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

Phototrophic and heterotrophic nanoplankton (PNAN, HNAN; 2–20 μm protists) and microplankton (PMIC, HMIC; 20–200 μm 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 76°30′S. The colonial prymnesiophyte Phaeocystis antarctica (excluding mucus carbon) contributed significantly to community structure during both non-bloom and bloom periods (∼25% 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 <50% during any particular cruise, PNAN and HNAN biomass ranged more than three orders of magnitude among the four cruises (0.1–359 and 1.5–268 mmol C m−2, respectively). Cruise averages of PMIC biomass ranged from 2.5 to 530 mmol C m−2, and a maximum biomass of 1530 mmol C m−2 was observed during the bloom of colonial P. antarctica in summer. Average heterotrophic biomass was <30% of the total microbial biomass (excluding bacteria) from early austral spring through summer. This value rose to ≈87% in autumn following the decline and disappearance of P. antarctica. The contribution of total nano- and microplankton biomass to POC in the upper 60 m 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.