Modeling Anthropogenic Pressure with Synthetic Water and Wastewater Media: Growth Response of Lemna minor Under a Controlled Chemical Gradient

Anthropogenic pressure on aquatic environments is currently associated not only with the inflow of nutrients, but also with the presence of municipal micropollutants and complex mixtures of sewage origin. This study aimed to evaluate the growth response of Lemna minor (L. minor) and the behavior of the plant–medium system under conditions of a controlled chemical gradient reflecting an increasing degree of anthropogenic transformation of the aquatic environment. The experimental setup included four synthetic media: two variants of river water with varying anthropogenic loads and two variants of synthetic wastewater, one of which was enriched with caffeine as a marker of municipal pressure. The experiment was conducted in two independent 14-day series, analyzing changes in nitrate and orthophosphate concentrations, pH, conductivity, plant morphological condition, and final biomass. The most favorable growth conditions for L. minor were found in the river variants, especially in the medium with higher loading, whereas the wastewater variants caused growth inhibition, deterioration in plant condition, and greater instability of physicochemical parameters. In the wastewater media, nitrate concentrations did not show a consistent net decrease, while orthophosphate concentrations remained high. These observations indicate that the measured nutrient forms were not effectively reduced under the tested conditions. However, the processes responsible for their temporal patterns could not be identified from the monitored parameters. The addition of caffeine modified the system’s response, but its effect was not quantitatively consistent across both series. The results confirm the usefulness of synthetic media for modeling anthropogenic pressure gradients.