This study compares regional differences in benthic diatom communities exposed to similar stresses in Canada and Portugal. Diatoms were sampled in the Água Forte Stream, Aljustrel (SW Portugal) and in the Little River, New Brunswick (SE Canada), both streams surround the respective zinc mine and are subject to similar metal (e.g. Cd, Cu, Fe, Zn) and acidic (Água Forte pH = 1.9–2.9 vs. Little River pH = 2.2–5.5) stresses. In this kind of extreme environment, diatoms are frequently the main algae group in the streams, widely used as bioindicators. Diatom communities in the Água Forte Stream were dominated mostly by Pinnularia aljustrelica and Eunotia exigua (5% teratological forms), whereas communities in the Little River were more diverse (e.g. Achnanthidium minutissimum, Nitzschia palea, Eunotia sp.). Shannon-Wiener Index (H′) and percentage of taxa relative abundance were used to characterize the diversity and species composition of the diatom communities. Using canonical correspondence analysis (CCA), it was found that regional variation in acceptable in-stream concentrations of metals, conductivity and pH were the primary drivers of benthic diatom community. Mine remediation to decrease metal concentrations and increase pH in streams will increase diatom diversity even in highly impacted streams such as Água Forte.

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