Abstract
In late summer (Antiprod I cruise, March 1977), antarctic waters of the southern part of the Indian Ocean contain low amounts of dissolved Vitamin B12: 1 ng l-1 on average. However, higher concentrations were recorded in two areas: in the subantarctic convergence zone and between 50° and 57°S, with values greater than 3,5 ng l-1. Despite nutrient-rich waters, primary production was low and the phytoplankton sparse. Abundance of diatoms was related, to a certain extent, to the concentration of Vitamin B12. An experimental approach (Antiprod II cruise, February and March 1980) was applied to (1) artificially enriched natural sea water, and (2) cultures of two diatom species isolated from antarctic waters, Nitzschia turgiduloides and Chaetoceros sp. The experiments demonstrated that antarctic diatoms in general do not require any of the three investigated vitamins for growth. However, Vitamin B12 has a slightly stimulating effect on growth. This may explain the observed relation between phytoplankton density and vitamin concentration in the Antarctic Ocean. This experimental approach elucidates the minor role of vitamins in the primary production of antarctic waters and provides indirect evidence of the major importance of turbulence as a limiting factor.
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