Polar Biol (1995) 15:465-475 © Springer-Verlag 1995 Carlo Andreoli. Claudio Tolomio. Isabella Moro Marco Radice • Emanuela Moschin - Sabrina Bellato Diatoms and dinoflagellates in Terra Nova Bay (Ross Sea-Antarctica) during austral summer 1990 Received: 14 July 1994/Accepted: 19 December 1994 Abstract During January/February 1990 the floristic composition and biomass of diatoms and dinoflagel- lates in Terra Nova Bay (Ross Sea) were investigated. The study area included eight stations, seven of which were inshore and one offshore. Of the 94 taxa identified, 58 were diatoms and 34 were dinoflagellates. Cell dens- ities were higher in surface layers and particularly at two stations, one located at the centre of the bay, and the other to the south of the Terra Nova station. While density data showed that the diatoms predominated over the dinoflagellates and over Phaeocystis sp., the biomass values were largely made up of the dinoflagel- lates contribution. Introduction Antarctic phytoplankton research has been developed since the beginning of the nineteenth century (see re- views in E1-Sayed and Fryxell 1993). Most of these investigations were carried out in the marginal ice-edge zone which, unlike the open waters, is recognized as the major site of increasing phytoplankton biomass and primary productivity (Kang and Fryxell 1993). Several reports, also, have discussed the phytoplankton assem- blage and seasonal succession of the Southern Ocean nearshore sites (Everitt and Thomas 1986; Perrin et al. 1987; Tanimura et al. 1990; Davidson and Marchant 1992b; McMinn and Hodgson 1993) and certain re- ports refer to McMurdo Sound in the Ross Sea (Raw- lence et al. 1987; Knox 1990; E1-Sayed and Fryxell 1993). Diatoms are generally considered as the major C. Andreoli (l~)- C. Tolomio. I. Moro" M. Radice" E. Moschin S. Bellato Dipartimento di Biologia, Universitfi di Padova, Via Trieste 75, 1-35121 Padova, Italy taxon and their densities are often connected with krill grazing (E1-Sayed and Fryxell 1993). They are espe- cially represented by the Nitzschia genus (Pseudonit- zschia and Fragilariopsis sections), which has been re- ported as abundant in the pack ice and in the water column near the ice edge. Phaeocystis sp. (Prym- nesiophyceae) is also an important taxon often respon- sible for blooms, which precede those of the diatoms (Knox 1990). Little information about heterotrophic and autotrophic dinoflagellates is available even if they play an important role in the Southern Ocean ecosys- tem (Balech 1970; Jacques et al. 1979; Dodge and Priddle 1987; Garrison and Gowing 1993). Helbling et al. (1993), for example, observed, around Elephant Is- land, that dinoflagellates of the Amphidinium and Gym- nodinium genera dominated the microplankton frac- tion. Diatoms were also present but they accounted for a low percentage of the total carbon compared with the flagellate < 20-gm percentage. Moreover, many auto- trophic athecate dinoflagellates were found in the sea ice (Stoecker et al. 1992). This paper follows a previous one on the coastal photoautotrophic picoplankton of Terra Nova Bay (Andreoli et al. 1993) and the main objective has been to gain quantitative information on the composition, abundance, and biomass of the diatoms and dinoflagel- lares of that area. There is little information on the phytoplankton of Terra Nova Bay and most of that available refers to offshore areas during the austral summer 1987-88 (Innamorati et al. 1990; Nuccio et al. 1992). Materials and methods Sampling was carried out in open water from 10 January to 8 Febru- ary 1990 at the same eight stations at which the photoautotrophic picoplankton and some environmental parameters had already been investigated (Andreoli et al. 1993) (Fig. 1). Seven stations were in- shore, where the water depth was i00 m or less. Station 1 was the only one offshore, with a depth of more than 500 m. Samples were