Spectral Bio-optical Properties of Waters in the Bransfield Strait and Powell Basin

Abstract

In this research, we examined variability of the spectral absorption coefficients of optically significant components in Antarctic waters (Bransfield Strait and Powell Basin) based on the data collected during cruise 79 of the R/V “Akademik Mstislav Keldysh” in the austral summer of 2020 (January 11–February 4). Chlorophyll a concentration, spectral light absorption coefficients of phytoplankton, non-algal particles (NAP), and colored dissolved organic matter (CDOM) varied by more than one order of magnitude. Vertical distribution of chlorophyll a concentration depends on hydrophysical characteristics. In the case of the depth-dependent distribution of chlorophyll a concentration specific light absorption coefficient of phytoplankton (${a}_{ph}^{\ast}\left(\lambda \right)$(λ)) decreased with depth and especially in the blue spectrum domain, which resulted in the decreasing of blue-to-red peak ratio (R). Values of ${a}_{ph}^{\ast }(678)$ varied from 0.017 to 0.025 m² mg⁻¹. The value of R was in the range 1.5–2.8. At one station (the most trophic) in the upper layer (0–15 m), a local maximum was observed at a wavelength of ~545 nm, corresponding to the absorption band of phycoerythrin. The relative NAP absorption at 440 nm was equal to 25 ± 12% of particulate absorption. The spectral slope coefficient (S_NAP) varied from 0.006 to 0.016 nm⁻¹ with the mean value equal to 0.010 ± 0.002 nm⁻¹. The a_CDOM(440) varied from 0.016 to 0.19 m⁻¹ without any correlation with chlorophyll a concentration. The spectral slope coefficient (S_CDOM) varied from 0.009 to 0.022 m⁻¹ with an inverse trend of a_CDOM(440). The mean value of the S_CDOM was 0.013 ± 0.003 nm⁻¹. The absorption budget at 438 nm showed that phytoplankton and CDOM were the main optically active components.