An evaluation of visual and measurement-based methods for estimating deposited fine sediment

A wide range of methods are commonly used to measure deposited fine sediment and quantify substrate quality in rivers as part of bioassessment or monitoring programmes. In this laboratory-based experiment known amounts of three sediment types (sand, topsoil, peat) were added to mesocosms and four methods of measuring deposited fine sediment; turbidity, estimation of released sediment, Turner-Hillis deposited sediment sampler (DSS) and visual estimation of % surface cover were evaluated. The objective of the study was to evaluate which of these methods for estimating deposited sediment best discriminates between levels of deposited fine sediment added and assesses the effects of inter-observer variability between % surface cover estimations. While turbidity measurement and the resuspension method were strongly related to levels of added sediment, it proved difficult using the two methods to resolve differences between adjacent levels of added sediment e.g. 50 g and 100 g. Surface cover estimations were also strongly related to added sediment levels and were better able to distinguish between adjacent levels of added sediment. Furthermore, we found no significant differences between the % surface cover estimations between observers. Results from this laboratory experiment strongly endorse the use of visual estimation of surface cover in field studies. Further work evaluating the turbidity and re-suspension methods under field conditions would also be beneficial.