Bacterial metacommunity organization in a highly connected aquatic system

The spatial structure and underlying assembly mechanisms of bacterial communities have been studied widely across aquatic systems, focusing primarily on isolated sites, such as different lakes, ponds and streams. Here, our main aim was to determine the underlying mechanisms for bacterial biofilm assembly within a large, highly connected lake system in Northern Finland using associative methods based on taxonomic and phylogenetic alpha-and beta-diversity and a large number of abiotic and biotic variables. Furthermore, null model approaches were used to quantify the relative importance of different community assembly processes. We found that spatial variation in bacterial communities within the lake was structured by different assembly processes, including stochasticity, species sorting and potentially even dispersal limitation. Species sorting by abiotic environmental conditions explained more of the taxonomic and particularly phylogenetic turnover in community composition compared with that by biotic variables. Finally, we observed clear differences in alpha diversity (species richness and phylogenetic diversity), which were to a stronger extent determined by abiotic compared with biotic factors, but also by dispersal effects. In summary, our study shows that the biodiversity of bacterial biofilm communities within a lake ecosystem is driven by within-habitat gradients in abiotic conditions and by stochastic and deterministic dispersal processes.Peer reviewe

Excess of nitrogen reduces temporal variability of stream diatom assemblages

Abstract Nutrient enrichment degrades water quality and threatens aquatic biota. However, our knowledge on (dis)similarities in temporal patterns of biota among sites of varying level of nutrient stress is limited. We addressed this gap by assessing temporal (among seasons) variation in algal biomass, species diversity and composition of diatom assemblages in three streams that differ in nutrient stress, but are otherwise similar and share the same regional species pool. We monitored three riffle sections in each stream bi-weekly from May to October in 2014. Temporal variation in water chemistry and other environmental variables was mainly synchronous among riffles within streams and often also among streams, indicating shared environmental forcing through time. We found significant differences in diatom assemblage composition among streams and, albeit less so, also among riffles within streams. Diatom assemblages in the two nutrient-enriched streams were more similar to each other than to those in the nutrient-poor stream. Taxa richness did not differ consistently among the streams, and did not vary synchronously at any spatial scale. Temporal variation in diatom assemblage composition decreased with increasing DIN:TotP ratio, likely via a negative effect on sensitive taxa while maintaining favorable conditions for certain tolerant taxa, irrespective of season. This relationship weakened but remained significant even after controlling for stochastic effects, suggesting deterministic mechanisms between nutrient levels and diatom assemblage stability. After controlling for stochastic effects temporal variability was best explained by DIN suggesting that excess of nitrogen reduces temporal variability(intra-annual beta diversity) of diatom assemblages. The high temporal variation, and especially the lack of temporal synchrony at the within streams scale, suggests that single sampling at a single site may be insufficient to reliably assess and monitor a complete stream water body. Our results also showed that measures including species identity outperform traditional diversity metrics in detecting nutrient stress in streams