Diatom biogeography in freshwaters–new insights from between-region comparisons and the role of unmeasured environmental factors

Diatom biogeography has attracted increased attention especially over the past two decades. However, due to covariance between space and environment, diatom biogeographical studies suffer from the fundamental problem of reliably evaluating the relative roles of dispersal processes versus local environment in shaping diatom distributions. Using between-region comparisons, we investigate whether diatom community differences stem partly from dispersal effects or are purely driven by the local environment. The data used comprise diatom presence–absence records and associated environmental-variable data from streams in Iceland and in northern Fennoscandia (Finland and Norway), and from ponds and lakes in Italy and northern Fennoscandia. We found that between-region differences were larger for diatom community composition than for local environmental variables, perhaps suggesting that biogeographical factors such as dispersal limitation or dispersal history influenced diatom communities causing their spatial segregation. We further discuss methods for disentangling the roles of spatial and environmental effects in shaping diatom communities. Finally, we highlight the possible role of unmeasured environmental factors in diatom biogeographical research.