1. The biological response to in situ Southern Ocean iron fertilization. Richard T. Barber and SOFeX Team SOFeX: Southern Ocean Iron Experiments, An Overview of the Biological Responses Richard Barber and the SOFeX Group

2. Primary productivity, photosynthetic performance and quantum yield team on the three ships: R/V Revelle: Barber, Lance, Stuebe R/V Melville: Hilting, Hiscock, Apprill, Delizo, Tanner USCG Polar Star: Van Hilst FRRF data provided by Sasha Tozzi. PAM data provided by Jill Peloquin. POC flux data provided by Ken Buesseler.

3. North Patch Temp. 7.5ºC Si(OH) 4 2.0μM NO 3 21μM +Fe 1.2nM South Patch Temp. -0.5ºC Si(OH) 4 63μM NO 3 27μM +Fe 0.7nM

4. NP SP Underway nutrients and temperature figure provided by W. T. Hiscock, U. Miami.

5. Surface Chl (mg Chl m -3 ) 0 1 2 3 4 Year Day 2025303540455055 SOFeX Productivity DUKE - MBARI - VIMS - SIO South Patch Fertilization Melville Revelle Polar Star

6. South Patch Year Day 2025303540455055 Popt (mmolC m -3 d -1 ) 0 1 2 3 4 SOFeX Productivity DUKE - MBARI - VIMS - SIO Melville Revelle Fertilization

7. Z morel (m)

10. No. Patch revisit by Revelle.

11. SOFeX Productivity DUKE - MBARI - VIMS - SIO North Patch Year Day 101520253035404550 Surface Chl (mg Chl m -3 ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Melville Revelle Fertilization

12. North Patch Year Day 101520253035404550 Popt (mmolC m -3 d -1 ) 0 1 2 3 4 5 6 7 SOFeX Productivity DUKE - MBARI - VIMS - SIO Melville Revelle Fertilization

16. Weeks Post First Fertilization 123 Binned Int PP (mmol C m -2 d -1 ) 10 20 30 40 Out 10% In 10%

21. PAM- bulk assemblage for South Patch, bars are iron additions, circled data are OUTs. Cells never reached photochemical maximum for 20 m water for PAM (0.70). Outs

25. Results 10x increase in Chl in both N and S patch 10x increase in productivity (P opt ) in both N and S patch 3 to 4x increase in integrated primary productivity 3 to 4x increase in quantum yield in South patch, but no differential enhancement of quantum yield in large (> 20 μm) phytoplankton Big change in size composition in N patch despite low Si conditions, but only modest change in size composition in S patch with high Si conditions In patch POC flux about 30% of In patch PP integrated to 10% light level; Out flux about 40% of Out PP integrated to 10% light level

26. Remaining Questions 1.Depth of export 2.Fate of exported organic carbon at depth 3.Cellular mechanism of “fast” photosynthetic efficiency response to Fe 4.Nature of temperature vs. photosynthetic efficiency relationship

29. South Patch Year Day 2025303540455055 % Popt >20 μm 0 10 20 30 40 50 60 70 80 SOFeX Productivity DUKE - MBARI - VIMS - SIO Melville Revelle Fertilization

30. Prim Prod (mmol C m -3 d -1 )