Biodiversity in Enclosed Seas and Artificial Marine Habitats - Proceedings of the 39th European Marine Biology Symposium, held in Genoa, Italy, 21-24 July 2004 - Foreword

The pack ice surrounding the Antarctic continent is one of the most dynamic marine ecosystems. Frozen seawater appears as a semisolid matrix permeated by a network of channels and micrometric pores filled with salt brine. Apart from a highly variable heterotrophic community including viruses, bacteria, protozoan, flatworms and small crustaceans, the pack-ice is inhabited by micro-algae, mainly diatoms. While the major contribution of diatoms to the Antarctic primary production is well-known, floristic investigations on pack ice communities are very scarce. We analysed phytoplankton biomass and vertical distribution of diatom species in the annual pack ice in Terra Nova Bay (Antarctica) during the austral spring 1999. One 240 cm-long ice core was divided into five sections: four 55 cm-long and a 20 cm section at the water interface (bottom-ice). In addition, the platelet ice, constituted by discoid ice crystals present under the consolidated pack ice, was collected. Spectrofluorometric analyses of total biomass (Chlorophyll a) were carried out on different sections. Quantitative analysis and taxonomic identification of diatoms were performed using light and electron microscopy. Biomass concentrations slightly increased from the surface to the bottom layer (from less than 1 to 1.30 g l-1 Chla); very high concentrations (90 g l-1 Chla) were recorded in the platelet ice. Diatom abundances ranged from 0.3 x 106 to 3.4 x 106 cells l-1 in the consolidated pack ice and up to 22.7 x 106 cells l-1 in the platelet ice. Planktonic species constituted about 98% of the diatom assemblage from the surface layer of the ice core down to 220 cm. Some species, such as Fragilariopsis curta, F. rhombica, F. sublinearis, F. obliquecostata, F. cylindrus, Thalassiosira gracilis and T. lentiginosa, were present along the entire ice core. Other species, such as Asteromphalus parvulus, T. gravida and T. maculata, were found only in the upper layers of the ice-core. Benthic diatoms (e.g. Berkeleya sp., Entomoneis sp., Navicula directa, Navicula sp., Nitzschia stellata) were exclusively present in the bottom and platelet ice, where they constituted about 30% of the diatom population. The vertical distribution of diatoms along the ice core reflects the seasonal occurrence of the different species at sea. Planktonic species are trapped in the ice as the water freezes in autumn and most likely play a role in seeding the water column when the ice melts. In particular, Fragilariopsis curta, reported as dominating pelagic spring blooms, constituted about 50% of the total diatom assemblage along the entire ice core. The presence of benthic diatoms in the bottom and platelet ice is probably due to colonization and/or in situ growth.