Cross frontal variability in bio-optical characteristics in the Indian sector of the Southern Ocean during an austral summer

Abstract

Bio-optical properties, which play a crucial role in the understanding of the radiation budget of optical active substances (OAS), phytoplankton community, biological pump, and detection of algal blooms have been sparsely studied in the Southern Ocean (SO). We assessed how bio-optical characteristics change across the frontal regions and during phytoplankton bloom and non-bloom domains in the Indian sector of the SO (ISSO) during the austral summer of 2011. In both the meridional transect (47 oE and 57.5 oE), the surface Chlorophyll-a, and phytoplankton absorption coefficient (aph(443)) increased with latitude. Whereas Chlorophyll-a specific phytoplankton absorption (a*ph(443)), an index for the “pigment package effect”, decreased with latitude, implying that the size of phytoplankton increases with latitude. The 57.5 oE transect showed higher quantities of OAS than 47 oE, except for Polar Front 2 (PF-2). Moreover, the vertical profiles of Chlorophyll-a in PF-2 at 47 oE showed a massive phytoplankton bloom with a thickness of 60 m, which could be confirmed in the surface Chlorophyll-a images from the MODIS Aqua satellite. The massive bloom occurred due to distinct physical processes (nutrients supplied by pack ice), causing high productivity. The vertical profiles of Chlorophyll-a, aph(443), and a*ph(443) identified subsurface chlorophyll maxima (SCM) and different phytoplankton communities across the fronts. The interrelationships between bio-optical properties exhibited distinct features in bloom and non-bloom domains. The aCDOM(443) and its spectral slope (S300−500) increased and decreased with latitude, respectively. Using the vertical distribution of CDOM and its relationships with slope and Chlorophyll-a, multiple sources of CDOM could be identified. The percentage-contribution of OAS in the Polar Front-1 (PF-1) exhibited lower aph, lower detrital absorption (ad), and higher aCDOM indicating its low productivity compared to other fronts. This study facilitates the understanding of the phytoplankton size-class (pico, nano and micro) and productivity using the bio-optical properties in this lesser-studied polar region.