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Abstract: Redox cycling of iron in natural seawater is an important process that can affect iron availability to marine phytoplankton. In this work, luminol chemiluminescence was used to measure picomolar Fe(II) oxidation rate constants in continental shelf waters and oceanic surface (upper 200m) waters of the iron-limited eastern subarctic Pacific. In both cases, Fe(II) oxidation rate constants were faster within the chlorophyll maximum than in UV oxidized seawater (UVOS), while rate constants were comparable to UVOS rate constants in waters from below the mixed layer. The larger Fe(II) oxidation rate constants in surface waters converged with UVOS rate constants upon stepwise additions of either Fe(II) or Fe(III), while Fe titrations did not affect Fe(II) oxidation rate constants in waters from below the mixed layer. Direct measurements of hydrogen peroxide, a common Fe(II) oxidant in surface waters, do not explain the high Fe(II) oxidation rate constants. We hypothesize that excess concentrations of strong Fe(III)-complexing organic ligands measured in surface seawater increase Fe(II) oxidation, and that titration of these ligands with added Fe explains the observed slowing of Fe(II ) oxidation rates. Given that Fe(III)-complexing ligands are found in both surface and deep waters, and higher Fe(II) oxidation rate constants were measured only near the chlorophyll maximum, we suggest that the chemical nature and perhaps origin of natural Fe(III)-complexing organic ligands differ between in surface and deep waters. [Copyright &y& Elsevier]

Abstract: The cumulative evidence from more than a dozen mesoscale iron-enrichment studies in high nitrate low chlorophyll (HNLC) waters demonstrates that iron limitation is widespread and very likely affects atmospheric carbon dioxide and thus global climate. However, the responses of microphytoplankton (>20μm), predominantly diatoms, vary greatly among these mesoscale experiments even though similar amounts of iron were added, making it difficult to quantitatively incorporate iron effects into global climate models. Nowhere is this difference more dramatic than between the massive bloom observed during Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS) I and the order of magnitude smaller ecosystem response in SEEDS II; two mesocale experiments performed in the same HNLC region of the western subarctic Pacific in different years. Deckboard incubation experiments initiated during the early, middle, and late stages of the 32-day SEEDS II experiment show that while the two iron infusions increased phytoplankton growth, diatoms remained significantly limited by iron availability, despite total dissolved Fe concentrations in the patch being well above the diffusion-limited threshold for rapid diatom growth. This iron limitation was apparent <6 days after the initial iron infusion and was not alleviated by the second, smaller iron infusion. In contrast, smaller phytoplankton (<20μm) showed a more restricted response to further iron amendments, indicating that their iron nutrition was near optimal. Iron complexed to desferrioximine B, a commonly available siderophore produced by at least one marine bacterium, was poorly available to diatoms throughout the patch evolution, indicating that these diatoms lacked the ability to induce high-affinity iron uptake systems. These results suggest that the strong organic complexation of Fe(III) observed in the SEEDS II-fertilized patch was not compatible with rapid diatom growth. In contrast, iron associated with protoporphyrin IX, a weaker iron complexing ligand of a class hypothesized to be representative of recycled iron species, was readily available to diatoms. Our findings demonstrate that a persistence of iron limitation was the primary factor underlying the comparatively small diatom response during SEEDS II. This continued growth limitation would have increased the importance of mesozooplankton grazing as a controlling factor in the SEEDS II ecosystem response. [Copyright &y& Elsevier]

Presents research which studied the iron concentrations needed to fuel diatom production across the eastern equatorial Pacific Ocean. Phytoplankton growth in this region; Episodic increases in iron concentrations in the past; Records of cycles of plankton productivity; Influence of tectonic processes in the Papua New Guinea region and the effects of global climate forcing.

Tests the hypothesis that iron limits phytoplankton production and biomass in regions of the world's oceans otherwise rich in plant nutrients. Causes of the difference between bottle-scale and mesoscale iron enrichments; Increase in phytoplankton production and biomass upon iron enrichment.