Using DNA taxonomy to investigate the ecological determinants of plankton diversity: explaining the occurrence of Synchaeta spp. (Rotifera, Monogononta) in mountain lakes

1. The occurrence of unresolved complexes of cryptic species may hinder the identification of the main ecological drivers of biodiversity when different cryptic taxa have different ecological requirements. 2. We assessed the reliability of the influence of ecological drivers on the occurrence of Synchaeta species (monogonont rotifers) in seventeen water bodies of the Trentino-South Tyrol region in the Eastern Alps. To do so, we compared the results by using unresolved complexes of cryptic species, as is common practice in limnological studies based on morphological taxonomy, and by resolving cryptic complexes, possible by DNA taxonomy. 3. The DNA taxonomy approach we used to identify cryptic species was the generalized mixed Yule coalescent (GMYC) model. We, thus, investigated the relationship between the environment and the occurrence of Synchaeta by multivariate ordination using two definitions of the units of diversity, namely (i) unresolved species complexes (morphospecies) and (ii) putative cryptic species (GMYC entities). Our expectation was that solving complexes of cryptic species could provide more information than using morphospecies. 4. DNA taxonomy provided higher taxonomical resolution than morphological taxonomy. As expected, environmental-based multivariate ordination on cryptic species explained a significantly higher proportion of variance than the one on morphospecies. Occurrence of GMYC entities was related to total phosphorus (TP), whereas no relationship could be found between morphospecies and the environment. Moreover, different cryptic species within the same morphospecies showed different, and even opposite, preferences for TP. Additionally, the large geographical distribution of haplotypes and cryptic species indicated the absence of limiting barriers to dispersal in Synchaeta.