Iron in the Ross Sea:: 1.: Impact on CO2 fluxes via variation in phytoplankton functional group and non-Redfield stoichiometry -: art. no. C03009

We present new model results examining nutrient and carbon biogeochemistry within the Ross Sea, focusing on the sensitivity of ecosystem dynamics to taxon-specific nutrient utilization parameters and the impact of alleviating Fe limitation of phytoplankton growth. The coupled ice-atmosphere-ocean ecosystem (CIAO) model of the Ross Sea was modified to include air- sea CO2 exchange and non-Redfield C/N/P uptake ratios of the dominant phytoplankton taxa. Model results show that the Ross Sea was a substantial sink for atmospheric CO2, driven by the high primary productivity prior to the onset of Fe limitation. Taxon-specific C/ N/ P uptake ratios controlled the relative rate of removal of each macronutrient, while Fe availability constrained the absolute magnitude of utilization. When Redfield C/ N/ P stoichiometry was applied to both phytoplankton taxa, net primary production ( NPP) was overestimated in areas normally dominated by diatoms and underestimated in regions of Phaeocystis antarctica dominance, and macronutrient dynamics were misrepresented. Simulated shifts in phytoplankton taxonomic composition significantly altered uptake of atmospheric CO2 when the phytoplankton were dominated by diatoms (-70%) or P.antarctica (+35%). The ability to bloom later in the season afforded P.antarctica a relatively greater role than diatoms in controlling the air- sea flux of CO2 in the Ross Sea. In response to alleviation of Fe limitation, both total Ross Sea NPP and CO2 uptake increased by 30%. The response of the carbon cycle to Fe fertilization was predicted to be complex, and its magnitude and nature were dictated by patterns of macronutrient utilization.