Seasonal and spatial variability in bio-optical properties of the Persian Gulf: Implications for ocean color remote sensing

In this study, the absorption coefficients of phytoplankton community (a(Ph)(lambda)), Colored Dissolved Organic Matter (CDOM) [alpha CDOM(lambda)], and Non-Algal Particles (NAP) [alpha Nap(lambda)] were analyzed in the northern Persian Gulf, a Mediterranean-type semi-enclosed marginal sea, through seven cruise expeditions conducted from May 2018 to February 2022. The highest and lowest aPh(443) values were observed in the shallow area (depth <= 20 m) during the wet season (November-April) (0.043 +/- 0.004 m(-1)) and within the deep area (depth >20 m) through the dry season (July-September) (0.024 +/- 0.003 m(-1)), respectively. The chlorophyll-specific absorption (a*Ph(lambda)) values increased from the deep to shallow areas as well as the wet to dry seasons. A significant difference was further detected in a*Ph(lambda) magnitude at the blue (443 nm) and red (675 nm) wavelengths, with the mean values of 0.0124/0.0223 m2 mg 1 and 0.0069/0.0135 m2 mg 1 in the wet/dry seasons, respectively. The mean values of aCDOM(443) in the shallow area during the wet season (0.10 +/- 0.001 m(-1)) were significantly larger than those in the deep area during the dry season (0.06 +/- 0.002 m(-1)). Similarly, aNAP(443) decreased from the shallow (0.056 +/- 0.021 m(-1)) to deep areas (0.019 +/- 0.011 m(-1)), and from the wet (0.035 +/- 0.020 m(-1)) to dry (0.029 +/- 0.015 m(1)) seasons. The statistical analysis revealed that the differences in aCDOM(lambda) and aNAP(lambda) spectral distribution were not statistically significant for the duration of the wet and dry seasons, whereas significant differences were observed in the shallow and deep samples. During both wet and dry seasons, the magnitude of the blue-green absorption spectra was dominated by CDOM, but the red bands were associated with phytoplankton dominated samples. The ratio of aCDOM(lambda)/a*Ph(lambda) was utilized as an index for uncertainty assessment to estimate Chla concentrations by the National Aeronautics and Space Administration (NASA) standard Chl-a algorithms for the Moderate Resolution Imaging Spectrometer (MODIS; OC3M) and Sentinel-3 Ocean and Land Color Instrument (OLCI) (OLCI; OC4v6). The results additionally demonstrated that the satellite-derived Chl-a was overestimated due to higher aCDOM(lambda) and underestimated owing to lower a*Ph(lambda) at the low (<0.5 mg m 3) and high (>3 mg m 3) concentrations of Chl-a, respectively. The study findings established that the red bands should be considered for tuning or developing new Chl-a algorithms in the Persian Gulf.