Iron-mediated changes in phytoplankton photosynthetic competence during SOIREE

Active #uorescence (fast repetition rate #uorometry, FRRF) was used to follow the photosynthetic response of the phytoplankton community during the 13-day Southern Ocean Iron RElease Experiment (SOIREE). This in situ iron enrichment was conducted in the polar waters of the Australasian}Paci"c sector of the Southern Ocean in February 1999. Iron fertilisation of these high nitrate low chlorophyll (HNLC) waters resulted in an increase in the photosynthetic competence (F /F ) of the resident cells from around 0.20 to greater than 0.60 (i.e. close to the theoretical maximum) by 10/11 days after the "rst enrichment. Although a signi"cant iron-mediated response in F /F was detected as early as 24 h after the initial fertilisation, the increase in F /F to double ambient levels took 6 days. This response was "ve-fold slower than observed in iron enrichments (in situ and in vitro) in the HNLC waters of the subarctic and equatorial Paci"c. Although little is known about the relationship between water temperature and F /F , it is likely that low water temperatures* and possibly the deep mixed layer* were responsible for this slow response time. During SOIREE, the photosynthetic competence of the resident phytoplankton in iron-enriched waters increased at dissolved iron levels above 0.2 nM, suggesting that iron limitation was alleviated at this concentration. Increases in F /F of cells within four algal size classes suggested that all taxa displayed a photosynthetic response to iron enrichment. Other physiological proxies of algal iron stress (such as #avodoxin levels in diatoms) exhibited di!erent temporal trends to iron-enrichment than F /F during the time-course of SOIREE. The relationship between F /F , algal growth rate and such proxies in Southern Ocean waters is discussed. 2001 Elsevier Science Ltd. All rights reserved. 0967-0645/01/$ see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 0 6 4 5 ( 0 1 ) 0 0 0 0 7 8