In this study, seawater optical data was collected between 1996 and 2018 to discuss relationship between observed Secchi depth and measured light transmission in İzmir Bay. The turbidity values increase from inner bay (İzmir city centre and harbour), where eutrophication conditions were reported throughout the year, towards outer bay (Aegean Sea opening). In the present study, a relation with a very good agreement in space and time was found between light transmission and Secchi depth. Pearson correlation coefficients were calculated as 0.924, 0.96 and 0.948 between Secchi disk and light transmission data for inner, middle and outer bay respectively. The model equations were fixed for space-based regression analysis. Therefore, the space-based regression models could be used to estimate unmeasured Secchi depths from frequently measured light transmission values in the Bay. Use of this regression will assist the interpretation of historical data base where Secchi depth only available and also to calculate unmeasured Secchi depths from measured light transmission data.