The CO uptake by the Southern Ocean (<35°S) varies substantially on all timescales and is a major determinant of the variations of the global ocean carbon sink. Particularly strong are the decadal changes characterized by a weakening period of the Southern Ocean carbon sink in the 1990s and a rebound after 2000. The weakening in the 1990s resulted primarily from a southward shift of the westerlies that enhanced the upwelling and outgassing of respired (i.e., natural) CO. The concurrent reduction in the storage rate of anthropogenic CO in the mode and intermediate waters south of 35°S suggests that this shift also decreased the uptake of anthropogenic CO. The rebound and the subsequent strong, decade-long reinvigoration of the carbon sink appear to have been driven by cooling in the Pacific Ocean, enhanced stratification in the Atlantic and Indian Ocean sectors, and a reduced overturning. Current-generation ocean models generally do not reproduce these variations and are poorly skilled at making decadal predictions in this region.

Several explanations for the 200 to 280 ppm glacial/interglacial change in atmospheric CO2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients. An hypothesis is presented herein in which arguments are made that new productivity in today's southern ocean (7.4 × 1013g yr−1) is limited by iron deficiency, and hence the phytoplankton are unable to take advantage of the excess surface nitrate/phosphate that, if used, could result in total southern ocean new production of 2−3 × 1015 g C yr−1. As a consequence of Fe‐limited new productivity, Holocene interglacial CO2 levels (preindustrial) are as high as they were during the last interglacial (≈ 280 ppm). In contrast, atmospheric dust Fe supplies were 50 times higher during the last glacial maximum (LGM). Because of this Fe enrichment, phytoplankton growth may have been greatly enhanced, larger amounts of upwelled nutrients may have been used, and the resulting stimulation of new productivity may have contributed to the LGM drawdown of atmospheric CO2 to levels of less than 200 ppm. Background information and arguments in support of this hypothesis are presented.

. Since the start of the industrial revolution, human activities have caused a\nrapid increase in atmospheric carbon dioxide (CO2) concentrations, which have, in\nturn, had an impact on climate leading to global warming and ocean\nacidification. Various approaches have been proposed to reduce atmospheric\nCO2. The Martin (or iron) hypothesis suggests that ocean iron\nfertilization (OIF) could be an effective method for stimulating oceanic\ncarbon sequestration through the biological pump in iron-limited,\nhigh-nutrient, low-chlorophyll (HNLC) regions. To test the Martin hypothesis,\n13 artificial OIF (aOIF) experiments have been performed since 1990 in HNLC\nregions. These aOIF field experiments have demonstrated that primary\nproduction (PP) can be significantly enhanced by the artificial addition of iron.\nHowever, except in the Southern Ocean (SO) European Iron Fertilization Experiment (EIFEX),\nno significant change in the effectiveness of aOIF (i.e., the amount of\niron-induced carbon export flux below the winter mixed layer depth,\nMLD) has been detected. These results, including possible side effects, have been debated\namongst those who support and oppose aOIF experimentation, and many questions\nconcerning the effectiveness of scientific aOIF, environmental side effects, and\ninternational aOIF law frameworks remain. In the context of increasing global\nand political concerns associated with climate change, it is valuable to\nexamine the validity and usefulness of the aOIF experiments. Furthermore, it\nis logical to carry out such experiments because they allow one to study how\nplankton-based ecosystems work by providing insight into mechanisms operating\nin real time and under in situ conditions. To maximize the\neffectiveness of aOIF experiments under international aOIF regulations in the\nfuture, we therefore suggest a design that incorporates several components. (1) Experiments\nconducted in the center of an eddy structure when grazing\npressure is low and silicate levels are high (e.g., in the SO south of the polar front during early summer). (2) Shipboard observations\nextending over a minimum of ∼40 days, with multiple iron injections (at\nleast two or three iron infusions of ∼2000 kg with an interval of ∼10–15 days to fertilize a patch of 300 km2 and obtain a ∼2 nM\nconcentration). (3) Tracing of the iron-fertilized patch using both physical\n(e.g., a drifting buoy) and biogeochemical (e.g., sulfur hexafluoride,\nphotosynthetic quantum efficiency, and partial pressure of CO2) tracers.\n(4) Employment of neutrally buoyant sediment traps (NBST) and application of the\nwater-column-derived thorium-234 (234Th) method at two depths (i.e., just\nbelow the in situ MLD and at the winter MLD), with autonomous profilers equipped with an underwater video profiler\n(UVP) and a transmissometer. (5) Monitoring of side effects on marine/ocean\necosystems, including production of climate-relevant gases (e.g.,\nnitrous oxide, N2O; dimethyl sulfide, DMS; and halogenated volatile organic compounds, HVOCs), decline in\noxygen inventory, and development of toxic algae blooms, with\noptical-sensor-equipped autonomous moored profilers and/or autonomous benthic vehicles.\nLastly, we introduce the scientific aOIF experimental design guidelines for a\nfuture Korean Iron Fertilization Experiment in the Southern Ocean (KIFES).\n

