The ecological characteristics of ostracods in Lake Sunnet were monitored monthly between May 2005 and September 2007; 9 living and 5 subfossil ostracod taxa were recorded. Among the species, Ilyocypris getica was reported in Turkey for the first time; while Stenocypria fischeri was found for the second time in the country in 55 years. The most abundant species, Limnocythere inopinata, was collected in dry and wet months, while I. bradyi was the only species found in only the wet months. The 9 species clustered into 3 groups based on their ecological preference. Species composition and occurrences were closely related to the lake water level fluctuation, from 7 to 13 m within a year. These fluctuations played a significant role in changes in the physicochemical characteristics of the lake, along with ostracod species composition (P < 0.01). A strong negative correlation (P < 0.05) between precipitation and both ambient temperature and SO4 was also interpreted as the outcome of such fluctuations. The number of species corresponded with salinity changes. Most of the ostracod species appearing in Lake Sunnet were tolerant to the large water level fluctuation. These results correspond with the idea of 'pseudorichness,' when species composition is dominated by cosmopolitans over noncosmopolitans, an indication of low water quality in a lake. Changes in the lake have been accelerated by anthropogenic activities such as water diversions for chicken farms and agricultural fields during dry climatic conditions. The occurrence of these activities appears to have produced seasonal differences in the lake's water quality and species composition.