Phytoplankton community responses to iron and CO.sub.2 enrichment in different biogeochemical regions of the Southern Ocean

The ongoing rise in atmospheric CO.sub.2 concentration is causing rapid increases in seawater pCO.sub.2 levels. However, little is known about the potential impacts of elevated CO.sub.2 availability on the phytoplankton assemblages in the Southern Ocean's oceanic regions. Therefore, we conducted four incubation experiments using surface seawater collected from the subantarctic zone (SAZ) and the subpolar zone (SPZ) in the Australian sector of the Southern Ocean during the austral summer of 2011-2012. For incubations, FeCl.sub.3 solutions were added to reduce iron (Fe) limitation for phytoplankton growth. Ambient and high (~750 [micro]atm) CO.sub.2 treatments were then prepared with and without addition of CO.sub.2-saturated seawater, respectively. Non-Fe-added (control) treatments were also prepared to assess the effects of Fe enrichment (overall, control, Fe-added, and Fe-and-CO.sub.2-added treatments). In the initial samples, the dominant phytoplankton taxa shifted with latitude from haptophytes to diatoms, likely reflecting silicate availability in the water. Under Fe-enriched conditions, increased CO.sub.2 level significantly reduced the accumulation of biomarker pigments in haptophytes in the SAZ and AZ, whereas a significant decrease in diatom markers was only detected in the SAZ. The CO.sub.2-related changes in phytoplankton community composition were greater in the SAZ, most likely due to the decrease in coccolithophore biomass. Our results suggest that an increase in CO.sub.2, if it coincides with Fe enrichment, could differentially affect the phytoplankton community composition in different geographical regions of the Southern Ocean, depending on the locally dominant taxa and environmental conditions.