The influence of phytoplankton assemblage composition on biogeochemical characteristics and cycles in the southern Ross Sea, Antarctica

To test the hypothesis that phytoplankton assemblages dominated by differen t taxa have distinct biogeochemical characteristics and cycles, the tempora l and spatial variations in phytoplankton biomass and composition were stud ied within the Ross Sea polynya, where diatoms and the haptophyte Phaeocyst is antarctica are thought to have spatially distinct distributions. Two cru ises were completed, with the first conducted in spring, 1994, and the seco nd in late spring-early summer, 1995/1996. Ice concentrations decreased sub stantially from spring to summer. Mixed layer depths for the region decreas ed markedly in early spring and were relatively invariant thereafter; the s trength of the stratification varied both in time and space. Mixed layers w ere greater in spring in assemblages dominated by diatoms (as determined by HPLC pigment concentrations) than those dominated by Phaeocystis antarctic a, whereas in summer no difference was observed. Nutrient concentrations we re initially high and near winter values, but decreased throughout November and December. Nitrate:phosphate removal ratios varied widely, with ratios exceeding 20 in spring but decreasing below 14 in summer. N : P removal rat ios at stations dominated by diatoms were less than the Redfield ratio in b oth spring and summer, and at those stations dominated by P. antarctica the N : P removal ratio was ca. 19 in both seasons. Chlorophyll and particulat e matter concentrations increased as nutrients decreased. Spatial and tempo ral variations of phytoplankton pigments occurred, with 19'-hexa-noylfucoxa nthin, a pigment of P. antarctica, exceeding 3.9 mug 1(-1) during spring in the south-central polynya, and fucoxanthin, an accessory pigment of diatom s, found in concentrations > 1 mug 1-1 in the western Ross Sea. The distrib utions were not mutually exclusive, and concentrations of both pigments wer e greatest in spring. The early growth of P. antarctica appears to be relat ed to earlier stratification and disappearance of ice from the south-centra l Ross Sea. Ratios of FUCO/CHL were relatively invariant, but substantial c hanges in the HEX/CHL and POC/CHL ratios were observed through time. A one- dimensional nitrogen budget for the spring-early summer period suggests tha t much of the surface production was partitioned into particles, most (53%) of which remained in the upper 200 m. The rest was partitioned into dissol ved organic matter (14%), remineralized as ammonium (19%), or sank from the surface layer as particles (13%). The region may serve as a useful analog to other polar systems, and an understanding of the processes controlling a ssemblage composition, production, and biomass accumulation may provide ins ights into biogeochemical cycles of other Antarctic environments, (C) 2000 Elsevier Science Ltd. All rights reserved.