Abstract
The Proportion of Sediment-sensitive Invertebrates (PSI) index is a biomonitoring tool that is designed to identify the degree of sedimentation in rivers and streams. Despite having a sound biological basis, the tool has been shown to have only a moderate correlation with fine sediment, which although comparable to other pressure specific indices, limits confidence in its application. The aim of this study was to investigate if the performance of the PSI index could be enhanced through the use of empirical data to supplement the expert knowledge and literature which were used to determine the original four fine sediment sensitivity ratings. The empirical data used, comprised observations of invertebrate abundance and percentage fine sediment, collected across a wide range of reference condition temperate stream and river ecosystems (model training dataset n=2252). Species were assigned sensitivity weights within a range based on their previously determined sensitivity rating. Using a range of weights acknowledges the breadth of ecological niches that invertebrates occupy and also their differing potential as indicators. The optimum species-specific sensitivity weights were identified using non-linear optimisation, as those that resulted in the highest Spearman's rank correlation coefficient between the Empirically-weighted PSI (E-PSI) scores and deposited fine sediment in the model training dataset. The correlation between percentage fine sediment and E-PSI scores in the test dataset (n=252) was eight percentage points higher than the correlation between percentage fine sediment and the original PSI scores (E-PSI rs=−0.74, p<0.01 compared to PSI rs=−0.66, p<0.01). This study demonstrates the value of combining a sound biological basis with evidence from large empirical datasets, to test and enhance the performance of biomonitoring tools to increase confidence in their application.
Highlights
Fine sediment (
The results of this study show that modelling using an extensive empirical sediment-invertebrate dataset in order to find optimum species-specific sensitivity weightings, has increased the sediment specificity of the E-Proportion of Sediment-sensitive Invertebrates (PSI) index in comparison to the PSI index
An increased specificity is shown by the results of the Kruskal–Wallis test which demonstrated an increase in the number of significant differences between fine sediment values in grouped Empirically-weighted PSI (E-PSI) scores, compared to grouped PSI scores
Summary
When levels deviate from natural conditions, ecological degradation can occur (reviewed in Bilotta and Brazier, 2008). The PSI index is a pressure-specific biomonitoring tool, designed to identify the impacts of deposited fine sediment, using standardised kicksamples of the benthic invertebrate community (Extence et al, 2011). The sensitivity ratings are used to assign abundance-weighted scores, which are used to calculate (Eq (1)) PSI scores ranging from 0 (heavily sedimented) to 100 (unsedimented). Given that rivers vary in their natural sediment conditions/dynamics (Bilotta et al, 2012; Grove et al, 2015), the index is designed to be used alongside a reference-based model (e.g. River Invertebrate Classification Tool), where observed PSI scores
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