Hiscock¹, Michael R., Veronica P. Lance¹, Richard T. Barber¹, Amy Apprill², Robert R. Bidigare², Steve Lindley³ and B. Greg Mitchell⁴

¹Duke University Nicholas School of the Environment and Earth Sciences, 135 Duke Marine Lab Road, Beaufort, NC 28516 USA, Tel: 252-504-7641, Fax: 252-504-7648, E-mail: hiscock@duke.edu, ²University of Hawaii, 1000 Pope Road, Honolulu, HI 96822 USA, ³NOAA Fisheries, Santa Cruz Laboratory, 110 Shaffer Road, Santa Cruz, CA 95060 USA and ⁴Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 USA

Maximum quantum yield responses of phytoplankton in the Southern Ocean and eastern equatorial Pacific

We measured the size-fractionated (<5μm, >20μm, and the fraction between 5 and 20 μm) maximum quantum yield of photosynthesis during two iron enrichment experiments in the Pacific Sector of the Southern Ocean. The south patch area (~66°S, 172°W), south of the Southern Boundary of the Antarctic Circumpolar Current (ACC), is characterized by high nitrate and high silicic acid, while the north patch area (~56°S, 172°W), within the Subantarctic Zone, is characterized by high nitrate and low silicic acid. Both regions responded to iron addition with an increase in maximum quantum yield (j_m, mol C mol photons^-1), however the response within size fractions differed between the two sites. In the south patch, there was a large, rapid, and sustained size-independent increase in j_m. In the north patch, there was little response in j_m to iron enrichment in the smaller size fractions, however the j_m of the >20 μm size fraction was significantly higher inside the patch than outside the patch. These results are consistent with the hypothesis that smaller phytoplankton in the Subantarctic Zone are not iron limited, while the larger size fractions of phytoplankton in the Subantarctic Zone and all size fractions of open-ocean phytoplankton south of the Southern Boundary of the ACC are strongly iron limited. We compare the Southern Ocean iron enrichments with earlier iron enrichment experiments in the eastern equatorial Pacific in which j_m increased 5-fold and iron exerted a strong control over phytoplankton community structure.