Drivers predicting biodiversity and community assembly of benthic diatoms in a karst spring environment

Geographical location, landscape heterogeneity, and their position at the edge of Quaternary glaciations are the major reasons why the river basins of the Western Carpathians are of the utmost importance for research on European aquatic biodiversity. Moreover, unlike streams, springs have several features determining their insularity in the river continuum. This study aimed to uncover the mechanism forming the diatom assemblages in the karst springs environment, using a two-dimensional approach, including taxonomic and trait data. Thus, (1) the congruence between observed and expected values of null models of taxonomic/functional diversity and functional dispersion and (2) Generalized mixed models and Variation partitioning were used to assess how taxonomic/functional diversity and composition were associated with the local conditions, regional affiliation and spatial springs distance. In 64 rheocrene karst springs (10 karst mountain ranges) of the Western Carpathians, 138 epilithic diatom taxa were found. Null model analyses did not uncover any clear evidence of non-random events forming the spring diatom community. Although, the observed functional diversity did not differ from the null model for most springs, most observed values were lower than the mean value of the null model, suggesting that, except for random events, environmental filtering was an important driver of community assembly. Substratum type, spring connectivity and phosphorus concentration drove the value of taxonomic richness, whereas functional diversity was controlled by silica and bicarbonate concentration as well as spring size. In both models, affiliation to the mountain range (defined as a random factor) was significant. The Variation Partitioning Analysis showed that diatom assemblage composition was mainly determined by local and regional environmental conditions, whilst the distance between springs had only a weak influence.