Trait-based community assembly of epiphytic diatoms in saline astatic ponds: a test of the stress-dominance hypothesis.

The stress dominance hypothesis (SDH) postulates that strong environmental gradients drive trait convergence in communities over limiting similarity. Previous studies, conducted mostly with terrestrial plant communities, found controversial evidence for this prediction. We provide here the first test for SDH for epiphytic diatoms. We studied community assembly in diatom communities of astatic ponds. These water bodies serve as a good model system for testing SDH because they exhibit stress gradients of various environmental factors. Functional diversity of diatom communities was assessed based on four traits: (1) combined trait reflecting the trade-off between stress tolerance and competitive dominance, (2) cell size, (3) oxygen requirement and (4) N-uptake strategy. According to our results, salinity, pH and the width of the macrophyte belt appeared as significant predictors of the trait convergence/divergence patterns presumably acting through influencing the availability of carbon dioxide and turbidity. Lower trait diversity was found in turbid, more saline and more alkaline ponds and functional diversity was higher in transparent, less saline and less alkaline ponds. Overall, our results supported the stress dominance hypothesis. In habitats representing increased environmental stress, environmental filtering was the most important community assembly rule, while limiting similarity became dominant under more favourable conditions.

Biogeography and phylogenetic position of a warm-stenotherm centric diatom, Skeletonema potamos (C.I. Weber) hasle and its long-term dynamics in the River Danube.

Skeletonema potamos is a poorly known freshwater species in the ancestrally and predominantly marine genus Skeletonema. With phylogenetic analysis of two nuclear {partial SSU (18S) and partial LSU (28S) rDNA)} and two chloroplast (rbcL and psbC) genes, we verified its placement within the genus Skeletonema and identified the mostly brackish species, Skeletonema subsalsum, as its closest known relative. Comparisons of SSU and LSU rRNA genes from S. potamos populations from Europe and North America revealed no intraspecific variation. Skeletonema potamos can be a dominant element of the phytoplankton community in various ecosystems, including the River Danube. We tracked phytoplankton composition in the River Danube weekly from 1979 to 2 012, and throughout this period, S. potamos exhibited a strong increase in proportion of total phytoplankton abundance and biomass - an increase that was correlated with increasing water temperature over the same time period. Current records indicate a temperate distribution of S. potamos, but ecological data predict possible expansion of its geographic range and increase in seasonal duration within existing habitats in response to the warming of surface waters.