. Twenty-seven 10-km grid squares were selected from three areas; north-west, west-central and south-west Scotland, with nine 'squares' 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 predictive models of dipper density based on altitude and gradient. Comparisons of predicted with observed densities revealed no significant relationships, 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 indicators 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 between 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 counts could allow the general biotic and chemical status of flowing waters to be assessed at the river or catchment level. 7. Although critical load (CL) models do not capture all the detail of ecosystem effects within catchments, they provide a means of predicting the current biological status of surface waters at coarser resolutions, such as the 10-km grid square, and of anticipating some consequences of any future changes in acid deposition.