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[1] An improved knowledge of iron biogeochemistry is needed to better understand key controls on the functioning of high-nitrate low-chlorophyll (HNLC) oceanic regions. Iron budgets for HNLC waters have been constructed using data from disparate sources ranging from laboratory algal cultures to ocean physics. In summer 2003 we conducted FeCycle, a 10-day mesoscale tracer release in HNLC waters SE of New Zealand, and measured concurrently all sources (with the exception of aerosol deposition) to, sinks of iron from, and rates of iron recycling within, the surface mixed layer. A pelagic iron budget (timescale of days) indicated that oceanic supply terms (lateral advection and vertical diffusion) were relatively small compared to the main sink (downward particulate export). Remote sensing and terrestrial monitoring reveal 13 dust or wildfire events in Australia, prior to and during FeCycle, one of which may have deposited iron at the study location. However, iron deposition rates cannot be derived from such observations, illustrating the difficulties in closing iron budgets without quantification of episodic atmospheric supply. Despite the threefold uncertainties reported for rates of aerosol deposition (Duce et al., 1991), published atmospheric iron supply for the New Zealand region is ∼50-fold (i.e., 7- to 150-fold) greater than the oceanic iron supply measured in our budget, and thus was comparable (i.e., a third to threefold) to our estimates of downward export of particulate iron. During FeCycle, the fluxes due to short term (hours) biological iron uptake and regeneration were indicative of rapid recycling and were tenfold greater than for new iron (i.e. estimated atmospheric and measured oceanic supply), giving an “fe” ratio (uptake of new iron/uptake of new + regenerated iron) of 0.17 (i.e., a range of 0.06 to 0.51 due to uncertainties on aerosol iron supply), and an “Fe” ratio (biogenic Fe export/uptake of new + regenerated iron) of 0.09 (i.e., 0.03 to 0.24).

[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 highnitrate, 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 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 55 Fe: 14 C uptake were highest for picophytoplankton (median: 17 mmol:mol) and declined to � 5 mmol: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.