From perennial to temporary streams: an extreme drought as a driving force of freshwater communities ’ traits

Abstract. Alterations in trait proportions of diatom and aquatic macroinvertebrate communities were used to evaluate the consequences of the drying of temperate perennial streams due to an uncommon drought event. Four Atlantic-temperate Portuguese streams were sampled on three occasions; spring before the drought, 2 weeks after the return of water to the streambed, and spring after the drought. The traits from diatom and macroinvertebrate communities from temperate streams were also compared with those from Mediterranean systems to assess possible shifts in traits. The effects from stream drying were long-lasting for macroinvertebrates; however, only the diatom trait proportions shifted towards proportions similar to those occurring in Mediterranean streams. There was a significant reduction of macroinvertebrates with the ability to produce cocoons, to disperse actively by flight and passively disperse in the water and of swimmers following the return of water. A decrease in stalked species immediately after drought was evident in diatom communities. In contrast to temperate communities, Mediterranean ones were characterised by larger and less mobile diatoms, and smaller-bodied macroinvertebrates with shorter life cycles. Additional studies are required to determine the long-term effect of uncharacteristic drought events on biological traits and their ecosystem functions in typically perennial streams