Since the mid-1980s, our understanding of nutrient limitation of oceanic primary production has radically changed. Mesoscale iron addition experiments (FeAXs) have unequivocally shown that iron supply limits production in one-third of the world ocean, where surface macronutrient concentrations are perennially high. The findings of these 12 FeAXs also reveal that iron supply exerts controls on the dynamics of plankton blooms, which in turn affect the biogeochemical cycles of carbon, nitrogen, silicon, and sulfur and ultimately influence the Earth climate system. However, extrapolation of the key results of FeAXs to regional and seasonal scales in some cases is limited because of differing modes of iron supply in FeAXs and in the modern and paleo-oceans. New research directions include quantification of the coupling of oceanic iron and carbon biogeochemistry.

. South Georgia phytoplankton blooms are amongst the largest of the Southern Ocean and are associated with a rich ecosystem and strong atmospheric carbon drawdown. Both aspects depend on the intensity of blooms, but also on their regularity. Here we use data from 12 yr of SeaWiFS (Sea-viewing Wide Field-of-view Sensor) ocean colour imagery and calculate the frequency of bloom occurrence (FBO) to re-examine spatial and temporal bloom distributions. We find that upstream of the island and outside the borders of the Georgia Basin, blooms occurred in less than 4 out of the 12 yr (FBO &lt; 4). In contrast, FBO was mostly greater than 8 downstream of the island, i.e., to the north and northwest, and in places equal to 12, indicating that blooms occurred every year. The typical bloom area, defined as the region where blooms occurred in at least 8 out of the 12 yr, covers the entire Georgia Basin and the northern shelf of the island. The time series of surface chlorophyll a (Chl a) concentrations averaged over the typical bloom area shows that phytoplankton blooms occurred in every year between September 1997 and September 2010, and that Chl a values followed a clear seasonal cycle, with concentration peaks around December followed in many years by a second peak during late austral summer or early autumn, suggesting a bi-modal bloom pattern. The bloom regularity we describe here is in contrast with results of Park et al. (2010) who used a significantly different study area including regions that almost never exhibit bloom conditions.\n

. The island of South Georgia is situated in the iron (Fe)-depleted Antarctic\nCircumpolar Current of the Southern Ocean. Iron emanating from its shelf\nsystem fuels large phytoplankton blooms downstream of the island, but the\nactual supply mechanisms are unclear. To address this, we present an\ninventory of Fe, manganese (Mn), and aluminium (Al) in shelf sediments, pore\nwaters, and the water column in the vicinity of South Georgia, alongside data\non zooplankton-mediated Fe cycling processes, and provide estimates of the\nrelative dissolved Fe (DFe) fluxes from these sources. Seafloor sediments,\nmodified by authigenic Fe precipitation, were the main particulate Fe source\nto shelf bottom waters as indicated by the similar Fe ∕ Mn and Fe ∕ Al ratios for\nshelf sediments and suspended particles in the water column. Less than 1 %\nof the total particulate Fe pool was leachable surface-adsorbed (labile) Fe\nand therefore potentially available to organisms. Pore waters formed the\nprimary DFe source to shelf bottom waters, supplying 0.1–44 µmol DFe m−2 d−1.\nHowever, we estimate that only 0.41±0.26 µmol DFe m−2 d−1 was transferred to the surface mixed layer by vertical\ndiffusive and advective mixing. Other trace metal sources to surface waters\nincluded glacial flour released by melting glaciers and via zooplankton\negestion and excretion processes. On average 6.5±8.2 µmol m−2 d−1 of labile particulate Fe was supplied to the surface\nmixed layer via faecal pellets formed by Antarctic krill (Euphausia superba), with a further 1.1±2.2 µmol DFe m−2 d−1\nreleased directly by the krill. The faecal pellets released by krill included\nseafloor-derived lithogenic and authigenic material and settled algal debris,\nin addition to freshly ingested suspended phytoplankton cells. The Fe requirement of the phytoplankton blooms ∼ 1250 km\ndownstream of South Georgia was estimated as 0.33±0.11 µmol m−2 d−1, with the DFe supply by horizontal/vertical mixing, deep\nwinter mixing, and aeolian dust estimated as ∼0.12 µmol m−2 d−1. We hypothesize that a substantial contribution of DFe was\nprovided through recycling of biogenically stored Fe following luxury Fe\nuptake by phytoplankton on the Fe-rich shelf. This process would allow Fe to\nbe retained in the surface mixed layer of waters downstream of South Georgia\nthrough continuous recycling and biological uptake, supplying the large\ndownstream phytoplankton blooms.\n

