Factors controlling the distribution of diatoms and Phaeocystis in the Ross Sea

The spatial and temporal distributions of phytoplankton pigments were investigated in the western and south central Ross Sea during austral spring 1994 and summer 1990. Large gradients in biomass and phytoplankton community composition were observed both in the east–west and south–north directions, in relation to differences in water column structure and stability, which themselves depend on the processes of ice retreat within the different areas. Important are melting in the western Ross Sea, which induces strong stratification, and ice breakup and wind stress in the south central Ross Sea, which result in deep mixing and weakly stratified waters. In the western Ross Sea, the highest chlorophyll a concentrations observed in this study were tightly coupled to the stratified region of meltwater influence and were dominated by diatoms, as indicated by elevated fucoxanthin concentrations. In the diatom bloom area, high levels of phaeophorbides a indicated that the dominant grazers, identified as Limacina helecina and copepods [Hecq, J.H., Magazzù, G., Goffart, A., Catalano, G., Vanucci, S., Guglielmo, L., 1992. Distribution of planktonic components related to vertical structure of water masses in the Ross Sea and the Pacific sector of the Southern Ocean. In: Anonymous, Atti del 9° congresso A.I.O.L., Santa Margherita Ligure, 20–23 Novembre 1990, 665–678], transferred a sustained part of the diatom production to the herbivore trophic level. Synthesis of our data with published information suggests that the diatom bloom we observed in the western Ross Sea was dominated by the species Fragilariopsis curta, and occurs annually off the coast of Victoria Land from 72°302S to 77°S within 100–250 km from the coastal ice edge. It is assumed to persist on the order of 2.5–3 months during summer. In the south central Ross Sea, which was characterized by a poorly or unstratified water column, moderate chl. a concentrations were found in the polynya and in the ice-edge area in early spring. The 192-hexanoyloxyfucoxanthin (192-HF), a biomarker for Phaeocystis, was the major contributor to the total carotenoid abundance, corroborating the hypothesis that Phaeocystis are well adapted to develop in a relatively mixed water column. At the northern limit of the polynya, a quite narrow (ca. 30 km) diatom bloom overlapped with the Phaeocystis bloom in the slightly stratified marginal ice zone. This was an area of increased grazing pressure by various types of herbivorous zooplankton, among which very large amounts of krill were observed.