Biomass and composition of size fractionated phytoplankton in the Weddell-Scotia Confluence area

Abstract

The measurement of Chl a, Chl b and Chl c contents in four size fractions (Nuclepore filters of 10 μm, 3μm, 1 μm and 0.2 μm pore-size) together with microscopic examination illustrate the structure and the relative importance of the micro-, nano and pico-phytoplankton in the production system in the Weddell/Scotia Confluence area. In the Scotia Sea, large diatoms were prevalent and their biomass increased during the six week cruise period, exceeding 1 mg Chl a m−3 at the beginning of January. In contrast, in the Marginal Ice Zone of the Weddell Sea, the biomass remained low, up to 0.3 mg Chl a m−3. A diversified nanoplankton community accounted for more than 90% of this biomass: small diatoms, naked dinoflagellates, cryptophyceans, prymnesiophytes and green flagellates which increased the Chl b/Chl a ratio to values >0.20. An important trend affected the Confluence area, where a high biomass net-plankton community (4 mg Chl a m−3) rapidly changed towards a uniform nanoplankton system of the same kind as in the Weddell Sea. At times, autotrophic cryptophyceans were almost dominating (>4.106 cells/l), with a biomass up to 2 mg Chl a m−3 and a low phaeopytin ratio (<10%). This situation probably arises because of a grazing pressure by krill. However, due to the geographic and oceanographic peculiarities of this area, it is not possible to extrapolate these observations concerning the size structure of the primary producers to the Southern Ocean in general.