Bio-optical depiction of a polar ocean under global change: Exploring the regional absorption traits

Abstract

The Southern Ocean (SO) is a vital ecoregion that plays a crucial role in modulation of the global carbon cycle and changing climate. Light absorption properties of clear waters are challenging to study due to very low absorption coefficients typical for these waters, and the SO is no exception to this. The present study examines the bio-optical variability of light absorption coefficients of phytoplankton and non-phytoplankton constituents during the Indian Scientific Expedition to the Southern Ocean (ISESO) in the austral summer 2017–18. We characterize the light absorption coefficients of bio-optical constituents, quantify their relative contributions to total absorption, derive phytoplankton size classes from two absorption-based approaches and compare the results with hyperspectral radiometric observations. Our observations suggest a prevalence of the pigment package effect (phenomenon interfering with photosynthetic efficiency of phytoplankton) at both, the frontal (40°S–60°S) and coastal (60°S–69°S) domains, inducing deviations from the expected bio-optical classification of the study region. The total light absorption budget indicated a clear dominance of phytoplankton absorption at the frontal (41.93–85.27%) as well as coastal (36.77–93.71%) region. Further, the dominance of microphytoplankton indicated by the aph (443)/aph (675) Blue/Red spectral ratios supported the impact of pigment package on phytoplankton absorption at both, the frontal (0.5–3.2) and coastal (0.9–3.7) regions. The global absorption-based (that links the phytoplankton absorption to their size classes) model captured smaller sized (pico and nano) phytoplankton better than the larger (micro). A satisfactory agreement was observed between the Quasi-Analytical Algorithm (radiometer based) and Quantitative Filter Technique (in-situ measurements) for aph retrieval. The OC5 algorithm of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) was well-correlated with in situ Chlorophyll-a in the region. Our findings improve the existing understanding of bio-optical variability in the SO and highlight the essential considerations that should be taken while assessing the study region through global ocean colour platforms. They also highlight the necessity to assess the impact of pigment package on optical variability while designing satellite data tools at regional a level.