Limnetica 33

Portugal faced an uncharacteristic hydrological drought in the fall/winter of 2011-2012. Small, typically perennial streams were affected by this extreme event and many dried out. Five of these streams were examined during six sampling events between spring 2011 (pre-drought) and spring 2012 (post-drought) to analyse the effects of this disturbance on the macroinvertebrate and diatom communities. Two weeks after dewatering, macroinvertebrate metrics exhibited accentuated decreases in the total abundance, EPT taxa and the scores of the Portuguese Index of Invertebrates but exhibited increases in equitability. The post-drought diatom assemblages showed no significant difference in abundance or evenness in relation to pre-drought conditions, but the diatom quality index (“Indice de Polluosensibilité Spécifique-IPS”) decreased. Four weeks after the drought, the invertebrate communities progressively recovered, whereas the diatom metrics were already at pre-drought values, except for the IPS, which improved slowly over time. The benthic communities recovered faster in streams with higher Habitat Quality Assessment scores. The EPT taxa, Echinogammarus spp., Hydroporus spp. and Ancylus fluviatilis, did not recover to pre-drought values, whereas Lumbricidae and Orthocladiinae increased. For diatoms between the pre- and post-drought years, there was a shift from more sensitive to more tolerant taxa (e.g. the disappearance of Cocconeis euglypta and the appearance of Nitzschia palea). This study showed that an extreme off-season drought had immediate effects on both of the analysed benthic communities, but the diatoms recovered faster. Over one year, none of the studied benthic communities returned to the same pre-drought conditions, but the differences were stronger for invertebrates. In both cases, the a priori habitat condition of streams appeared to control the reaction and recovery patterns of the benthic communities to drought in the studied temperate streams.