Interannual variability of surface bio-optical characteristics in the frontal zones of the Indian sector of the Southern Ocean during austral summer

Abstract

Surface bio-optical constituents (Chl-a, light-absorption coefficient of phytoplankton (a_ph (443)), detritus (a_d(443)), coloured dissolved organic matter (a_CDOM(443))), and hydrographic variables were studied across the fronts along 57.5°E during 2011, 2013 and 2015 in the Indian sector of Southern Ocean to understand their interannual variability and phytoplankton pigment package effect. Latitudinal increases of Chl-a, a_ph(443), and a_CDOM(443) were observed in 2011 and 2013. Mean Chl-a was higher in 2013 than in 2015 and 2011. The Chl-a specific absorption (a*_ph(443)) latitudinally decreased implying ‘pigment package effect’ with increasing latitude. Interannually increase of a_ph blue (443)/red (675) ratio suggested higher non-photosynthetic absorption during 2013 and 2015 than in 2011. The relative contributions of bio-optical constituents to total absorption (a_t) showed interannual decrease in a_ph(443) and increase in a_CDOM(443) and a_d(443) inferring phytoplankton growth and decay phases. The robust a_ph(443)-Chl-a relationship during 2011 was used to derive a_ph(443) for other years, which holds good for in situ and satellite-derived counterparts. a_CDOM(443) positively correlated with Chl-a and negatively with CDOM-spectral slope (S_300-500), suggesting autochthonous sources. Satellite-retrieved Chl-a, a_ph(443) showed significant correlation (p < 0.001) with in situ data. This study provides baseline bio-optical data for understanding phytoplankton size-class, and improvement of bio-optical algorithms in this relatively inaccessible polar region.