Summary Most metacommunity studies aim to explain variation in community structure using environmental and spatial variables. An alternative is to examine patterns emerging at the level of an entire metacommunity, whereby six models of metacommunity structure (i.e. random, chequerboards, nestedness, evenly spaced, Gleasonian gradients and Clementsian gradients) can be examined. We aimed to test the fit of six competing models of metacommunity structure to extensive survey data on diatoms, bacteria, bryophytes and invertebrates from three drainage basins in Finland, along a latitudinal gradient from 66 °N to 70 °N. Species were mainly distributed independently of one another (following the Gleasonian model) in the southernmost drainage basin (66 °N), whereas there were discrete community types, with sets of species responding similarly along environmental gradients (following the Clementsian model), in the northernmost drainage basin (70 °N). The patterns found were not directly related to an expected relationships between environmental heterogeneity and metacommunity structures, but rather to the geographical location of the drainage basin. There is evidently among‐region variation in the best‐fit models of metacommunity structure of stream organisms. These metacommunity patterns may show some similarities among biologically disparate organismal groups sampled at the set of the same sites, although the underlying environmental drivers of those patterns may vary between the groups.

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The International Union for Conservation of Nature (IUCN) Red List Index (RLI) is recognized as one of the key indicators of trends in the status of species. The red-list assessment done by Finnish authorities of species in Finland is taxonomically one of the most extensive national assessments. We used the Finnish Red Lists from 2000 and 2010 to calculate for the first time the national RLIs for 11 taxonomic groups at different trophic levels and with different life cycles. The red-list index is calculated on the basis of changes in red-list categories and indicates trends in the status of biological diversity of sets of species. The RLI value ranges from 0 to 1. The lower the value the faster the set of species is heading toward extinction. If the value is 1, all species in the set are least concern and if the value is 0, all species are (regionally) extinct. The overall RLI of Finnish species decreased. This means that, in Finland, these taxonomic groups were heading toward extinction faster in 2010 than in 2000. Of the analyzed groups of organisms, RLIs of 5 decreased and RLIs of 6 increased. At the national level, the RLIs and status trends varied markedly between species groups. Thus, we concluded that generalizations on the basis of RLIs of a few taxa only may yield a biased view of ongoing trends in the status of biological diversity at the species level. In addition, one overall RLI that includes many different species groups may also be misleading if variation in RLI among species groups is not considered and if RLI values are not presented separately for each group.

Diatoms are widely used in stream quality assessment due to their response to the local environment. Diatoms are also influenced by many large-scale processes and so the diatom communities of boreal streams incorporate a strong spatial component at a regional level. What is not properly known yet is whether the variation in diatom communities between regions is larger than the variation in measured environmental variables. We studied the roles of environment and space in accounting for variability in stream diatom communities across four regions in Finland. According to canonical correspondence analysis, geographical coordinates, nutrient concentrations (total N and P), and water conductivity were the most important factors affecting variation in diatom community composition. Of physical factors, depth and current velocity were also significant. According to Mantel tests, both environmental and geographical distances were related to dissimilarity in diatom community composition. Analysis of Similarities indicated that the regional differences in diatom community composition were larger than the regional differences in environmental variables. We also found many indicator species confined to certain regions. Our results suggest that the four study regions differ in their diatom species composition more than in their environmental features and that diatoms are structured not only by the local environment but also by large-scale processes, possibly related to history, climate and dispersal. These results imply that, while diatom species composition reflects well the environmental differences between regions, future bioassessments would benefit from regional stratification. Otherwise, relationships with environmental variables may be masked by trans-regional differences in species pools caused by the largescale processes.

Water quality in streams typically changes fast, and sensitive biological indicators are crucial for monitoring these changes in water quality. Diatoms are widely used in biological stream quality assessment. However, interannual and intra-annual variation of diatom population densities is large, often hampering a reliable quality assessment. We studied the importance of different species traits on temporal occurrence of diatom species. We also examined whether temporal occurrence of diatoms is related to species' local abundance or regional distribution using a data set collected in Finnish streams. According to the general linear model (GLM), temporal occurrence of diatoms increased with increasing local abundance and regional distribution. Species that occurred more frequently also had larger niche breadths and nonmarginal niche positions. In addition, cell size and attachment ability were positively related to species temporal occurrence. Our results imply that abundant and widely distributed species with large niches and ability to attach sustain persistent populations in varying environmental conditions typical for streams. We suggest that future studies could concentrate on monitoring these common (abundant) species when detecting the possible changes in the biological state of streams. We advise, however, to consider a relatively large number of species as many of the most common species may have low indicator values.