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

• Published in: Journal of Plankton Research
• Main Authors: Sigleo, A.C., Neale, P.J., Spector, A.M.
• Format: Text
• Language: English
• Published: Oxford University Press 2000
• Subjects: ORIGINAL ARTICLES; Southern Ocean; Austral; Weddell
• Online Access: http://plankt.oxfordjournals.org/cgi/content/short/22/10/1989; https://doi.org/10.1093/plankt/22.10.1989

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 gravida -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 ( t 1/2 = 9.5 min) than the second ( t 1/2 = 55 min). Fucoxanthin, 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.