Quantifying Abrasion of Stable Substrata in Streams: A New Disturbance Index for Epilithic Biota

Abstract

Abrasion is a significant disturbing force to which the streambed epilithic biota is exposed during high flow periods. However, abrasion of biota in the field has not been measured, and effects of abrasion on biota are usually inferred. We describe a method that quantifies abrasion of a standardised substrate in the field. This provides an index of the level of abrasion to which flora and fauna are exposed. The method uses autoclaved lightweight aerated concrete (ALC) blocks that are fixed in place on the streambed and subsequently abraded. Two methods were used to quantify abrasion: the change in mass of the block from before to after deployment, and the reduction in cross-sectional area of the corners of the blocks as measured by image analysis. We performed preliminary trials in the laboratory to quantify the amount of work required to remove mass from the blocks. These measurements confirmed that ALC was a consistent substrate with regards to the amount of work required to remove a given mass. The technique was field-tested during a year-long survey at six sites in the Acheron valley, south-eastern Australia. Blocks were deployed for approximately 2 months at a time at each of six sites. The method was able to distinguish average differences in abrasion between trials and sites, but neither of these factors was constant. The variability of abrasion within sites was also high. Results show that the technique in its current form can be used to provide an index of the amount of abrasion to which the epilithic biota is exposed in streams and rivers. An assessment of the strengths and weaknesses of the technique is provided, and suggestions made for further improvements and research.