The Time-Dependent Dynamics of Trophic State Index Related with Some Rehabilitation Approaches: Lake Mogan over 30-Year Period

In this study, the temporal changes of the trophic state in Lake Mogan were examined to evaluate the results of various rehabilitation approaches, including dredging, over a 30-year period. Water samples were taken from two different points seasonally between May 2022 and February 2023. The Carlson’s Trophic State Index (TSI) was calculated using total phosphorus, Secchi depth and chlorophyll a values, and the trophic status of the lake was estimated. Furthermore, TSI was also calculated using data obtained from previous studies conducted in the lake, and a 34-year change was evaluated based on TSI, OECD, and Turkish Surface Water Regulation (SWQR). In this study, the average TSI value was calculated as 52.46, and the trophic status of the lake was determined as eutrophic. Differences in TSI (AVG) values among the months and sampling points were found statistically significant (p<0.05). Based on the TSI values calculated using data from previous years, it was determined that the trophic status of the lake still maintained a eutrophic level in a 34-year period despite the rehabilitation efforts. A similarity test (Bray-Curtis Similarity Index) applied to the TSI values obtained for different years showed a strong similarity between the values in 2023 and those in 1989. As a result, preparing precautionary plans against eutrophication of Lake Mogan and monitoring the trophic status via TSI is recommended.