MEPS 129:253-267 (1995)  -  doi:10.3354/meps129253

The Bransfield Strait region shows complex patterns of water circulation due to mixing of diverse water masses. Physicochemical properties of the different water types should affect the distribution, biomass, and species composition of the phytoplankton assemblages. We examined these features of phytoplankton in the western Bransfield Strait region during early February 1993. Diversity of the phytoplankton species was low. Only 5 or 6 species accounted for more than 95% of the total phytoplankton carbon biomass. Integrated phytoplankton biomass in the upper 100 m ranged from 407 to 3605 mg C m^-2 (average 1906 mg C m^-2). Areas with the higher biomass values (2000 to 3500 mg C m^-2) were located in the northern part of the study area including waters to the north of the South Shetland Islands, and were generally related to increased stability of the surface water originating from the Bellingshausen Sea featuring lower salinity (33.69 to 33.78 ) and higher temperature (1.89 to 2.36*C). Distribution of autotrophic nanoflagellates (<20 um) showed a marked contrast to that of diatoms. Waters in the Bransfield Strait region were characterized by a dominance of nanoflagellates such as Cryptomonas sp. and Phaeocystis antarctica (motile stage) accounting for 83% of the total phytoplankton carbon. In the Drake Passage area, however, diatoms such as the nanoplanktonic Fragilariopsis pseudonana and the microplanktonic (>20 um) Rhizosolenia antennata f. semispina accounted for 84% of the total phytoplankton carbon. From measurements of upper ocean physical parameters and nutrients in combination with the data on composition, biomass, and distribution of the major phytoplankton species, it was found that mesoscale distributions of phytoplankton species reflect the physical conditions of the upper water column, particularly the profile of vertical stability of the water.

Antarctic phytoplankton . Bransfield Strait . Cryptomonas sp. . Phaeocystis antarctica (motile stage)