This study aimed to evaluate the biological quality of the water and the biodiversity of Lake Parches in the pre-delta area, Tulcea County, using phytoplankton as indicator organisms. The phytoplankton samples were collected at the end of September 2021. Species composition plays a key role in the functioning of ecosystems. Diversity is a functional and structural parameter of ecosystems, a descriptor of their health. Phytoplankton Alpha diversity was calculated using the Shannon (H’), Gini-Simpson Index (HS), Brillouin index (HB), Berger-Parker dominance Index (d), and the Evenness Index (J’) (the natural logarithm was used for calculations). Results obtained in all four stations suggest that the phytoplankton diversity in the Parches Lake aquatic ecosystem is good. The saprobiological analysis is based on the results - the assessment criteria delivered by the classic "saprobe system" (1977). Based on the list of indicators, the saprobic index is calculated according to the Pantle Bulk method. The values obtained after calculating the SI index (Saprobity index Pantle and Bulk) suggested that the water fell into the second class of quality.