Mining activities affect a landscape in different ways, including by the formation of subsidence troughs, which after being inundated form wetlands and lakes. The development of new mining reservoirs may give a unique opportunity to study the early stages of colonization by various freshwater communities, including phytoplankton. Our paper presents the results of phycological research undertaken in newly formed subsidence reservoir near the coal mine “Bogdanka” (Polesie Lubelskie) a few months after its filling with water. The reservoir represented a unique, rare limnologic type due to the fact that it disappeared as a result of intended melioration works in the autumn of the same year. The study focused on morphometric measurements of the episodic reservoir, determination of general physicochemical parameters of water, and qualitative and quantitative structure of the phytoplankton community, was undertaken in five separated basins. A total of 80 algal taxa were determined. Most of them belonged to euglenoids (Euglenophyta-36) and green algae (Chlorophyta-26). Among euglenoids, most species were represented by Trachelomonas genus (14), while among green algae, most species were assigned to Scenedesmus genus (8). Several rare species were found, including: Scenedesmus bacillaris Gutw., Dinobryon petiolatum Willén, and Trachelomonas botanica Playfair. Green algae and euglenoids also had major contributions to the total phytoplankton abundance, which in all study sites did not exceed 2.2×10⁶ ind.·dm^-3. This phytoplankton structure was probably influenced by the high water color related to particular and dissolved organic matter from pre-existing alder forests. Some differences in phytoplankton structure found among sampling sites were probably connected with habitat differentiation in terms of exposure to light.