The relationship between primary production in the surface ocean and export of particulate organic carbon (POC) on sinking particles is examined in studies that have utilized 234Th as a tracer of upper ocean export. Comparisons between production and export are made in a wide range of open ocean settings and seasons. The results indicate that much of the ocean is characterized by low POC export relative to primary production (export/production = ThE &lt; 5–10%). Exceptions to this pattern are found during blooms at high latitudes, accompanying spring blooms at midlatitudes, and perhaps in association with more episodic export pulses. These sites of high export are most often characterized by food webs dominated by large phytoplankton, in particular diatoms. These results can be used to better parameterize surface export in biogeochemical models in order to provide a more accurate prediction of the flow of C and associated nutrients in the oceans.

Cellular nutrient ratios are often applied as indicators of nutrient limitation in phytoplankton studies, especially the so‐called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellular C:N:P stoichiometry of benthic microalgae in response to different levels and types of nutrient limitation and a variety of abiotic conditions in laboratory experiments with natural inocula. C:N ratios increased with decreasing growth rate, irrespective of the limiting nutrient. At the highest growth rates, the C:N ratio ranged uniformly around 7.5. N:P ratios &lt;13 indicated N limitation, while N:P ratios &gt;22 indicated P limitation. Under P limitation, the C:P ratios increased at low growth rate and varied around 130 at highest growth rates. For a medium with balanced supply of N and P, an optimal stoichiometric ratio of C:N:P = 119:17:1 could be deduced for benthic microalgae, which is slightly higher than the Redfield ratio (106:16:1) considered typical for optimally growing phytoplankton. The optimal ratio was stable against changes in abiotic conditions. In conclusion, cellular nutrient ratios are proposed as an indicator for nutrient status in periphyton.

SeaWiFS estimates of surface chlorophyll concentrations are reported for the region of the U.S. JGOFS study in the Southern Ocean (∼ 170°W, 60°S). Elevated chlorophyll was observed at the Southern Ocean fronts, near the edge of the seasonal ice sheet, and above the Pacific‐Antarctic Ridge. The elevated chlorophyll levels associated with the Pacific‐Antarctic Ridge are surprising since even the crest of the ridge is at depths &gt; 2000 m. This elevated phytoplankton biomass is likely the result of mesoscale physical‐biological interactions where the Antarctic Circumpolar Current (ACC) encounters the ridge. Four cruises surveyed this region between October 1997 and March 1998, as part of the U.S. JGOFS. Satellite‐derived chlorophyll concentrations were compared with in situ extracted chlorophyll measurements from these cruises. There was good agreement (r² of 0.72, from a linear regression of shipboard vs. satellite chlorophyll), although SeaWiFS underestimated chlorophyll concentrations relative to the ship data.

. In high-nutrient low-chlorophyll waters of the western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia. Multiple sources, including shallow sediments and atmospheric dust deposition, are thought to introduce iron to the region. However, the relative importance of each source is still unclear, owing in part to the scarcity of dissolved iron (dFe) measurements in the South Georgia region. In this study, we combine results from a recently published dFe data set around South Georgia with a coupled regional hydrodynamic and biogeochemical model to further investigate iron supply around the island. The biogeochemical component of the model includes an iron cycle, where sediments and dust deposition are the sources of iron to the ocean. The model captures the characteristic flow patterns around South Georgia, hence simulating a large phytoplankton bloom to the north (i.e. downstream) of the island. Modelled dFe concentrations agree well with observations (mean difference and root mean square errors of ~0.02 nM and ~0.81 nM) and form a large plume to the north of the island that extends eastwards for more than 800 km. In agreement with observations, highest dFe concentrations are located along the coast and decrease with distance from the island. Sensitivity tests indicate that most of the iron measured in the main bloom area originates from the coast and very shallow shelf-sediments (depths &lt; 20 m). Dust deposition exerts almost no effect on surface chlorophyll a concentrations. Other sources of iron such as run-off and glacial melt are not represented explicitly in the model, however we discuss their role in the local iron budget.\n

