The aim of the investigations was a recognition of the complex of physico-chemical parameters affecting the dynamics and structure of phytoplankton. The taxonomical composition, abundance and biomass of nano- and phytoplankton has been determined. In the development of the reservoir four stages were distinguished. In stage I of the reservoir filling, the temperature and pH value of water as well as the level of PO4-P, had a decisive influence on the process of phytoplankton formation. A sudden change in the damming ordinate in stage II of the reservoir studies contributed to the development of nanoplanktonic diatoms, and the refilling of the reservoir ( stage III) caused an abundant development of green algae. After the reservoir had been filled (stage IV) the planktonic forms of cryptophytes group dominated. The initial dominance of taxons had no influence on its structure after a four-year searching. A very similar pattern of proportionally altering phytoplankton communities in the investigated basins (BM and BD) suggests that both the depth and availability of nutrient compounds were not the limiting factors in the initial period of functioning of the Dobczyce Reservoir.