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The spatial structure significantly affected the community structure of desmids and diatoms at the study sites. Interestingly, the geographic distances were slightly more pronounced in diatoms, suggesting that, compared with desmids, their community structure was relatively more strongly structured by large scale processes, such as dispersal, climate, or history. Similar significant effects on the community structure at the regional level were reported in several studies of benthic diatoms from streams (Soininen et al., 2004; Heino et al., 2010; Smucker & Vis, 2011; Virtanen & Soininen, 2012). Individual geographically constrained distribution areas were also recently detected for several large desmid taxa in the genus M craster as (Neustupa et al., 2011b). In this study, the large species of desmids and diatoms were clearly more geographically restricted than taxa with smaller cell biovolumes. The significant effect of spatial distances among localities, which was not correlated with environmental data, although more important for large diatoms, was still highly significant in large desmids. By contrast, the datasets of small species lacked any significant spatial structure that was not accounted for by environmental factors. This pattern may suggest that the spatial pattern in the species data was actually related to dispersal limitations, which should be considered as a structuring factor for phytobenthic communities in peatlands. At a regional scale, dispersal limitations are probably more important for large species, which may have less effective passive dispersal (Heino & Soininen, 2006; Passy, 2007; Vanormelingen et al., 2008a). Overall, our results showed that local environmental parameters are important for structuring the phytobenthic assemblages of peatlands, but they may mask the important effects of dispersal-related processes at regional scales, which are related to the cell sizes of individual taxa. However, we should also note that species concepts of microalgae, including desmids and diatoms, are notoriously unstable, and numerous recent studies have detected cryptic or pseudocryptic diversity within traditional morphospecies (Vanormelingen et al., 2008b; Evans et al., 2009; Poulickova et al., 2010). Small desmid and diatom species typically have fewer conspicuous morphological discriminatory characters. Thus, there may be more cryptic species in the relatively small taxa compared with larger species. This may lead to an underestimation of species diversity among small taxa in ecological studies, including this one, based on morphological species concepts. Thus, the lack of significant geographic structure among the small taxa in the current study may be explained by the low reliability of taxonomic concepts in these species. To the best of our knowledge, there have been no rigorous analyses of the level of cryptic species differentiation relative to the cell size of individual traditional taxa. However, such data would be very useful for estimating the size-related dispersal limits of microalgae.Acknowledgments This study was supported by Grant No. 13-29315S from the Czech Science Foundation. The authors are indebted to Magda Skaloudova for her sampling assistance. The authors thank Bioedit proofreading service for the language and style corrections. We thank the anonymous reviewers for their recommendations that led us to the improvements of the manuscript.
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