Complex patterns in phytoplankton and microeukaryote diversity along the estuarine continuum

Estuaries and coastal lagoons are included within the transitional waters category, according to the Water Framework Directive. However, criteria for their differentiation and characterisation are still under discussion and require more research. In particular, detailed observations of biodiversity in more complex transitional and coastal waters are lacking. Microscopic and molecular analyses were therefore used to investigate phytoplankton diversity and spatial community structure, in early spring, along the freshwater-to-marine continuum of the Segura River (Spain), an intensively regulated semiarid basin discharging into the Mediterranean Sea. In addition to the salinity gradient as the major factor determining taxa distribution, influence of multiple anthropogenic and climatic impacting factors (drought, confined waters, irrigation canal) leads to a significant spatial heterogeneity of the aquatic habitat types associated with variations in community composition. Several shifts within the phytoplankton distribution pattern along the continuum are revealed using multivariate analyses. An impressive bloom of the cryptophyte Plagioselmis prolonga occurred in the mixing zone, associated with a typical euryhaline community indicative of eutrophication. The 18S rDNA diversity revealed a microeukaryotic richness including several little-known groups, heterotrophic representatives, and potential parasites. By combining morphological and molecular approaches we revealed the presence of a ‘hidden’ diversity often neglected in traditional surveys.