MEPS 190:1-16 (1999)  -  doi:10.3354/meps190001

Primary production, physiological state and composition of phytoplankton in the Atlantic Sector of the Southern Ocean

A. U. Bracher1,*, B. M. A. Kroon1, M. I. Lucas2,3

1Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, 27515 Bremerhaven, Germany
2Zoology Department, University of Cape Town, Rondebosch 7700, South Africa
3George Deacon Division, Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, United Kingdom

ABSTRACT: Phytoplankton species composition and primary production were studied in the Atlantic Sector of the Southern Ocean in early austral summer 1995/1996. Results from photosynthesis-irradiance experiments (P vs E curves) were used to examine photosynthetic adaptation in this part of the ocean. The study area comprised 3 different provinces: the Antarctic Polar Front (APF), the Antarctic Circumpolar Current (ACC) beyond the influence of frontal systems, and the marginal ice zone (MIZ). Phytoplankton composition derived from HPLC data, P*m (maximum biomass-specific photosynthetic production rate) values and areal daily primary production (ADP) rates showed different features for these zones. The central core of the APF was dominated (60%) by a bloom of large (>20 µm) diatoms (Thalassiothrix spp., Pseudonitzschia cf. lineola and Chaetoceros spp.), equal values for P*m at the surface and 1% light depths and ADP rates exceeding 900 mg C m-2 d-1. At the fringes of the APF core, phytoplankton were smaller, diatom abundance decreased and dinoflagellates, prymnesiophytes and chrysophytes became more important within the community. Chlorophyll a concentrations and ADP rates were low and comparable to values for the ACC outside the front: <0.5 mg m-3 and <300 mg C m-2 d-1 respectively. Beyond the frontal systems, P*m values from the 1% light depth were significantly higher than at the surface. There was also a bloom of large phytoplankton species within the MIZ, dominated in contrast by Phaeocystis spp.; this province was characterised by ADP rates of 558 mg C m-2 d-1. Vertical mixing processes, temperature, silicate concentrations and zooplankton grazing seem to be the factors controlling production and growth of phytoplankton at this time.

KEY WORDS: Southern Ocean · Phytoplankton · Photosynthesis · Photoacclimation · Pigment composition

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