Environmental and Spatial Control of Freshwater Diatoms—a Review

The examination of correlates of diatom community structure has long been centred on local environmental factors instead of broad-scale biogeographical factors largely because diatom distribution has been considered cosmopolitan. However, a number of recent publications have stressed the role of regional constraints such as historical factors, evolution and variability in modern day dispersal ability in determining the diatom community structure. This study aims to review the literature examining freshwater diatom community structure in relation to environmental and spatial gradients, and to consider the results in a wider ecological context. Diatom literature stresses the importance of ion concentration and trophic status as major environmental drivers of diatom distribution in lakes and streams, while physical factors typically have a lesser effect on community structure. Moreover, studies where spatial configuration accounted for a significant proportion of the community variance clearly outnumbered the studies suggesting strict local environmental control. Freshwater diatom communities seem also to be strongly spatially structured. According to studies from both lakes and streams, pure spatial factors account for about 20–30% of the total explainable community variation, and thereby suggest that diatoms lack strict ubiquitous dispersal. For the studies conducted in lakes, a meta-analysis showed that the proportion of variance explained by local environmental factors decreased, though non-significantly, as study extent increased. Although the pattern was relatively weak, this may imply that the role of spatial factors increased with increasing study area. These results suggest that bioassessment programs utilising diatoms should consider spatial factors explicitly using, for example, regional stratification as diatom communities are strongly spatially structured.