Biological, Biogeochemical, Bio-Optical, and Physical Variability of the Southern Ocean Along 150°W and Its Relevance to the Great Calcite Belt

Abstract

We report hydrographic and biogeochemical measurements from a meridional transect performed along 150°W, 30°S to 60°S in the Southern Ocean, plus Polar waters to the east. Both of these areas are sites of annual high-reflectance features in ocean color remote sensing, which were heretofore never confirmed with in situ measurements. This study aimed to document factors driving phytoplankton productivity and coccolithophore calcification within the circumpolar coccolithophore-rich band known as the Great Calcite Belt (GCB). We measured concentrations of particulate inorganic carbon (PIC) and biogenic silica (BSi), two common biominerals, sources of ballast for organic matter, and contributors to optical reflectance. Results demonstrated that integrated euphotic standing stocks of PIC were highest in the GCB and at the Polar Front south of 54°S. BSi concentrations were highest south of 54°S. Integrated calcification rates were highest near the Polar and Subantarctic Fronts, whereas peak photosynthesis rates were observed in Subantarctic waters of the GCB, near the site of Subantarctic Mode Water formation. South of ∼54°S, optical particulate backscattering (b_bp) of BSi dominated over PIC b_bp by 10×, while in the GCB, PIC b_bp dominated over BSi b_bp by a similar magnitude. The slope of the particle size distribution function became less negative with depth, a trend that reflects larger particles becoming more abundant relative to smaller particles. Moreover, typical relationships between the particle size distribution slope and beam attenuation were only observed in the top 50 m depth, suggesting a fundamental difference in particle composition and size for deeper suspensions in this region.