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International audience; Climatic extreme events such as droughts (unpredictable), dry periods (predictable) or even flush floods, threaten freshwater ecosystems worldwide. The filtering mechanisms of these events and their strength on communities, however, can be different among regions. While time-for-adaptation theory defines whether or not water scarcity can be considered as disturbance, the stress-dominance theory predicts an increase in importance of environmental filtering and a decrease in the role of biotic interactions in communities with increasing environmental stress. Here, we tested whether environmental filtering (leading to trait convergence) or limiting similarity (leading to trait divergence) is the main assembly rule shaping the structure and trait composition of benthic diatom assemblages in Mediterranean (Portuguese) and continental (Hungarian) temporary and perennial streams. We assumed that the trait composition of diatom assemblages in the two stream types would be less different in the Mediterranean than in the continental region (addressed to time-for-adaptation theory). We also hypothesized that trait composition would be shaped by environmental filtering in the Hungarian streams while by biotic interactions in Portuguese streams (addressed to stress-dominance theory). Our results supported our first hypothesis since traits, which associated primarily to temporary streams were found only in the continental region. Our findings, however, only partially proved the stress-dominance hypothesis. In the continental region, where drying up of streams were induced by unpredictable droughts, biotic interactions were the main assembly rules shaping community structure. In contrast, environmental filtering was nearly as important as limiting similarity in structuring trait composition in the Mediterranean region during the predictable dry phase with no superficial flow. These analyses also highlighted that drought events (both predictable and unpredictable ones) have a complex and strong influence on benthic diatom assemblages resulting even in irreversible changes in trait composition and thereby in ecosystem functioning.

International audience; Benthic diatoms evolved in a biofilm structure, at the interface between water and substrata. Biofilms can adsorb toxicants, such as herbicides, but little is known about the exposure of biofilm organisms, such as benthic diatoms, to these adsorbed herbicides. We assessed the sensitivity of 11 benthic diatoms species to 6 herbicides under both planktonic and benthic conditions using single-species bioassays. The concentration that reduced the growth rate of the population by 10% (EC10) and 50% (EC50), respectively, varied depending on the species, the herbicides, and the growth forms involved. As a general trend, the more hydrophobic the herbicide, the more species were found to be sensitive under benthic growth conditions. Statistical differences (alpha < 5%) were observed between the sensitivities under planktonic and benthic growth conditions for many hydrophobic herbicides. A protective effect of the biofilm against herbicides was observed, and this tended to decrease (at both the EC10 and EC50 levels) with increasing hydrophobicity. The biofilm matrix appeared to control exposure to herbicides, and consequently their toxicity towards benthic diatoms. For metolachlor, terbutryn and irgarol, benthic thresholds derived from species sensitivity distributions were more protective than planktonic thresholds. For hydrophobic herbicides, deriving sensitivity thresholds from data obtained under benthic growth seems to offer a promising alternative. (C) 2013 Elsevier B.V. All rights reserved.