Application of eDNA metabarcoding to phytoplankton research in freshwater and saline lakes

Freshwater lakes have been heavily altered by multiple stressors for decades, including a wide range of harmfulhuman activities. Similarly, it is estimated that most of continental saline lakes will have undergone changes in theirfunctioning by 2025. In the European Water Framework Directive (WFD), the ecological status assessment of lakesis based on the survey of key communities (i.e., Biological Quality Elements, BQEs). Due to its ecological features,phytoplankton is one of the key BQEs required by the WFD. In current assessments, phytoplankton is studiedusing a standardised microscopy approach. The advent of methods based on molecular techniques (e.g., eDNAmetabarcoding) provides an alternative to overcome issues associated with microscopy-based assessment. Theaim of our study was to compare phytoplankton data obtained by microscopy approach with eDNA metabarcodingbased on the 23S RNA marker gene. Water was sampled in eight sites: four sites in Savsko lake (urban freshwaterlake) and four sites in Plava Banja (saline clay pit pond) during spring, summer, and autumn 2023. The waterquality analyses showed that nutrient concentrations (especially total nitrogen), chlorophyl a and conductivity wereseveral times lower in the freshwater lake than in the saline lake. The very high total dissolved mineral content inPlava Banja had a substantial impact on the phytoplankton composition The results of lake species inventoriesobtained using both approaches will be presented. Particular attention will be paid to the level of congruencebetween the fl oristic lists.