Higher Chl a concentrations are indicative of high phytoplankton activity. These thresholds were chosen as they corresponded to patterns in benzene and toluene depth profile distributions, and reflect the distribution of Chl a in the Southern Ocean . The highest surface concentration measured during the cruise was 1 mg m -3 . Grouped stations were averaged in depth bins to help tease out patterns (Figure ). Cruise track with CTD sampling locations (Figure ) and individual vertical profiles of benzene, toluene, Chl a and seawater density are shown in the supplement (Figure and S3), while additional details regarding the Chl a data, analytical chemistry and measurement uncertainty are provided in the supplementary text.