Impact of phytoplankton on the biogeochemical cycling of iron in subantarctic waters southeast of New Zealand during FeCycle

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Abstract

[1] During austral summer 2003, we tracked a patch of surface water infused with the tracer sulfur hexafluoride, but without addition of Fe, through subantarctic waters over 10 days in order to characterize and quantify algal Fe pools and fluxes to construct a detailed biogeochemical budget. Nutrient profiles characterized this patch as a high-nitrate, low-silicic acid, low-chlorophyll (HNLSiLC) water mass deficient in dissolved Fe. The low Fe condition was confirmed by several approaches: shipboard iron enrichment experiments and physiological indices of Fe deficiency (Fv/Fm < 0.25, Ferredoxin Index < 0.2). During FeCycle, picophytoplankton (0.2–2 μm) and nanophytoplankton (2–20 μm) each contributed >40% of total chlorophyll. Whereas the picophytoplankton accounted for ∼50% of total primary production, they were responsible for the majority of community iron uptake in the mixed layer. Thus ratios of 55Fe:14C uptake were highest for picophytoplankton (median: 17 μmol:mol) and declined to ∼5 μmol:mol for the larger algal size fractions. A pelagic Fe budget revealed that picophytoplankton were the largest pool of algal Fe (>90%), which was consistent with the high (∼80%) phytoplankton Fe demand attributed to them. However, Fe regenerated by herbivory satisfied only ∼20% of total algal Fe demand. This iron regeneration term increased to 40% of algal Fe demand when we include Fe recycled by bacterivory. As recycled, rather than new, iron dominated the pelagic iron budget (Boyd et al., 2005), it is highly unlikely that the supply of new Fe would redress the imbalance between algal Fe demand and supply. Reasons for this imbalance may include the overestimation of algal iron uptake from radiotracer techniques, or a lack of consideration of other iron regeneration processes. In conclusion, it seems that algal Fe uptake cannot be supported solely by the recycling of algal iron, and may require an Fe “subsidy” from that regenerated by heterotrophic pathways.

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