We investigated phytoplankton Fe limitation using shipboard incubation experiments in the high-nutrient South American eastern boundary current regime. Low ambient Fe concentrations (ϳ0.1 nM) in water collected from the Humboldt and Peru Currents were supplemented with a range of added Fe levels up to 2.5 nM. Phytoplankton chlorophyll a, photosystem II photosynthetic efficiency, and nitrate and phosphate drawdown increased in proportion to the amount of Fe added. The Humboldt Current algal community after Fe additions included colonial and flagellated Phaeocystis globosa and large pennate diatoms, whereas the Peru Upwelling assemblage was dominated by coccolithophorids and small pennate diatoms. Apparent half-saturation constants for growth of the two communities were 0.17 nM Fe (Humboldt Current) and 0.26 nM Fe (Peru Upwelling). Net molar dissolved Si(OH) 4 : NO drawdown ratios were low in both experiments (ϳ0.2-0.7), but net particulate silica to nitrogen production Ϫ 3 ratios were higher. Fe limitation decreased net NO : PO utilization ratios in the Humboldt Current incubation to

Studies in high nutrient, low chlorophyll (HNLC) regions have demonstrated that increased Fe availability results in an increase in phytoplankton biomass and changes in community composition. Here we present experiments in which the availability of iron (Fe) was increased or reduced to monitor the response of individual groups of phytoplankton (large eukaryotes, picoeukaryotes and cyanobacteria) by flow cytometry. Additions (0.5 to 5.0 nM Fe) and reductions in available Fe (through addition of 1 to 10 nM of the fungal siderophore desferrioxamine B) were made to enclosed communities from the South American eastern boundary current off Peru, where ambient dissolved Fe concentrations were <100 pM. As predicted, chlorophyll concentrations increased in the added Fe treatments relative to the control, indicative of Fe limitation. Flow cytometry demonstrated that this was due to increases in the abundance of large eukaryotes that are Fe-starved under ambient conditions. Cyanobacterial abundance increased and decreased linearly with Fe availability, suggesting that cyanobacteria were Fe-limited but not Fe-starved. In contrast, picoeukaryote cell abundance increased with decreasing Fe availability, although chlorophyll cell -1 in this group responded in an inverse manner. The results demonstrate that members of the marine phytoplankton community respond differently to Fe availability, which may influence the outcome of biological competition among organisms in Fe-limited environments. KEY WORDS: Fe limitation · Fe Uptake · Desferrioxamine B · PicoeukaryotesResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 35: 79-91, 2004 areas support ca. 10% (~0.8 Pg C yr -1 ) of the total global new production (Chavez & Toggweiler 1995). The Humboldt Current (which runs northward along the western coast of South America) is extremely productive; 19.9 g C m -2 d -1 are fixed by photosynthesis in the south and central fishing areas, and 9.3 g C m -2 d -1in the Antofagasta upwelling to the south (Daneri et al. 2000). Recent estimates suggest that the Humboldt Current produces 7 million tonnes in annual fish catch (Daneri et al. 2000).In pelagic HNLC regions of the equatorial Pacific, picoplanktonic cyanobacteria such as Synechococcus and Prochlorococcus are the most abundant photosynthetic organisms in the water column (Chavez et al. 1991, Fogg 1995. Although no direct evidence exists to account for their success, their ability to alter cellular Fe quotas, their small size, and their ability to use high-affinity Fe-acquisition mechanisms may allow for their persistence in Fe-limited HNLC environments (Brand 1991, Wilhelm & Trick 1994, Hutchins et al. 1999b). These studies, mostly in laboratory settings, suggest that these cyanobacteria assume an alternate physiology that allows them to persist when Fe availability may be growth limiting. In field studies, where HNLC environments have been fertilized with Fe, a shift in species composition is usually observed, with large diatoms r...

Phytoplankton community structure in the eastern Equatorial Pacific (EEP) and the Peru upwelling system was determined using ChemTax analysis of pigment data. Photosynthetic efficiencies (F v : F m ), concentrations of macronutrients, iron (Fe), and the redox proteins flavodoxin (Flv) and ferredoxin (Fd) were used to assess the physiological status of the algal communities with respect to Fe. Diel periodicity in