Antarctic phytoplankton assemblage in the western Bransfield Strait region, February 1993:composition, biomass, and mesoscale distributions

Abstract

The Bransfleld Strait region shows complex patterns of water circulation d u e to mixing of dlverse water masses Physicochemical properties of the different water types should affect the distnbution biomass and speclcs composition of the phytoplankton assemblages We examined these features of phytoplankton in the western Bransficld Strait region durlng 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--(average 1906 mg C m Areas w ~t h the higher biomass values (2000 to 3500 mg C m 2, were located in the no] thern part of the study area including waters to the north of the South Shetland Islands and wele generally related to increased stabil~ty of the surface water orig~nating from the Belhngshausen Sea featuring lolver salinity (33 69 to 33 78' " ) and higher temperature (1 89 to 2 36C) Distribution of autotrophic nanoflagellates ( < 2 0 pm) showed a marked contrast to that of diatoms Waters in the Bransfleld Strait region were charactenzed by a dominance of nanoflagellates such as Cryptornonas s p and Phaeocystis antarctrca (motile stage) accounting for 83 % of the total phytoplankton carbon In the Diake Passage area hoxvever, diatoms such as the nanoplanktonic Fragilanopsls pseudonana and the microplanktonic (>20 pm) Rhlzosolenla antennata f semlsplna accounted for 84 % ot the total phytoplankton carbon From measurements of upper ocean phys~cal parameters and nutrients in cornblnatlon with the data on composition, biomass, and distribution of the mdjor phytoplankton specles, it was found that mesoscale dlstnbutions of phytoplankton species reflect the physical con-d~tlons of the upper water coluinn particularly the proflle of vertlcal stability of the water