Biogeographic structure of the microphytoplankton assemblages of the south Atlantic and Southern Ocean during austral summer

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Abstract
Microphytoplankton distribution in the Atlantic sector of the Southern Ocean was investigated along a transect during the SAAMES II cruise undertaken in late austral summer (January/ February) 1993. Samples were collected at ∼60 km intervals between 34 and 70°S for the analysis of mineral nutrients, and the identification and enumeration of microphytoplankton. Peaks in microphytoplankton abundance were recorded in the neritic waters of Africa and Antarctica, at all major oceanic fronts, and in the marginal ice zone (MIZ). Partial correlation analysis indicated that 45% of the total variance associated with microphytoplankton abundance could be explained by silicate and phosphate concentrations, while temperature accounted for 65% (P<0.001). Cluster and ordination analyses identified two major groups of stations, one north and one south of the Subantarctic Front (SAF). This division appears to be related to differences in temperature and silicate concentrations. Each region comprised distinct microphytoplankton subgroups associated with specific water masses or hydrological features. Indicator species could be identified for some water masses. In the MIZ, microphytoplankton species composition and succession were strongly affected by sea-ice throughout the summer.