The abundance and biomass of choanoflagellates and other nanoflagellates in waters of contrasting temperature to the north-west of South Georgia in the Southern Ocean

The abundance and biomass of different categories of autotrophic and heterotrophic nanoflagellates were determined in surface waters along a transect crossing the Polar Front to the north-west of South Georgia in the Southern Ocean during the austral summer 1996. In the nanoflagellate community 14 choanoflagellate species and 11 other taxonomic categories were recognised. Total nanoflagellate abundance and biomass during the study varied from 4-56 x 10(5) litre(-1) and from 3-20 mug C litre(-1) respectively, with highest abundance and biomass of both autotrophic and heterotrophic nanoflagellates recorded in surface waters to the north of the Polar Front. The most common choanoflagellates were Sicosta spinifera, Calliacantha spp., Diaphanoeca pedicellata and Parvicorbicula socialis. Of these Calliacantha spp. dominated warmer waters north of the Polar Front with B. spinifera dominating colder waters to the south. Positive relationships between several nanoflagellate categories and temperature were identified but no relationships were found with either chlorophyll a or bacterial biomass. Overall the study has revealed significant differences in the abundance, biomass and quantitative importance of different taxonomic categories of nanoflagellates in offshore waters of differing physico-chemical character north and south of the Polar Front. However, the factors responsible for the distribution of nanoflagellates remain unresolved.