Monitoring of Optically Active Constituents (OACs) in the Southern Caspian Sea: Concurrent Match-up Between In-Situ and Satellite Observations

Field sampling for this research was conducted in the Southern Caspian Sea with careful planning to ensure broad spatial coverage and high-quality data collection. Sampling stations along each transect were spaced 4 km apart, with adjacent transects separated by 10 km. Six research cruises in the summer of 2011 under varying environmental conditions provided a representative dataset of the marine ecosystem. Key bio-optical components, including Colored Dissolved Organic Matter (CDOM) absorption, Chlorophyll-a concentration (Chl-a), and turbidity, were analyzed. CTD sensors (Idronaut OCEAN SEVEN 316) were used to measure physico-chemical parameters such as salinity, temperature, and dissolved oxygen, while surface water samples were collected for laboratory analysis. CDOM absorption was measured using the PERKIN ELMER Lambda 25 spectrophotometer, and the data were fitted with an exponential function to calculate parameters like the spectral slope (S) and aCDOM(375). In situ Chl-a measurements were taken with the Seapoint Chlorophyll Fluorometer, and water turbidity was assessed with the Seapoint Turbidity Meter. Higher values of aCDOM(375) (ranging from 0.38–3.52 m⁻¹) were observed at nearshore stations, indicating significant freshwater influence. Linear regression analysis showed a weak inverse relationship between aCDOM(375) and salinity (r = -0.08; R² = 0.007), suggesting minimal salinity impact on CDOM variability. The match-up analysis revealed that turbidity and dissolved organic matter products tended to underestimate values, while chlorophyll was overestimated using satellite data. The lowest mean absolute error was found for CDOM (0.84). The spatial variability of bio-optical components highlights the influence of freshwater inputs and nearshore processes. The weak correlation with salinity suggests that other factors, such as terrestrial inputs and biological activity, may affect CDOM variations in the region.