| Resumo: | The surrounding streams of the Aljustrel mining area (Alentejo Region, Southern Portugal) are strongly affected by acid mine drainage (AMD), as a result of the weathering of the tailing's sulphide minerals. Benthic diatoms (Bacillariophyceae) and water samples were collected seasonally, during one year (2008/09), at 13 sampling sites (5 impacted and 8 unimpacted). Benthic diatom assemblages were studied using Pulse Amplitude Modulated (PAM) fluorometry to assess the effects of the metals/acidity on the photosynthetic efficiency. The information provided by Chl-a fluorescence parameters (e. g. alpha, ETRmax) is an early warning signal of biological effects after acute exposure to chemicals, damaging either directly or indirectly Photosystem II (PSII). Statistical analysis (Two-way and Factorial ANOVAs) found significant differences on Chl-a fluorescence parameters of impacted and unimpacted sites. Photosynthetic efficiency and biomass (Chl-a) were lower for benthic diatom assemblages in impacted streams (Agua Forte and Agua Azeda), especially during summer, due to very low pH (2.3) and the concentration effect of Cu (348 ppm), Fe (6173 ppm) and Zn (1728 ppm) in waters and sediments. Diatom diversity was low at these sites, mainly dominated by Pinnularia aljustrelica, the main contributor to the high similarity between impacted sites (81 % of similarity by SIMPER). This is a typical species from acidic waters with high metal concentrations. Maximum values of Chl-a fluorescence parameters (alpha, ETRmax) were found for winter at the unimpacted sites in the Canal da Barrada and Roxo streams due to: 1) higher distance from mine input, 2) pH differences, 3) the existence of brackish/marine species (e. g. Entomoneis paludosa and Haslea spicula), supported by high ions content in the waters. When the analysis was based on the structure of diatom assemblages dominated by Achnanthidium minutissimum, two groups were highlighted: JU-PB and RJ-BF (uncontaminated), indicating seasonal differences. This study confirmed PAM fluorescence as an effective method for the rapid and non-destructive estimation of photosynthetic performance in diatom communities of AMD impacted streams from pyritic mining areas.
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