Eutrophication has become a widespread matter of concern during the last 50 years. Because of the relationship between biotic and abiotic factors, the determination of the trophic level of lakes is important. In this study, the trophic level of Lake Uluabat was determined using different trophic states indices (TSISD, TSICHL, TSITp, and TSITN) in 8 stations between May-2008 and May-2009. As a result, the indices were above the eutrophic level during the year. With regard to the seasonal variations of the indices, the highest values were in the summer and autumn (hypertrophic), while the lowest values were in the spring and winter seasons (hypertrophic-eutrophic). It was seen that in addition to nitrogen and phosphorus, temperature affected the growth of algae in the summer. For this reason, Chl-a (maximum 43.87 mg/m(3)) and alkalinity (maximum 282.85 mg/L) concentrations raised, and the dissolved oxygen (minimum 5.08 mg/L) concentrations decreased in the summer. It was observed that the diversity of the aquaculture species (fish and algae) decreased and the species having low economic value were dominant with an increase of the eutrophication level. According to the ratio between the nitrogen and phosphorus and a significant statistical correlation between nitrogen forms and chl-a, the algae species were co-limited by nitrogen with phosphorus the whole year and might be species that were each limited by different nutrients. The high diversity of species in the Lake, discharging of untreated domestic wastewater and chemical fertilizer usage were determined as the reasons of the co-limitation of the nitrogen with phosphorus. Also according to the trophic level and concentration values, Lake Uluabat exceeded the limit for potable water and water-oriented recreation.