Variability of dissolved organic matter sources in the eastern Arabian Sea: impacts of physical and biological processes

To understand the variation in dissolved organic matter (DOM) source characteristics in relation to the dynamics of physical and biological processes in marine environments, fluorescence spectroscopy was utilized as a measurement tool. Observations were carried out in the mixed layer waters of the eastern Arabian Sea (EAS), from coastal to offshore waters, spanning from south to north during the winter and spring inter-monsoon (WM and SIM) periods. Parallel factor analysis of excitation-emission matrix fluorescence spectra revealed protein (Tryptophan, T, and Tyrosine, B) and humic-like (UV-visible humic, C, and marine humic, M) components. T, M, and C components showed a significant negative correlation with salinity in the coastal waters of north EAS during the WM, attributed to external DOM sources. In contrast, offshore waters had in situ DOM, showing a positive correlation of Chl a with C and B due to convective mixing. During the SIM, in situ DOM sources prevailed, with a significant positive correlation of humic-like components with Chl a and heightened B component intensity. T was low in the south EAS during WM but increased in T and B during SIM, linked to reduced Bay of Bengal water intrusion via the west Indian coastal current (WICC). However, the Central EAS exhibited mixed characteristics compared to the north and south EAS, accumulating bacteria-derived and humic-like DOM, reflected in T: B and M: C ratios decreasing in WM and SIM, respectively.