Underwater light field and phytoplankton absorbance in different surface water masses of the Atlantic sector of the Southern Ocean

Spectral water transparency and phytoplankton light absorbance were studied in the Atlantic sector of the Southern Ocean during the Southern Ocean JGOFS ANT XIII/2 cruise in early austral summer 1995/1996. The study area comprised three zones, which differed markedly with respect to their hydrographic and planktological characteristics: the Antarctic Polar Frontal Zone with adiatom bloom, the Antarctic Circumpolar Current outside frontal systems with phytoplankton-poor water and a higher flagellate abundance than in the other two areas, and the marginal ice zone with a Phaeocystis bloom. The influence of phytoplankton on spectral water transparency was assessed by two independent procedures: the pigment-specific beam absorption coefficient, a_J*[5], at all stations, as estimated by spectroscopy of in vivo light absorption of plankton on glass fibre filters, and the pigment-specific light attenuation, (k_c[5]), as derived by regression analysis of spectral in situ vertical light attenuation coefficients in the sea against concomitant pigment concentrations. Values of a_J*[5] and vertical profiles of light attenuation by phytoplankton exhibited regional differences that corresponded with the three zones from which samples had been collected. These differences can be related to the specific characteristics of the three zones with respect to cell size distribution, pigment composition and biomass. The observed variations in a_J*[5] values should be considered when oceanic primary production is to be estimated by biooptical modelling.