How does water scarcity affect spatial and temporal patterns of diatom community assemblages in Mediterranean streams?

Mediterranean streams are mostly characterised by intermittent flow regime, with droughts in summer followed by floods in autumn. During droughts, the creation of lentic habitats (i.e. lentification process) and isolated pools in the riverbeds (i.e. fragmentation process) represents a strong selective pressure on biotic communities, which might be intensified by human-induced water scarcity. In the highly heterogeneous habitat patches created by these processes, species sorting may be driven by stochastic mechanisms, as well as by environmental filtering. In this work, we investigated the spatial and temporal patterns in the assemblage of diatom communities during water scarcity periods in Mediterranean streams. Five oligotrophic streams, which were similar in terms of physical and chemical parameters, were sampled in the Ligurian Alps (NW Italy). Data were collected during moderate flow in April 2014 and then at least monthly during water scarcity, from June to September 2014, in: (1) lentic sites, where the water flow became slower but was permanent all year around, (2) fragmented sites, where the riverbed stretch dried out creating isolated pools. For each sampling site, we collected six benthic diatom samples from different microhabitats. We examined which type of factors, i.e. spatial or environmental, better explained the temporal and spatial variation in diatom communities. We then calculated the turnover and nestedness components of dissimilarity by comparing samples collected during moderate flow with samples obtained during a period of water scarcity. We further investigated if the idiosyncrasy of diatom species, i.e. a measure of their discrepancy from a nested pattern, could be explained by the ecological niche width of species and/or by species traits. Our results showed a contribution of both environmental and spatial factors in determining species composition during water scarcity, with the overall trends in community structure being driven by species turnover. The percentage of idiosyncratic species (those departing from the nested pattern) increased with increasing water scarcity, but showed no differences in terms of species traits. According to our results, species turnover, with replacement of sensitive species by tolerant species, seemed to be the dominating process in diatom benthic communities during water scarcity in the streams under investigation. A better evaluation of species autecology could improve the evaluation of water quality in Mediterranean streams.