Bio-optical relationships in the surface waters of the Indian Sector of the Southern Ocean during austral summer

The bio-optical properties of the Indian Sector of the Southern Ocean (ISSO) differ significantly from those of temperate and tropical latitudes due to distinct environmental and geographical conditions. These differences often lead to the failure of standard ocean colour algorithms in the ISSO waters. To enhance the accuracy of regional ocean colour algorithms, comprehensive in-situ measurements of chlorophyll-a (Chl) concentration, particle absorption, and backscattering coefficients were conducted in the surface waters of the ISSO region during the austral summer of 2017 to 2018. Our analysis indicated that Chl concentrations derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Visible Infrared Imaging Radiometer Suite (VIIRS) exhibit noticeable uncertainties, which may result from sensor-specific characteristics and differences in retrieval methodologies. These findings emphasize the need for improved regional atmospheric correction and refinement of standard algorithms. Accuracy assessment of existing Chl algorithms suggested the development of a new two-band ratio algorithm specific to the ISSO region, using the remote sensing reflectance ratio at 443 nm and 555 nm. This study also explains the bio-optical relationships, which show that the variation in the blue-to-green reflectance ratio with Chl was primarily driven by Chl-dependent changes in the green-to-blue absorption ratio of pure seawater and particles, and secondarily by the blue-to-green backscattering ratio. Further analysis indicated that surface waters in the ISSO were dominated by phytoplankton (Chl, 58%), followed by coloured dissolved organic matter (CDOM, 35%) and non-algal particles (NAP, 7%) during this austral summer. Analysis of phytoplankton biomass and the absorption ratio between 443 nm and 667 nm indicates that pico and nanophytoplankton are the dominant groups in the surface waters of the study area.