Abstract. Antarctic near-shore waters are amongst the most sensitive in the world to
ocean acidification. Microbes occupying these waters are critical drivers of
ecosystem productivity, elemental cycling and ocean biogeochemistry, yet
little is known about their sensitivity to ocean acidification. A six-level,
dose–response experiment was conducted using 650 L incubation tanks
(minicosms) adjusted to a gradient in fugacity of carbon dioxide
(fCO2) from 343 to 1641 µatm. The six
minicosms were filled with near-shore water from Prydz Bay, East
Antarctica,
and the protistan composition and abundance was determined by microscopy
during 18 days of incubation. No CO2-related change in the protistan
community composition was observed during the initial 8 day acclimation
period under low light. Thereafter, the response of both autotrophic and
heterotrophic protists to fCO2 was species-specific.
The response of diatoms was mainly cell size related; microplanktonic diatoms
(> 20 µm) increased in abundance with low to moderate
fCO2 (343–634 µatm) but decreased at
fCO2 ≥ 953 µatm. Similarly, the abundance
of Phaeocystis antarctica increased with increasing
fCO2 peaking at 634 µatm. Above this threshold
the abundance of micro-sized diatoms and P. antarctica fell
dramatically, and nanoplanktonic diatoms (≤ 20 µm) dominated,
therefore culminating in a significant change in the protistan community
composition. Comparisons of these results with previous experiments conducted
at this site show that the fCO2 thresholds are similar,
despite seasonal and interannual differences in the physical and biotic
environment. This suggests that near-shore microbial communities are likely
to change significantly near the end of this century if anthropogenic
CO2 release continues unabated, with profound ramifications for
near-shore Antarctic ecosystem food webs and biogeochemical cycling.