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Southern Ocean Phytoplankton Responses to Atmospheric CO2 Enrichment
Tortell, P.D., Payne, C.D., Li, Y., Trimborn, S., Rost, B., Smith, W.O., Riesselman, C., Dunbar, R.B., Sedwick, P. and DiTullio, G.R. 2008. CO2 sensitivity of Southern Ocean phytoplankton. Geophysical Research Letters 35: 10.1029/2007GL032583.

What was done
The authors measured CO2 uptake of in situ phytoplankton assemblages collected at 35 stations in the Ross Sea polynya during Austral spring and summer, together with 14C uptake for a subset of 11 station samples, while they conducted CO2 manipulation experiments with phytoplankton collected at three Ross Sea locations via shipboard incubations using a semi-continuous batch-culture technique.

What was learned
The researchers report that "for the Phaeocystis-dominated springtime phytoplankton assemblages, there was a statistically significant increase in 14C fixation between 100 and 380 ppm CO2, but no further effects observed at 800 ppm CO2." In the case of the diatom-dominated summertime phytoplankton assemblages, however, the CO2-induced increase in both relative growth rate and primary productivity continued all the way out to the highest CO2 concentration investigated, i.e., 800 ppm, and it promoted "a shift towards larger chain-forming species."

What it means
Noting that the larger chain-forming species of diatoms "are prolific bloom formers with a very high capacity for organic carbon export to the sediments (Stickley et al., 2005)," Tortell et al. concluded that "potential CO2-dependent productivity increases and algal species shifts could thus act to increase the efficiency of the biological pump, enhancing Southern Ocean CO2 uptake and contributing to a negative feedback on increased atmospheric CO2."

Stickley, C.E., et al. 2005. Deglacial ocean and climate seasonality in laminated diatom sediments, Mac Robertson Shelf, Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology 227: 290-310.

Reviewed 14 May 2008