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Bioassessment of Organically Polluted Spanish Rivers, Using a Biotic Index and Multivariate Methods

In this study we compared water-quality results obtained by a modified version of the Biological Monitoring Working Party Score System (BMWP′, a multimetric method) with those from multivariate methods--TWINSPAN and Canonical Correspondence Analysis (CCA)--in a river basin in southern Spain affected by organic pollution. Main environmental variables correlated to macroinvertebrate distributions were identified by CCA. Biological quality of the water was assessed by the BMWP′ biotic index, which classified the sites according to the tolerance of taxa to organic pollution. TWINSPAN was used to classify the sites according to benthic communities, and CCA was performed to establish the relationships among groups of sites, taxa, and abiotic variables. The results showed a clear separation of river sites according to their water composition and degree of pollution; nutrient content and water hardness were the main factors influencing the faunal distribution. Most of the sites in the study area were polluted, and consequently the most frequently encountered taxa were tolerant species. Moreover, the worsening of the water quality downstream was accompanied by a substitution of species, even within the same genus, from less to more pollution tolerant. The results obtained by the multivariate analyses used were highly satisfactory for correctly interpreting the assemblages and substitutions of macroinvertebrates, according to the main environmental variables measured in the basin. Moreover, the classification of sites according to macroinvertebrate requirements, provided by TWINSPAN analysis, was closely related to the classification according to tolerance to water pollution, provided by the biotic index. In addition, a significant part of the variance of each taxon as a pollution bioindicator, in the sites where they were found, was explained by the main canonical axis. Thus, the BMWP′ biotic index, of easy applicability in river basins, proved to be an easy tool for water-quality assessment.