Phytoplankton pigments at the Weddell-Scotia confluence during the 1993 austral spring

During a 1995 austral spring cruise, a complex biomass was encountered near South Orkney Island that ranged from a low-biomass, Chaetoceros tortissimu s assemblage south of the front towards the ice edge, to a high-biomass, Th alassiosira gravida-dominated assemblage at the northern edge. The maximum levels of chlorophyll (Chl) a (up to 6 mg m(-3)) were higher than those obs erved in previous high-performance liquid chromatography-based studies of p igments in the pelagic Southern Ocean. The non-photosynthetic pigment chlor ophyllide a comprised up to 75% of the chlorophyllous pigments in the south ern assemblage, but < 5% in the northern assemblage. Concentrations of the xanthophylls diadinoxanthin (DD) and diatoxanthin (DT), used as indicators of mean irradiance, indicated low-light-adapted populations. Low-light DD DT/Chl a ratios in surface waters indicated that vertical mixing limited p hytoplankton residence time in the near-surface layer, and thus limited exp osure to maximum irradiance. Deck incubations of natural assemblages indica ted that the dark epoxidation reaction (i.e. the return of DT to DD) was a two-step reaction with the initial rate being more rapid (t(1/2) = 9.5 min) than the second (t(1/2) = 55 min). Fucoxanthin, a major diatom pigment, wa s more stable chemically in the water column than Chi a, and the vertical p rofiles of fucoxanthin followed those of chlorophyllide a in some cases. Th e formation and apparent stability of chlorophyllide a and fucoxanthin are important considerations when estimating photosynthetically active biomass over large regions of the ocean.