Environmental heterogeneity among basins shapes phytoplankton patterns in a shallow, highly mixed reservoir

Understanding how basin-scale heterogeneity influences phytoplankton in regulated reservoirs is essential for reconciling water management and conservation objectives. We analysed phytoplankton assemblages across five environmentally distinct basins and the Kis-Tisza of Lake Tisza during the exceptionally dry summer of 2022, when water levels were reduced to winter stage. The aim of our study was to investigate how spatial heterogeneity among these areas and temporal environmental variability jointly shape phytoplankton assemblages in a large, shallow reservoir system. Altogether 387 taxa were identified from 79 samples, revealing high species richness despite strong hydrological regulation. Significant spatial differences occurred in transparency, COD and nutrient concentrations, while temporal effects explained a larger proportion of community variation than spatial factors. Rare taxa (≈25% of all records) substantially contributed to basin-level richness, particularly in the protected, shallow Tiszavalk Basin and the relatively isolated Tiszafüred Basin. A clearly distinct assemblage was detected only in the intensively used, deeper Abádszalók Basin, characterized by mixing-tolerant centric diatoms, cryptomonads and euglenoids. Five threatened taxa listed on the Hungarian Red List were recorded, with higher occurrence in protected or isolated basins. Our findings demonstrate that although strong hydrological connectivity constrains basin-scale differentiation, protected and relatively isolated basins can contribute disproportionately to phytoplankton biodiversity and the occurrence of rare taxa. Explicit integration of algological biodiversity into reservoir management is therefore crucial in multi-purpose artificial water bodies facing increasing climatic extremes.