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The tolerance of aquatic organisms to stressors is widely used to monitor and evaluate the condition of freshwater ecosystems. Tolerance values (TV) derived from analyses of the relationship between species and their environment are considered to be more objective than those that rely on expert opinion. We used principal component analysis (PCA) to derive a generalized stressor gradient based on physicochemical characteristics and physical habitat quality and structure. Scores of the first principal component axis (PC1) were used to estimate TV for 37 fish species collected from 54 sites in the Karun River basin, Iran. PCA of 17 variables identified stressors that were influential such as total phosphate, total nitrogen, total coliform, and habitat and morphological score. The species were separated into three categories on the stressor gradient: sensitive (18.9%), semi‐tolerant (48.6%), and tolerant species (32.4%). Based on these results we developed the Karun Fish Tolerance Index (KFTI) and demonstrated that it performed well in separating the least, moderate, and most disturbed sites in the study area. The discrimination efficiency of the KFTI was 82.5%, which makes it a robust management tool for the protection and conservation of streams and rivers in the Karun River watershed. TV developed here reflect objective characteristics of the sensitivity of fish species to the predominant stressors in the Karun and similar systems. [ABSTRACT FROM AUTHOR]

Bioassessment indices routinely used in freshwater assessments are calculated using enumeration methods that are often time‐intensive. For diatom assessments that are mainly based on time‐consuming valve counts, little attention has been devoted to optimizing current counting methods, despite the need for cost‐effective bioassessments and biomonitoring. With a focus on count size sufficiency, we examined diatom samples from >3000 rivers and streams across the United States to assess the effects of reducing the standard count size of 600 diatom valves on the recently published, genus‐level, trait‐based multimetric diatom indices (MMIs) and its component metrics. The diatom data were collected by the United States Environmental Protection Agency's National Rivers and Streams Assessment (NRSA). The MMIs, based on the NRSA data, were designed to assess river and stream ecological condition for three large ecoregions across the United States—East, Plains and West. We found that reducing the standard 600 valve counts introduced variability in component metric values, especially for % taxa metrics, but the variability had little effect on the performance of the actual MMI. More than 90% of all subsamples from the Plains and East ecoregions, and over 80% of West subsamples, were placed in the same condition class regardless of whether a 600‐ or 100‐count size was used. Moreover, the ability of each component metric and MMI to discriminate least‐ from most‐disturbed sites remained the same at 100 valves as that obtained using 600 valves. Synthesis and applications. Contrary to the general perception that counting a high number of diatom valves will lead to a more responsive and robust index, our research shows that a good index performance can still be obtained by counting considerably fewer diatom valves. A smaller count sum will not just reduce the analysis effort but also the quality assurance/quality control effort. For large‐extent bioassessment programmes, reducing the count size could lead to substantial savings in resources that could potentially allow the assessment of more sites. Our subsampling approach to determine an optimal count size is an effective way to achieve time‐ and cost‐effective sample processing that can be applied to any group of organisms. [ABSTRACT FROM AUTHOR]

A physical, chemical and biological characterization of river systems is needed to evaluate their ecological quality and support restoration programs. Herein, we describe an approach using water chemistry, physical structure and land use for identification of a disturbance gradient existing in the Karun River Basin. For this purpose, at each site, physical structure and physico-chemical data were collected once in each season for a total of 4 samples during the period (October 2018 - September 2019). Principal components analysis (PCA) of 17 variables identified five variables that were influential across all seasons: conductivity, total habitat score, stream morphology, clay & silt, and sand. Of the 54 sites, 14, 26 and 14 sites were classified as least, moderate and most disturbed sites, respectively. The metric Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa was used for validation of the classification. Results in different seasons showed that all the least disturbed sites (n=14) were significantly different from moderate and most disturbed sites (p < 0.01). In this study the validation process presented a good confirmation of a priori reference sites selection process, showing that the proposed criteria could be considered as appropriate tools for characterization of the existent disturbance gradient in the Karun River Basin.