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Although flow regime is one of the major drivers of riverine communities, not much is known about how inter‐annual variability and extremes of flow influence community assembly mechanisms. We used data on benthic macroinvertebrates and modelled flow regimes in 23 near‐pristine boreal streams to assess how community assembly mechanisms and species occupancy varied in response to inter‐annual variability in flow conditions across 11 successive years encompassing extreme (both low and high) flow events. A null model approach was used to test whether deterministic or stochastic processes dominated community assembly and how much regional (among‐stream) flow variability contributed to community variability (β‐diversity). Mean daily flow and the greatest rate of flow rise were the strongest flow‐related descriptors of invertebrate community composition. Communities were differentially assembled depending on the direction of change in flow magnitude: in high‐flow years, communities were more similar than expected by chance, while at low flows they tended to be more dissimilar than expected. Beta‐diversity of macroinvertebrate communities was related to among‐stream flow variability only at high flows. Common species correlated strongly with flow variability and contributed most to variation in β‐diversity, suggesting that changes in assembly mechanisms are mainly driven by common species. While homogenization of communities in high‐flow years reflected increased species occupancies and environmental sorting, increased turnover during low flows likely resulted from stochastic extinctions and dispersal limitation. Our findings suggest that extreme hydrological events exert a strong control over stream invertebrate community assembly, and their effect may be even more profound in the future as high and low‐flow spells are expected to occur more frequently, not allowing time for communities to recover.

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One of the key challenges to understanding patterns of β diversity is to disentangle deterministic patterns from stochastic ones. Stochastic processes may mask the influence of deterministic factors on community dynamics, hindering identification of the mechanisms causing variation in community composition. We studied temporal β diversity (among-year dissimilarity) of macroinvertebrate communities in near-pristine boreal streams across 14 years. To assess whether the observed β diversity deviates from that expected by chance, and to identify processes (deterministic vs. stochastic) through which different explanatory factors affect community variability, we used a null model approach. We observed that at the majority of sites temporal β diversity was low indicating high community stability. When stochastic variation was unaccounted for, connectivity was the only variable explaining temporal β diversity, with weakly connected sites exhibiting higher community variability through time. After accounting for stochastic effects, connectivity lost importance, suggesting that it was related to temporal β diversity via random colonization processes. Instead, β diversity was best explained by in-stream vegetation, community variability decreasing with increasing bryophyte cover. These results highlight the potential of stochastic factors to dampen the influence of deterministic processes, affecting our ability to understand and predict changes in biological communities through time.

A. M ä ki-Pet ä ys and T. Vehanen, Finnish Game and Fisheries Research Inst., Oulu Game and Fisheries Research, PO Box 413, Finland. Temporal coherence or spatial synchrony refers to the tendency of population, community or ecosystem dynamics to behave similarly among locations through time as a result of spatially-correlated environmental stochasticity (Moran eff ect), dispersal or trophic interactions. While terrestrial studies have treated synchrony mainly as a population-level concept, the majority of freshwater studies have focused on community-level patterns, particularly in lake planktonic communities. We used spatially and temporally hierarchical data on benthic stream invertebrates across six years, with three seasonal samples a year, in 11 boreal streams to assess temporal coherence at three spatial extents: 1) among regions (watersheds), 2) among streams within a region, and 3) among riffl es within a stream, using the average of correlation coeffi cients for stream/riffl e pairs across years. Our results revealed the primacy of strongly synchronized climatic factors (precipitation, air temperature) in inducing temporal coherence of macroinvertebrate assemblages across geographically distinct sites (i.e. Moran eff ect). Coherence tended to decrease with increasing spatial extent, but positive coherence was detected for most biological variables even at the largest extent (about 350 km). Th e generally high level of coherence refl ected the strong seasonality of boreal freshwater communities. A hydrologically exceptional year enhanced the synchrony of biological variables, particularly total macroinvertebrate abundance. Regionally low precipitation in that year led to a substantial decrease in benthic densities across a broad spatial extent, followed by a rapid post-drought recovery. Coherence at the among-riffl e (within-stream) extent was lower than expected, implying that local-scale habitat fi lters determine community dynamics at smaller spatial extents. Th us, temporal coherence of stream benthic communities appears to be controlled by partly diff erent processes at diff erent spatial scales.Ecological processes and patterns vary through space and time, and population and community ecologists since Elton (1958) and MacArthur (1955) have strived to understand the magnitude and causes of this variation. In his classical study, Moran (1953) documented inter-annual covariation between spatially distinct lynx populations that resulted from corresponding climatic variation among regions (i.e. Moran eff ect). Since then, the tendency of population, community or ecosystem dynamics to behave similarly among locations over time, referred to as spatiotemporal autocorrelation, spatial synchrony, or temporal coherence, has been studied in a variety of species and biological systems ranging from squirrels (Ranta et al. 1997) to butterfl ies (Powney et al. 2010), and from fossil algae (Patoine and Leavitt 2006) to grassland plants (Heisler and Knapp 2008).Interestingly, however, research on synchrony has tak...