Uzungöl Lake is situated in the eastern region of Turkey which is one of the most important ecological regions in terms of biological diversity. In this region important economic activities are tourism and fisheries. However, the lake is currently threatened by pollution because of; (i) poorly managed septic systems; (ii) landbased fish farms located on Haldizen Creek; (iii) sediment and debris flow to the lagoon through creeks; (iv) excessive erosion in the catchment area; and (v) flow of fertilizers, nutrients and pesticides into the lake due to agricultural activities. The objectives of the present study are to evaluate historical water quality data from 2006 to 2011 at four monitoring sites, three located at Haldizen Creek and one in the middle of Uzungöl Lake. Specific objectives are to: (i) determine concentration trends in Uzungöl Lake during the study period; (ii) estimate nutrient loads from Haldizen Creek to Uzungöl Lake; (iii) assess water quality classification of Haldizen Creek; (iii)evaluate how land-based fish farm affects Haldizen Creek’s water quality; (iv) use the Trophic State Index (TSI),Water Quality Index (WQI) and Total Nitrogen/Total Phosphorus (TN:TP) ratio to evaluate the eutrophicationtrends. A six-year data set showed that TSI values of Secchi Depth (SD) and TP remained within the range ofhypereutrophic and eutrophic category, respectively. TN/TP ratios suggest Uzungöl Lake can be considered Plimited and that the TN/TP ratio is generally always greater than 12 (57 out of 61 measurements). The WQI forUzungöl Lake shows some slight improvement from “POOR WQI” to “MARGINAL WQI” from 2006 to 2011.Haldizen Creek moderately influences Uzungöl Lake’s water quality in terms of ammonium–nitrogen, totalphosphate, and number of total and fecal coliforms. However, this study demonstrated the significant effect of fishfarm discharges in terms of NH₄-N and TP loadings during study period although its water quality class didn’tchange before and after fish farming.