The interannual variability of net community production (NCP) and air-sea CO2 flux in a naturally iron fertilized and productive area of the Southern Ocean (Kerguelen plateau) was investigated using a 1D biogeochemical model driven by satellite chlorophyll, sea surface temperature and wind speed data for the 1997–2007 period. The model simulates the low fCO2 and dissolved inorganic carbon (DIC) measured during summers 2004–05, 2005–06, 2006–07 and the high NCP derived from a seasonal carbon budget in the surface waters of these blooms. Although satellite data show high interannual variability in the dynamics and magnitude of the bloom during the 1997–2007 decade, the simulated interannual variability of the NCP was only ± 14%. This unexpected result could be due to the combined effect of both the duration and the start date of the bloom, the latter determining the depth of the mixed layer used to compute the NCP. In the productive area, the interannual variability of air-sea CO2 flux (± 13%) was not only driven by the biological effect but also by the solubility effect. Our results contrast with previous studies in the high nutrient, low chlorophyll regions of the Southern Ocean.

Antarctic krill () play a central role in the food web of the Southern Ocean, forming a link between primary production and large predators. Krill produce large, faecal pellets (FP) which can form a large component of mesopelagic particulate organic carbon (POC) fluxes. However, the patchy distribution of krill swarms, highly variable pellet composition, and variable sinking and attenuation rates means that these episodic, but potentially large, carbon fluxes are difficult to sample or model. We measured particle flux and type using Marine Snow Catchers (MSC) in the marginal ice zone near the South Orkneys, Antarctica. Krill FP were the dominant component of the POC flux in the upper 200 m (typically 60-85%). FP sinking velocities measured onboard were highly variable (15-507 m d) but overall high, with mean equivalent velocities of 172, 267, and 161 m d at our three stations. The high numbers of krill FP sinking through the mesopelagic suggest that krill FP can be transferred efficiently and/or that rates of krill FP production are high. We compared our direct MSC-derived estimates of krill FP POC flux (33-154 mg C m d) and attenuation to estimates of krill FP production based on previous measurements of krill density and literature FP egestion rates, and estimated net krill FP attenuation rates in the upper mesopelagic. Calculated attenuation rates are sensitive to krill densities in the overlying water column but suggest that krill FP could be transferred efficiently through the upper mesopelagic, and, in agreement with our MSC attenuation estimates, could make large contributions to bathypelagic POC fluxes. Our study contrasts with some others which suggest rapid FP attenuation, highlighting the need for further work to constrain attenuation rates and assess how important the contribution of Antarctic krill FP could be to the Southern Ocean biological carbon pump.© The Author(s) 2017.

. The biological carbon pump is responsible for much of the decadal\nvariability in the ocean carbon dioxide (CO2) sink, driving the\ntransfer of carbon from the atmosphere to the deep ocean. A mechanistic\nunderstanding of the ecological drivers of particulate organic carbon (POC)\nflux is key both to the assessment of the magnitude of the ocean CO2\nsink and for accurate predictions as to how this will change with\nchanging climate. This is particularly important in the Southern Ocean, a\nkey region for the uptake of CO2 and the supply of nutrients to the\nglobal thermocline. In this study we examine sediment-trap-derived particle\nfluxes and stable isotope signatures of carbon (C), nitrogen (N), and\nbiogenic silica (BSi) at a study site in the biologically productive waters\nof the northern Scotia Sea in the Southern Ocean. Both deep (2000 m) and\nshallow (400 m) sediment traps exhibited two main peaks in POC, particulate\nN, and BSi flux: one in austral spring and one in summer, reflecting periods\nof high surface productivity. Particulate fluxes and isotopic compositions\nwere similar in both deep and shallow sediment traps, highlighting that most\nremineralisation occurred in the upper 400 m of the water column.\nDifferences in the seasonal cycles of isotopic compositions of C, N, and Si\nprovide insights into the degree of coupling of these key nutrients. We\nmeasured increasing isotopic enrichment of POC and BSi in spring, consistent\nwith fractionation during biological uptake. Since we observed isotopically\nlight particulate material in the traps in summer, we suggest\nphysically mediated replenishment of lighter isotopes of key nutrients from\ndepth, enabling the full expression of the isotopic fractionation associated\nwith biological uptake. The change in the nutrient and remineralisation\nregimes, indicated by the different isotopic compositions of the spring and\nsummer productive periods, suggests a change in the source region of\nmaterial reaching the traps and associated shifts in phytoplankton community\nstructure. This, combined with the occurrence of advective inputs at certain\ntimes of the year, highlights the need to make synchronous measurements of\nphysical processes to improve our ability to track changes in the source\nregions of sinking particulate material. We also highlight the need to\nconduct particle-specific (e.g. faecal pellets, phytoplankton detritus,\nzooplankton moults) isotopic analysis to improve the use of this tool in\nassessing particle composition of the sinking material and to develop our\nunderstanding of the drivers of biogeochemical fluxes.\n