Floodplains are dynamic interfaces between aquatic and terrestrial ecosystems, where ecosystem functioning is strongly influenced by microbial communities. To investigate the composition of free‐living and particle‐associated prokaryotic and microbial eukaryotic communities, five interconnected study sites were sampled in one of the best‐preserved Danube floodplains and subsequently analyzed using 16S and 18S rRNA gene amplicon sequencing. We compared community dynamics across low‐water periods and minor to moderate floods and observed flooding to increase microbial diversity and promote gradual community shifts depending on flood intensity, whereas low‐water conditions limited microbial exchange and reduced compositional connectivity across floodplain ecosystems. Dispersal effects were particularly pronounced in microbial eukaryotes, including Perkinsea and Fungi, pointing to the importance of hydrological connectivity in structuring micro‐eukaryotic communities. Flooding also facilitated community mixing and more balanced interspecific interactions, while low‐water periods led to more compartmentalized networks. Core microbial community size increased with flooding intensity, reflecting the influence of ecosystem mixing, allochthonous inputs, and increased nutrient availability in shaping floodplain communities. This study highlights the effects of flooding intensity on both prokaryotic and microbial eukaryotic communities, advancing our understanding of how hydrological variability shapes microbial dynamics in riverine floodplains.

High doses of the antibiotic azithromycin in freshwater environments can impact planktonic organisms at both the individual and community levels, influencing interactions at the base of the food web. This study investigated the effects of azithromycin on the natural rotifer community feeding on phytoplankton from a eutrophic water body and its potential impacts on rotifer fitness (impaired mastax movement: slow, irregular or reduced frequency), grazing and mortality following acute exposure. The natural plankton community was exposed to three azithromycin concentrations based on the EC₅₀ value (EC₅₀, 1/2 EC₅₀ and 1/3 EC₅₀) and assessed at different exposure times (24, 48 and 72 h) in the microcosm experiments. The results showed that all azithromycin concentrations reduced the fitness of the rotifers, as indicated by impaired mastax movement and/or slow, irregular or reduced movement frequency. Impairment of mastax movement altered rotifer grazing and the abundance of phytoplankton. The rotifers in the control group suppressed abundant phytoplankton growth, suggesting that azithromycin impairs interspecific interactions between plankton species. Rotifer mortality occurred at 48 h after azithromycin exposure in all treated samples. These findings show that the effects of azithromycin can be observed at different trophic levels, affecting both phytoplankton and zooplankton through altered biotic interactions and suppressed grazing.

Natural ecosystems, especially those regulated by floods, are sensitive to prolonged temperature fluctuations that affect hydrology and the lateral connection between the river and its floodplain. Here, we analyzed a series of zooplankton data collected monthly from 2007 to 2016 during the ice-free period in Kopački Rit Nature Park in the Middle Danube, an area important as a food source and nursery area for fish stocks in the Danube. The aim was to find out how the long-term change in temperature and fluctuating environmental parameters affect the succession of zooplankton in the warmer (from April to September) and colder parts of the year (from October to March). Throughout the decade, total nitrogen concentrations showed significant differences between years, with an increase since 2012. Despite the increase in nitrogen levels and the expected increase in primary production, the higher nitrogen levels were accompanied by lower zooplankton biomass. A significant difference was found between the values of the zooplankton geometric mean index, with 73% of the variance explained by the difference between groups. In general, a trend toward a significant decrease in zooplankton biomass, with a simultaneous increase in the number of species and high turnover rates, was observed throughout the decade.