Abstract
Nitrogen is one of the essential elements limiting growth in aquatic environments. Being primarily of anthropogenic origin, it exerts negative impacts on freshwater ecosystems. The present study was carried out at the nitrate-vulnerable zone within the alluvial aquifer of the large lowland Drava River. The main aim was to investigate the ecosystem’s functionality by characterizing the bacterial and phytoplankton diversity of a small inactive gravel pit by using interdisciplinary approaches. The phytoplankton community was investigated via traditional microscopy analyses and environmental DNA (eDNA) metabarcoding, while the bacterial community was investigated by a molecular approach (eDNA). Variations in the algal and bacterial community structure indicated a strong correlation with nitrogen compounds. Summer samples were characterized by a high abundance of bloom-forming Cyanobacteria. Following the cyanobacterial breakdown in the colder winter period, Bacillariophyceae and Actinobacteriota became dominant groups. Changes in microbial composition indicated a strong correlation between N forms and algal and bacterial communities. According to the nitrogen dynamics in the alluvial aquifer, we emphasize the importance of small water bodies as potential buffer zones to anthropogenic nitrogen pressures and sentinels of the disturbances displayed as algal blooms within larger freshwater systems.
Highlights
Various aspects of nutrient dynamics in freshwater ecosystems are of paramount importance for understanding how the productivity of surface waters is controlled and provide the opportunity to analyse the current and future impacts of anthropogenic activities on freshwater ecosystems
The highest value of nitrates (NO3−) concentration was measured in March (38.4 mg L−1) and the lowest in June (0.62 mg L−1), whereas the maximum concentrations of ammonium (NH4+) and nitrites (NO2−) were recorded in July (2.75 mg L−1 and 0.17 mg L−1, respectively)
For electrical conductivity (EC) and bicarbonates (HCO3−), the maximum values were recorded in March (497 μS cm−1 and 249 mg L−1, respectively) and the minimum ones in October (252 μS cm−1 and 107 mg L−1, respectively)
Summary
Various aspects of nutrient dynamics in freshwater ecosystems are of paramount importance for understanding how the productivity of surface waters is controlled and provide the opportunity to analyse the current and future impacts of anthropogenic activities on freshwater ecosystems In such environments, a large part of the primary production may depend on the recycling of nutrients such as nitrogen compounds [1]. Nitrogen is an essential element that often limits growth in aquatic ecosystems, and a key compound in many biochemical processes that are important for life, but can be harmful in high concentrations [2,3,4] Nowadays, anthropogenic activities such as fertilizer synthesis and its widespread application on arable areas, as well as the burning of fossil fuels, significantly increase the N fluxes across different environmental compartments [3,4,5]. Alluvial groundwater is vulnerable to nitrate leaching from agricultural soils, since agricultural land is characterized by the presence of shallow groundwater and fertile soil suitable for farming [14,15]
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