Abundance and distribution of phototrophic and heterotrophic nano- and microplankton in the southern Ross Sea

Phototrophic and heterotrophic nanoplankton (PNAN, HNAN; 2–20 mm protists) and microplankton (PMIC, HMIC; 20–200 mm protists and micrometazoa) are major taxa involved in partitioning carbon and energy within the pelagic food web. In the Ross Sea, Antarctica, plankton biomass appears to be controlled by the seasonal recession of the sea ice and the formation of the Ross Sea polynya during the short austral spring-summer period. During four cruises in 1996–1997 within the southern Ross Sea as part of the US JGOFS program, we determined the abundances and biomasses of phototrophic and heterotrophic nanoplankton and microplankton primarily along a transect at 761300S. The colonial prymnesiophyte Phaeocystis antarctica (excluding mucus carbon) contributed significantly to community structure during both non-bloom and bloom periods (B25% and 90%, respectively, of microbial biomass). However, shifts occurred both seasonally and spatially between a diatom/heterotrophic dinoflagellate and a colonial P. antarctica-dominated assemblage. While nanoplankton biomass varied o50% during any particular cruise, PNAN and HNAN biomass ranged more than three orders of magnitude among the four cruises (0.1–359 and 1.5–268mmolCm , respectively). Cruise averages of PMIC biomass ranged from 2.5 to 530mmolCm , and a maximum biomass of 1530mmolCm 2 was observed during the bloom of colonial P. antarctica in summer. Average heterotrophic biomass was o30% of the total microbial biomass (excluding bacteria) from early austral spring through summer. This value rose to E87% in autumn following the decline and disappearance of P. antarctica. The contribution of total nanoand microplankton biomass to POC in the upper 60m over the three sampled seasons varied from 7% to 52.4% with an overall average of 21.8% for all four cruises which is comparable to contributions of these assemblages in other oceans even with the strong seasonal dominance of P. antarctica. r 2001 Elsevier Science Ltd. All rights reserved. *Corresponding author. Fax:+1-508-457-2169. E-mail address: [email protected] (M.R. Dennett). 0967-0645/01/$ see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 0 6 4 5 ( 0 1 ) 0 0 0 7 9 0