3 A new version of Trophic State Index for freshwater Himalayan lake (TSI FHL) has been 4 derived from Landsat 8 OLI to determine the aquatic health of the lake ecosystem. TSI FHL is 5 based on chlorophyll-a concentration (C Chl-a) which has been retrieved from Landsat 8 OLI 6 data and laboratory measurements using an empirical approach. Further, in-situ 7 measurements have also been taken with Secchi disk depth (Z SD) in a freshwater Himalayan 8 lake (FHL). The derived C Chl-a exhibited lower and upper limit of 25.81 µg/L and 207.96 9 µg/L respectively. The modelled Z SD values ranged between 0.18 m to 0.66 m with an 10 average depth of 0.50 m. The best-fitted regression model, developed for C Chl-a with R² = 11 0.89, exhibited model error of 0.77 µg/L for the Standard Error of Estimate (SEE). The Mean 12 Absolute Percentage Error (MAPE) and Nash-Sutcliffe coefficient (E) values were 5.83 % 13 and 0.98 µg/L respectively. For the Z SD, the best-fitted model showed errors of 0.11 µg/L 14 (SEE), 13.93 % (MAPE), and 0.77 µg/L (E) with R² = 0.84. 15