Eutrophication decreases distance decay of similarity in diatom communities

The distance-decay relationship has been claimed to be a predictor for biological diversity because it unites several ecological phenomena such as dispersal ability and environmental structure. The effect of long-term disturbances on distance decay, however, has been widely overlooked, especially for microorganisms. We examine the effect of eutrophication on the distance-decay relationship in communities of attached diatoms in three peatland areas: mesotrophic, eutrophic and hypertrophic. The study follows a spatially explicit sampling scheme, collecting evenly spaced samples along 6-km sampling tracks. The three areas shared 24% of the total number of species, but the different nutrient levels in the three areas are reflected by the prominence of low profile and planktonic diatom species. Our study demonstrates that eutrophication can affect distance-decay relationships by decreasing turnover rates in microorganisms. Diatom communities are shown to be constrained by both environmental and spatial features, whose relative importance depends on the degree of eutrophication. Under eutrophic conditions, species are filtered from the regional species pool and community structure responds strongly to environmental factors (water chemistry variables and depth), while in mesotrophic environments, purely spatial processes play a prominent role in structuring diatom communities. These findings reveal that homogenisation of communities triggered by environmental disturbance is an ecological phenomenon of importance in the microbial world.