Contents volume 49

Abstract

In the northwestern Weddell Sea and eastern Scotia Sea on a transect north from ice-covered, through ice-melt, to open-ocean stations, microalgae were compared to document an enhanced biological activity expected near the ice edge. The highest numbers of cells were found in open water, with 68.7 × 109 cells m−2 north of the ice edge, although melted ice cores and brine from pore water samples averaged more than an order of magnitude more cells per liter. The dominant taxa under the ice and in the ice-melt stations were the pennate diatom genus Nitzschia and the prymeesiophyte Phaeocystis. In the open ocean, the dominants were the centric diatom Thalassiosira gravida and Phaeocystis; both grew in gelatinous colonies, a growth habit that apparently gave competitive advantage and may have inhibited grazing. Phaeocystis could have been seeded from the melting ice and from the water under the ice; it seems less likely that T. gravida came only from those sources. Although Phaeocystis cells were frequently dominant in number, the genus represented far less carbon than T gravida in open waters; Phaeocystis presents a taxonomic and ecological enigma in comparison with accounts from the northern hemisphere in that it was abundant where diatoms also fluorished. Nitzschia survived in low light under the ice and in the brine pockets in the ice and increased steadily in abundance at the ice edge in the open water, whereas Thalassiosira was most abundant to the north and west where the water had recently been uncovered by the retreating ice edge. Of the three dominant taxa, Nitzschia appears to provide the best food base for the zooplankton such as krill.