Effects of iron on the elemental stoichiometry during EIFEX and in the diatoms <i>Fragilariopsis kerguelensis</i> and <i>Chaetoceros dichaeta</i>

International audienceThe interaction between iron availability and the phytoplankton elemental composition was investigated during the in situ iron fertilization experiment EIFEX and in laboratory experiments with the Southern Ocean diatom species Fragilariopsis kerguelensis and Chaetoceros dichaeta. Contrary to other in situ iron fertilization experiments we observed an increase in the bPSi : POC, bPSi : PON, and bPSi : POP ratios within the iron fertilized patch during EIFEX. This is possibly caused by a relatively stronger increase in diatom abundance compared to other phytoplankton groups and does not necessarily represent the amount of silicification of single diatom cells. In laboratory experiments with F. kerguelensis and C. dichaeta no changes in the POC : PON, PON : POP, and POC : POP ratios were found with changing iron availability in both species. BPSi : POC, bPSi : PON, and bPSi : POP ratios were significantly lower in the high iron treatments compared to the controls. In F. kerguelensis this is caused by a decrease in cellular bPSi concentrations and therefore possibly less silicification. In C. dichaeta no change in cellular bPSi concentration was found. Here lower bPSi : POC, bPSi : PON, and bPSi : POP ratios were caused by an increase in cellular C, N, and P under high iron conditions. We therefore assume that iron limitation does not generally increase silicification of diatoms and that changes in the bPSi : POC, bPSi : PON, and bPSi : POP ratios under iron fertilization in the field are caused by a variety of different mechanisms. These results imply that the effect of iron on nutrient uptake is more complex than hitherto assumed