Aquatic habitats are threatened by human activities in Turkey. These threats can lead to changes in ecological conditions and community composition of ostracod species. Overall, reduction in habitat quality may increase the number of tolerant (e.g., generalist) species in relation to the number of native (e.g., specialist) species. This phenomenon is called pseudorichness and is also characterized by a critical increase in the total numbers of colonial bacteria. A total of 16 ostracod species were encountered in Lake Abant (Bolu, Turkey) and its environs between 2001 and 2003. Two species (Psychrodromus fontinalis, Eucypris pigra) found outside the lake are new records for the region. About 80% of the species in this area have a cosmopolitan distribution throughout the Holarctic region. Four major clustering groups (UPGMA) were recognized based on species occurrences and ecological preferences. Each species was distinctly tolerant to different ecological variables though cosmopolitan species tended to have wider tolerance ranges compared to sensitive species. About 82% of the relationship between species and environmental variables was explained with Canonical Correspondence Analysis (P<0.05). Accordingly, temperature, dissolved oxygen and conductivity of water were found to be the three most influential factors affecting species composition. Spearman correlation analysis showed a significant positive relationship (P<0.01) between the number of individuals and both dissolved oxygen and the number of species. Generally, to provide long-term conservation of the Lake Abant Nature Park, immediate attention is needed for the wastewater treatment, and help from international organizations.