Assessing water color anomalies: A hue angle approach in the Gulf of Izmit

Understanding water color is important because it is influenced by biogeochemical constituents, directly affects human perceptions of water quality, and serves as a key water quality metric. This study evaluates the hue angle methodology for monitoring water quality dynamics, using the Gulf of Izmit in the Sea of Marmara as a case study. We employed in-situ water quality measurements, spectroradiometer data, and Sentinel-2 satellite imagery from campaigns in October 2021 and 2022. Hue angles were computed for these dates, and spatial distribution maps were generated to assess water quality dynamics and detect anomalies. The accuracy of these maps was validated using hue angle values from ASD FieldSpec 4 spectroradiometer measurements, with metrics including Mean Relative Error (MRE), Root Mean Square Error (RMSE), and Mean Absolute Error (MAE) demonstrating the method's feasibility (RMSE, MRE, MAE for 2021: 3.16°, 0.02 %, 2.91°; for 2022: 2.36°, 0.01 %, 1.65°). Correlation analyses with selected optically active constituents, such as Chlorophyll-a (Chl-a) and electrical conductivity (EC), as well as non-optically active constituents like Secchi Disk Depth (SDD) and silicate, revealed strong correlations, particularly improving from 2021 to 2022. The methodology's performance was further evaluated using Sentinel-2 data from April 29, 2021, in the Gulf of Gemlik, chosen for its similar environmental challenges. High correlations with in-situ measurements were observed, even without spectroradiometer validation, providing robust evidence of the method's effectiveness in detecting water quality anomalies across diverse coastal environments.