Biology and the Carbon Cycle
The assemblage of heter-
otrophic protists was domi-
nated by phagotrophic dino-
flagellates from 6 to >50 m
in size. Ciliates were also com-
mon in most samples. There
were few choanoflagellates in
the water column under the
ice pack, although choano-
flagellates were abundant in
ice-free waters of the Chukchi
and Greenland Seas, as others
have previously found. The
smallest protists observed were 12 m heterokont flagellates, which appeared to
Deck incubator for
productivity meas-
be feeding both on bacteria and on 1-m eukaryotic phytoplankton, tentatively
urements on the
identified as Micromonas pusilla, which were present in abundances of 103104
Polar Sea.
cells/mL in the upper 50 m. The protist biomass ranged from 0.006 to 0.1 g
C/m3 and often exceeded estimates of phytoplankton biomass determined
from chlorophyll a concentrations. All size classes and taxonomic groups of
heterotrophic protists appeared to be consuming phytoplankton cells.
Based on the data on standing stock abundance and activity of heterotrophic
microbes in the central Arctic Ocean, and on biomass and production of phyto-
plankton and ice algae, we developed a preliminary budget of carbon fluxes
through the pelagic microbial food web of the central Arctic Ocean. Integrated
over the upper 50 m, stocks of bacterioplankton ranged from 0.27 to 0.70 g
C/m2, and of heterotrophic protists from 0.37 to 0.79 g C/m2. Estimates of
productivity of autotrophs and heterotrophs in the upper 50 m were compar-
able: 0.020.14 g C/m2-day for bacteria, 0.030.25 g C/m2-day for phyto-
plankton plus ice algae, and 0.050.15 g C/m2-day for phagotrophic protists.
Assuming a gross growth efficiency of 40%, heterotrophic microbes could
consume on the order of 0.20.7 g C/m2-day. As primary production occur-
ring at the time of the cruise could not provide this daily amount of organic
carbon, we infer that the heterotrophic community was also supported by the
organic carbon present in the water as particulate material (3 g C/m2) and as
dissolved organic compounds (60 g C/m2). This detrital organic matter may
have been produced earlier in the growing season by blooms of ice algae in the
central Arctic.
Our results suggest that, contrary to the initial concept of a minor role of
heteorotrophic microbes in polar ecosystems, heterotrophic bacteria and phago-
trophic protists are significant consumers of plant production in the food webs
of the central Arctic Ocean.
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