Distribution and ecology of Pseudo-nitzschia species (Bacillariophyceae) in surface waters of the Weddell Sea (Antarctica)

The distribution of six Pseudo-nitzschia species and their relationship with environmental conditions were studied for the first time in a vast zone of the Weddell Sea (similar to 61-77 degrees S, Antarctica). Both qualitative and quantitative phytoplankton samples, collected during summer 2004, were examined using light and scanning electron microscopy. Phytoplankton abundance and composition showed great variability along our study area. Diatoms were the most conspicuous phytoplankton group in the northern area while small flagellates were generally dominant in the southern stations. The genus Pseudo-nitzschia was broadly distributed and significantly contributed to total diatom densities. A marked contrast in Pseudo-nitzschia species distribution was observed in three main zones divided by the Weddell Front (WF) and the Antarctic Slope Front (ASF). P. subcurvata and P. turgiduloides were the most abundant species in the neritic Weddell Sea zone, south of the ASF, mainly near the ice-edge in shallower waters and in conditions of long photoperiod. In contrast, P. prolongatoides and P. lineola dominated north of the ASF; the first was associated with deeper and nutrient-rich waters whereas the latter showed a weak relation with environmental variables examined. Finally, P. turgidula and P. heimii were mostly observed in the Weddell-Scotia Confluence Zone in the warmest and far from ice covered waters, north of the WF. A brief morphological Pseudo-nitzschia species description is given in the Appendix, including morphometrics and pictures.