THE EFFECTS OF A BASIC EFFLUENT ON MACROINVERTEBRATE COMMUNITY STRUCTURE IN A TEMPORARY MEDITERRANEAN RIVER

Macroinvertebrate communities and environmental variables were assesse d seasonally for I year in a temporary river in South Portugal receivi ng an effluent with high conductivity, pH, sulphates, nitrates and low oxygen content. The usefulness of the ordination method canonical cor respondence analysis (CCA) and the classification method cluster analy sis (UPGMA) were examined to evaluate the perturbation, Macroinvertebr ate samples were segregated along the first ordination axis by CCA, wh ich in turn correlated with sulphates and nitrates. CCA produced a two -dimensional distribution of sites similar to the grouping formed by c luster analysis. In general, three or four groups were distinguished. Immediately downstream of the effluent discharge point, only taxa tole rant to law oxygen, high pH and high sulphate and nitrate concentratio ns were present. Further downstream, sites had a community similar to the reference sampling locations. During flowing conditions the CCA or dination axis I was also correlated with several classic measures of w ater quality (i.e. taxon richness, diversity and biotic indices). In o ther periods, only the percentage of Ephemeroptera, Plecoptera and Tri choptera (%EPT) and the ratio EPT/(Chironomidae + EPT) were significan tly correlated with CCA axis one. This suggests that ordination method s outperform benthic indices in detecting pollution during low flows a nd segregated polluted from clean/recovered sites in all periods. (C) 1997 Elsevier Science Ltd.