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Sigleo, A.C., P.J. Neale, and A. Spector. Phytoplankton pigments at the Weddell-Scotia confluence during the 1993 austral spring. 2000. Journal of Plankton Research 22(10):1426-1441.

During a 1993 austral spring cruise, a complex biomass was encountered near South Orkney Island that ranged from a low-biomass, Chaetoceros tortissimus assemblage south of the front towards the ice edge, to a high-biomass, Thalassiosira gravada-dominated assemblage at the northern edge. The maximum levels of chlorophyll (Chl) a (up to 6 mg m-3) were higher than those observed in previous high-performance liquid chromatography-based studies of pigments in the pelagic Southern Ocean. The non-photosynthetic pigment chlorophyllide a comprised up to 75% of the chlorophyllous pigments in the southern 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 phytoplankton residence time in the near-surface layer and thus limited exposure to maximum irradiance. Deck incubations of natural assemblages indicated 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 (t1/2 = 9.5 min) than the second (t1/2 = 55 min). Fucoxauthin, a major diatom pigment, was more stable chemically in the water column than Chl a, and the vertical profiles of fucoxanthin followed those of chlorophyllide a in some cases. The formation and apparent stability of chlorophyllide a and fucoxanthin are important considerations when estimating photosynthetically active biomass over large regions of the ocean.

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