Comparing alternatives for combining invertebrate and diatom assessment in stream quality classification

Abstract

The present study aimed to determine if a predictive model integrating freshwater assemblages from different trophic levels (macroinvertebrates and diatoms) produces a more sensitive assessment of stream health than single-assemblage assessments combined a posteriori. For this purpose, individual and combined models based on two approaches (BEAST and RIVPACS) were developed for Portuguese streams: two for diatoms; two for macroinvertebrates; and two combining diatoms and macroinvertebrates as a single community. Twenty-three sites affected by organic contamination, industrial effluents and mine drainage were evaluated with the predictive models and also by the official biotic indices used in Portugal. The sensitivity of the RIVPACS assessment to disturbance was improved by the a priori combination of diatoms and macroinvertebrates, whereas for BEAST the a posteriori approaches were slightly more sensitive. Diatom and invertebrate indices combined a posteriori performed better than single-assemblage indices but with lower sensitivity than combined models. We conclude that the a priori combination of the two biological assemblages is valuable (more sensitive to disturbance) for the RIVPACS approach and that the a posteriori combination of assessments for individual biological elements may not always provide the most realistic indication of stream health.