Microplastics are among the most widespread pollutants in aquatic ecosystems. They accumulate in sediments, where they can influence biogeochemical processes and nutrient cycling. The aim of this bachelor's thesis was to investigate the effects of polyethylene terephthalate (PET) and polylactic acid (PLA) microplastics, applied at low and high concentrations, on phosphorus mobility in freshwater sediments. In addition to evaluating changes in phosphorus concentration in the medium, different phosphorus fractions in the sediment were determined, including total phosphorus, inorganic phosphorus bound to iron, aluminum and manganese, calcium-bound phosphorus, and organic phosphorus. Furthermore, concentration of dissolved oxygen in the sediment, activity of microorganisms based on the alkaline phosphatase enzyme, concentration of total organic carbon (TOC) in the medium, and pH were monitored.
The results showed that phosphorus concentration in the medium decreased throughout the experiment in all treatments, with a slightly greater decrease observed in samples containing microplastics. Sediment analysis revealed changes in the distribution of individual phosphorus fractions, with the most pronounced differences observed for calcium-bound phosphorus. In contrast, differences in total phosphorus between treatments could not be reliably confirmed due to the high standard deviations. The pH remained relatively stable throughout the experiment, whereas concentration of dissolved oxygen decreased in all samples, most notably in treatments with high concentrations of PET and PLA, indicating enhanced microbial activity. TOC decreased in all treatments except in the sample containing a high concentration of PLA, where it increased by more than 50%, suggesting initial steps of biodegradation, which increased the amount of organic carbon in the medium. Alkaline phosphatase activity was highest in the samples with high concentrations of PET and PLA, indicating that higher microplastic concentrations may influence the mineralization of organic phosphorus in the sediment.
Based on the obtained results, it can be concluded that microplastics do not act solely as inert pollutants but also affect phosphorus binding, redistribution, and retention in sediments, primarily through their influence on microbial communities, biofilm development, and changes in the physicochemical properties of the sediment. The effects were generally more pronounced at higher microplastic concentrations, whereas differences between PET and PLA were less evident.
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