Development of a biotic index using stream macroinvertebrates to assess stress from deposited fine sediment

Abstract

1. Detrimental impacts of excessive fine-grained sediment inputs to streams and rivers are well established. What is less well understood is the susceptibility of different elements of the freshwater biota to such perturbations and how such knowledge of their susceptibility could aid in identifying where excessive fine-grained sediment is impairing ecological condition. 2. Following the collection of biological and sediment data from 179 streams across England and Wales, representative of a range of river types over a gradient of fine sediment loading, objective statistical approaches were applied to establish relationships between the macroinvertebrate assemblage and fine-grained sediment inputs to river channels. 3. Having factored out that portion of the biological variation associated with natural environmental gradients, a model comprising mass of organic sediment in erosional areas of the stream bed [predominantly associated with the first axis of the partial canonical correspondence analysis (pCCA)], and mass of fine-grained sediment in the surface drape of depositional areas and % organic content in erosional areas (associated with the second axis of the pCCA) as explanatory variables best accounted for the residual variation in the macroinvertebrate assemblage. 4. The relative position of taxa along both axes of the pCCA, provided a ranking of taxa in relation to the two gradients of fine-grained sediment and provided the basis for a new empirically derived diagnostic index for fine-grained sediment stress in rivers. Two sub-indices were derived to capture the assemblage responses to both the gradient of organic sediment in erosional areas and the gradient of total fines in depositional areas. The two sub-indices were then combined to derive the new combined fine sediment index (CoFSI sp ). 5. The index was tested on an independent test data set (comprising 127 samples from 83 sites) and was found to provide a robust indication of benthic fine-grained sediment conditions (Spearman's rank correlations q = À0.519 to À0.703). The strength of correlation with the total fine-grained sediment gradient was always greater than that for other routinely used indices, confirming that CoFSI sp offered additional explanatory power when assessing this stressor of aquatic environments.

A sediment-specific family-level biomonitoring tool to identify the impacts of fine sediment in temperate rivers and streams

“…The re-suspension method of quantifying fine sediment was used, averaging samples from two erosional and two depositional patches to achieve a contemporaneous reach-scale average. Despite this technique necessitating some subjective assessment of "erosional" and "depositional" patches, moderate to strong correlations (r s = −0.54 to −0.70, p < 0.05) were observed between the resultant sediment data and CoFSI in their test dataset (Murphy et al, 2015). Due to the difficulties of selecting sites with minimal confounding pressures that at the same time represent a gradient of sediment pressures, their resulting test dataset (n = 83) is somewhat geographically restricted and is focused on agricultural streams, which may be impacted by an array of different pressures typically associated with agricultural practices (Allan, 2004;Matthaei et al, 2010;Weston et al, 2004).…”

“…The Lotic-invertebrate Index for Flow Evaluation (LIFE) (Extence et al, 1999) and the Combined Fine Sediment Index (CoFSI) (Murphy et al, 2015) were calculated and their relationship to both fine sediment, PSI and E-PSI indices were assessed in terms of their Spearman's rank correlation coefficients. LIFE was chosen to determine the influence of known interactions between flow and fine sediment deposition and re-suspension (Dewson et al, 2007).…”

“…The recently developed, predominantly species-and genuslevel CoFSI was included in this analysis, as it is a purely statistically based index designed to indicate fine sediment pressures (Murphy et al, 2015). All mixed-level indices were calculated using data of the same taxonomic detail, at the level used for Water Framework Directive reporting.…”

“…These approaches have achieved moderate-strong correlations with fine sediment but lack a biological basis and mechanistic linkage, which are important for disentangling the multiple pressures in rivers and streams (Friberg, 2014). Murphy et al (2015) developed CoFSI, an index that endeavours to take account of both the amount and organic content of fine sediment. The authors used partial canonical correspondence analysis to rank taxa in terms of their sensitivity to fine sediment, based on empirical data from 179 stream sites.…”

“…In addition, as the E-PSI index is not a purely statistically based tool, it is able to include relatively rare taxa (low abundances or small distribution range), which are often removed in the development stages of statistically based indices (Murphy et al, 2015;Relyea et al, 2012). These rare taxa, specifically those with small distribution ranges, may be important indicators of subtle environmental change as they are likely to be more specialised, having a narrower range of optimal conditions, and as such may be the first taxa to indicate the effects of a pressure (Cao et al, 1998;SchmidtKloiber and Nijboer, 2004).…”