Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the 'iron hypothesis'. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

[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).Citation: Boyd, P. W., et al. (2005), FeCycle: Attempting an iron biogeochemical budget from a mesoscale SF 6 tracer experiment in unperturbed low iron waters, Global Biogeochem. Cycles, 19, GB4S20,

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The flow of fluid associated with the impact of water drops of radius R at a speed V onto unyielding dry metal surfaces of known roughness R a is described. Spatial dimensions of the deforming drop are normalized by transformations of the kind x ' — x/R , and time scales are normalized according to t ' = tV/R , to permit comparison of events where or differ. It is shown that the primary influence of the surface roughness parameter R a is the determination of the condition for the ejection of secondary droplets by the excitation of an instability in the developing watersheet; provided R a ≪ R , it is possible to evaluate the condition to a high degree of accuracy, and for R a = 0.84 μm it is found to be α4/3 RV 1.69 > 7.4, where α is the eccentricity of the drop at the moment of impact. Deceleration of the drop apex does not commence until > 0.6, contrary to the prediction of Engel (1955) but in good agreement with that of Savic & Boult (1957). Close examination of the very early stages of impact suggests strongly that the so-called watersheet originates at a moment t ' — 0.01 after first contact, regardless of the absolute values of R, V or R a ; the initial normalized watersheet velocity is of order 5. Where there is ejected material, its normalized velocity at the moment of ejection is of the order of 20 % greater than that of the watersheet substrate. Simple calculations also suggest that initial fluid velocities greater than 10 are required immediately before the initiation of the watersheet ( t '< 0.01). Impacts at speeds considerably greater than the appropriate terminal fall speed in air show no deviations in character from those investigated at much lower speeds. A simple subsidiary experiment also suggests that greater impact velocities are required to produce splashing on inclined targets.

[1] Ocean color data from the 13 year Sea-viewing Wide Field-of-view Sensor mission are used to examine the distribution of surface chlorophyll a in the southwest Pacific Ocean. The mean surface chlorophyll field is similar to that found by previous workers, with elevated levels in the Subtropical Front and around the subantarctic islands that have an associated shelf. The annual cycle in surface chlorophyll shows a ubiquitous summer bloom in subantarctic water, with autumn, winter, and spring blooms variously in subtropical water and across the Subtropical Front. The autumn blooms progress equatorward with time at the same rate as wind stress. This supports the idea that the autumn bloom develops in response to increased wind stress, with a likely mechanism for the bloom being mixing to the surface of the deep chlorophyll maximum, and/or increased production due to entrainment of nutrients. The spring bloom progresses poleward with time. It starts after the mixed layer reaches its deepest, and its timing appears to be linked to the reduction in wind stress in spring. Under the assumption that all tracers are well mixed to the seasonal thermocline in autumn and winter, we conclude that vertically integrated chlorophyll increases at all latitudes in subtropical water during autumn and winter. Unfortunately, carbon-to-chlorophyll ratios are not known well enough to determine whether the same is true for vertically integrated carbon biomass. Individual spring blooms show significant spatial structure and are different from year to year. This leads to low spatial coherence for the temporal variability in surface chlorophyll.