Benthic Diatom Community Structure in Boreal Streams Distribution Patterns along Environmental and Spatial Gradients

The past decade has seen growing appreciation of the role of regional influences in determining the structure of local communities. An emerging view among ecologists is that local community composition is controlled by acting of nested filters which select species with suitable traits for prevailing conditions, thus leading communities regulated by local environmental factors and regional, mainly historical or dispersal related factors. Running waters are naturally open, hierarchical and heterogeneous ecosystems. This heterogeneity prevails in physical, chemical and biological elements across multiple spatial and temporal scales. The growing and prospering of benthic algae in streams is the outcome of complex interactions between hydrological, chemical and biotic factors. Diatoms constitute a major part of the cell and species number in benthic algal communities offering the most useful algal community for studying large-scale ecological patterns in stream ecosystems. The major aims of this thesis were (i) to find the main factors regulating benthic diatom community structure in boreal streams at different spatial scales, (ii) to test the correspondence between ecoregional delineations and spatial patterns in community structure, (iii) to assess seasonal community persistence and stability of benthic diatom communities and (iv) to investigate if benthic diatoms offer a usable tool for water quality assessment. Results of direct ordinations emphasized the predominance of chemical-constituent concentration and ion composition on structuring benthic diatom communities of running waters. Conductivity was the strongest environmental gradient explaining diatom distribution patterns in Finnish running waters at the national scale. The other important determinants of diatom community structure were latitude, pH, total P, and water colour. Results of this thesis showed that diatom communities exhibit a rather strong spatial component especially at a national scale. This was shown both by variation partitioning and by a direct test of congruence between diatom community structure and the spatial coordinates of the sampling sites. The proportion of variation explained independently by spatial factors was quite large, ca. 25 %, at the largest, national, scale. Furthermore, it seems that even at rather small spatial scales (ca. 10 km), pure spatial component still plays an role in regulating benthic diatom community composition. Moreover, data of this thesis support also the view that beta-diversity of benthic diatoms might be higher than previously believed. When studying temporal patterns of community structure, stability tended to be lowest among epiphytic communities. Moreover, species turnover seemed to be highest among epiphyton and lowest among epipelic communities. Although these differences could also result from lower diversity in epiphyton, they probably indicate lower persistence among epiphytic communities in boreal streams. For bioassessment needs, diatom-based weighted averaging models offer usable tool for water quality monitoring of boreal streams. Given the strong spatial patterns in community composition, it seems evident that bioassessment programs utilising lotic diatoms would benefit from geographical stratification, using e.g. ecoregions or subecoregions as regional delineations. However, since local in-stream factors were even more important than spatial factors in explaining diatom distributions, a combination of regional stratification and local environmental features might provide the most suitable framework for diatom-based bioassessment of boreal streams.