18S-V9 DNA metabarcoding of eukaryotic community structure in freshwater samples

Nitrogen is one of essential elements limiting growth in the aquatic environments. As being primarily of anthropogenic origin it exerts negative impacts on freshwaters ecosystems. The study was carried out at the nitrate vulnerable zone within the alluvial aquifer of a large lowland Drava River. The main aim was to investigate ecosystem resilience and functionality by characterizing the bacterial and phytoplankton diversity of a small inactive gravel pit by using interdisciplinary approaches. Phytoplankton community was investigated via traditional microscopy analyses and environmental DNA (eDNA) metabarcoding, while bacterial community was investigated by molecular approach (eDNA). Variations in algal and bacterial community structure indicated strong correlation with nitrogen compounds. Summer samples were characterized with high abundance of bloom-forming Cyanobacteria. Following the cyanobacterial breakdown in colder winter period, Bacillariophyceae and Actinobacteriota became dominant groups. Changes in microbial composition indicated strong correlation between N forms and algal and bacterial communities. According to the origin and transport of nitrate in the alluvial aquifer, we emphasize the importance of small water bodies as potential buffer zones to anthropogenic nitrogen pressures and sentinels of disturbances displayed as algal blooms within larger freshwater systems.