Spatiotemporal variability in the optical characteristics of dissolved organic matter in the coastal Bay of Bengal

The optical characteristics of dissolved organic matter (DOM) and physicochemical properties were studied in the coastal waters of the Bay of Bengal off Visakhapatnam for one year from April 2010 to March 2011. The objective was to test the ability of coloured dissolved organic matter (CDOM) as a signature constituent to record the environmental perturbations due to seasonal and spatial dynamics. The aCDOM(440) showed negative relationship with salinity during monsoon indicating conservative mixing of marine (autochthonous) and terrestrial (allochthonous) end members. During pre-monsoon, aCDOM(440) showed positive relationship with chlorophyll (chl-a) due to in situ primary production. The spectral slope (S300–500) which is an indicator of CDOM composition showed significant nonlinear relationship with aCDOM(440) for all the seasons and indicated mixing of different source waters arising from change of direction of East Indian Coastal Current (EICC). The aCDOM(440) showed positive relationship with dissolved inorganic nitrogen (DIN) for all seasons, which indicated that the DOM is non-labile. The spatial variability of CDOM in this region was affected by cyclonic and anticyclonic eddies. The annual rainfall was highest in 2010 for the 2000–2010 decade, and it showed a positive relationship with aCDOM(440), which indicated that rain water may be a secondary source of CDOM. The principal component analysis of optical and physicochemical properties clearly distinguished the sources and the seasonal classification and that the optical properties are useful as their proxy. Our study would help monitor the water quality and in improving the CDOM ocean colour algorithms in this region.