Optical properties of colored dissolved organic matter and their importance in the hypersaline Arabian Gulf to assess the status of the ecosystem and its influence on the northwestern Indian Ocean biogeochemistry: a review

This review summarizes the existing knowledge on the sources, composition, transformation, and biogeochemical importance of dissolved organic matter (DOM) in aquatic environments, based on observations of its optically measurable fraction, known as chromophoric dissolved organic matter (CDOM). Optical methods, including UV–visible absorption and fluorescence spectroscopy, spectral radiometry, and hyperspectral satellite remote sensing, provide valuable information on the distribution and dynamics of DOM, which are closely linked to physical transport, microbial transformations, and photochemical degradation in coastal and marine environments. The Arabian Gulf (AG) represents a distinct marine environment characterized by shallow bathymetry, hypersalinity, large seasonal temperature variability, semienclosed circulation, restricted water exchange, and relatively long residence times. These conditions, together with intense solar radiation, osmotic stress, limited terrestrial freshwater inputs, and increasing anthropogenic pressures, can strongly influence the sources, composition, and transformation of DOM. Within this dynamic environment, DOM may undergo transformations driven by seasonal biological production, benthic–pelagic interactions, sedimentary processes, and anthropogenic inputs, followed by further biological and photochemical processing. The formation of Persian Gulf water (PGW) mass in the AG and its export through the Strait of Hormuz into the subsurface waters of the northern Arabian Sea as a density-driven outflow may influence oxygen minimum zone (OMZ) dynamics and contribute to carbon biogeochemical cycling in the northern Indian Ocean. Despite the importance of these processes, significant knowledge gaps remain regarding the sources, composition, transformation, and fate of CDOM and its role in carbon cycling in the AG. Consequently, integrated in situ and remote sensing approaches for characterizing CDOM are essential to improve our understanding of DOM dynamics and carbon biogeochemical processes in the AG and in the northern Indian Ocean, particularly in the context of ongoing environmental and global climate change.