Spatial and Seasonal Dynamics of Microbial and Phytoplankton Communities and Factors Influencing the Surface Water and Sediments of the Ponjavica River in the Ponjavica Nature Park.

Microorganisms are key bioindicators of changes in biogeochemical cycles and the ecological status of aquatic ecosystems. In this paper, seasonal changes in culturable microbial groups, phytoplankton, and physicochemical water characteristics were examined as integrated indicators of environmental status in the Ponjavica River (Ponjavica Nature Park, Serbia). Sampling was conducted at six sites (P2-P7) in 2021, with two campaigns carried out in spring (May) and autumn (November). At all sites, the number of microorganisms was higher in sediment than in water, confirming the importance of sediment as a reservoir of microbial activity and a potential biomonitoring matrix. In autumn, the number of microorganisms in the water was higher than in spring, with anaerobic bacteria predominating. Phytoplankton communities exhibited pronounced seasonal dynamics. In autumn, Cyanobacteria dominated, accounting for 59-99% of total biomass, while in spring, Chlorophyta (26-97%) and Cryptophyta (2-37%) were more prevalent. The increased number and biomass of phytoplankton in autumn were accompanied by higher concentrations of phosphorus and nitrogen, which may reflect increased nutrient uptake and conditions favorable to phytoplankton development. Higher concentrations of iron, manganese, arsenic (except at P7), zinc, chromium, lead, and copper were recorded. These spatial and seasonal differences may be related to variations in nutrient inputs from the surrounding watershed, with agricultural activities as one potential source. Spatial analysis identified site P7, where higher conductivity, increased suspended matter, nitrate, calcium, sulfate, and greater phytoplankton abundance were recorded in autumn. The results confirm that seasonal nutrient input and local physicochemical conditions significantly shape culturable microbial groups and phytoplankton communities, highlighting their potential for biomonitoring pollution and assessing the ecological status of river ecosystems in protected areas.