Macey, A.I., Ryan-Keogh T J, Richier, S., Moore, C.M., Bibby, T.S.
Abstract

Iron availability influences phytoplankton physiology and growth over more than one-third of the surface oceans, with recent evidence even indicating iron stress during and following the latter stages of the spring bloom in the high latitude North Atlantic. The mechanistic basis of the phytoplankton physiological responses used for diagnosing iron stress and the broader ecophysiological consequences of iron stress within natural phytoplankton communities still remain unclear. We describe photosynthetic macromolecular and physiological responses of natural phytoplankton communities both in situ and within factorial nutrient-addition (iron and nitrogen) experiments over a seasonal growth cycle in the subpolar North Atlantic. The abundance of quantified photosynthetic proteins increased under relief of iron stress, with the synthesis of the associated protein catalytic complexes accounting for, ~ 3% of inorganic nitrogen drawdown. However, no significant differences in the stoichiometries of the photosynthetic complexes were observed, suggesting that re-modeling of the photosynthetic electron transport chain was not a significant influence on the community-level ecophysiological responses to iron stress. In marked contrast, iron stress resulted in significant increases in the cellular ratios of chlorophyll to the photosynthetic catalysts, including photosystem II (PSII), alongside a marked increase in PSII normalized chlorophyll fluorescence. Characteristic depressions in apparent photosynthetic energy conversion efficiencies in iron-limited oceanic regions are thus likely driven by a significant accumulation of partially energetically uncoupled chlorophyll-binding complexes. Such iron-stress–induced chlorophyll-binding proteins may contribute, ~ 40% of the total chlorophyll pool during iron-stressed periods.

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Macey-paper-fig

Incubation Experiment 4 (IE4) is shown as a typical example of the response of the phytoplankton community to the addition of Fe (white symbols) in relation to control samples (black symbols). (a–c) changes in the apparent photochemistry of PSII (Fv : Fm), chlorophyll a (μg L-1), and nitrate concentrations (μmol L-1). (d–f) changes in Fm normalized to chlorophyll (Fm : Chl), and accumulation of the peptide PsbA (a subunit of the photosynthetic complex PSII) normalized to total protein (fmol (μg protein)-1) and as total molar concentration (pmol L-1). a.u., arbitrary units.

Swart S., Liu, J., Bhaskar, P., Newman, L., Finney, K., Meredith, M., Schofield, O.
Abstract

The first Southern Ocean Observing System (SOOS) Asian Workshop was successfully held in Shanghai, China in May 2013, attracting over 40 participants from six Asian nations and widening exposure to the objectives and plans of SOOS. The workshop was organized to clarify Asian research activities currently taking place in the Southern Ocean and to discuss, amongst other items, the potential for collaborative efforts with and between Asian countries in SOOS-related activities. The workshop was an important mechanism to initiate discussion, understanding and collaborative avenues in the Asian domain of SOOS beyond current established efforts. Here we present some of the major outcomes of the workshop covering the principle themes of SOOS and attempt to provide a way forward to achieve a more integrated research community, enhance data collection and quality, and guide scientific strategy in the Southern Ocean.

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Map of the Southern Ocean and approximate location of regular shipping transects maintained by Asian nations.

Map of the Southern Ocean and approximate location of regular shipping transects maintained by Asian nations.

Tagliabue, A., Sallee, J. B., Bowie, A. R., Levy M., Swart S., Boyd. P. W.
Abstract

Low levels of iron limit primary productivity across much of the Southern Ocean. At the basin scale, most dissolved iron is supplied to surface waters from subsurface reservoirs, because land inputs are spatially limited. Deep mixing in winter together with year-round diffusion across density surfaces, known as diapycnal diffusion, are the main physical processes that carry iron-laden subsurface waters to the surface. Here, we analyse data on dissolved iron concentrations in the top 1,000 m of the Southern Ocean, taken from all known and available cruises to date, together with hydrographic data to determine the relative importance of deep winter mixing and diapycnal diffusion to dissolved iron fluxes at the basin scale. Using information on the vertical distribution of iron we show that deep winter mixing supplies ten times more iron to the surface ocean each year, on average, than diapycnal diffusion. Biological observations from the sub-Antarctic sector suggest that following the depletion of this wintertime iron pulse, intense iron recycling sustains productivity over the subsequent spring and summer. We conclude that winter mixing and surface-water iron recycling are important drivers of temporal variations in Southern Ocean primary production.

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A schematic representation of the seasonal variability in Southern Ocean Fe cycling

A schematic representation of the seasonal variability in Southern Ocean Fe cycling

Achterberg, E.P., Moore, C.M., Henson, S.A., Steigenberger, S., Stohl, A., Eckhardt, S., Avendano, L.C., Cassidy, M., Hembury, D., Lucas M., Ryan-Keogh T J, Et al.
Abstract

Aerosol deposition from the 2010 eruption of the Icelandic volcano Eyjafjallajökull resulted in significant dissolved iron (DFe) inputs to the Iceland Basin of the North Atlantic. Unique ship-board measurements indicated strongly enhanced DFe concentrations (up to 10 nM) immediately under the ash plume. Bioassay experiments performed with ash collected at sea under the plume also demonstrated the potential for associated Fe release to stimulate phytoplankton growth and nutrient drawdown. Combining Fe dissolution measurements with modeled ash deposition suggested that the eruption had the potential to increase DFe by > 0.2 nM over an area of up to 570,000 km2 . Although satellite ocean color data only indicated minor increases in phytoplankton abundance over a relatively constrained area, comparison of in situ nitrate concentrations with historical records suggested that ash deposition may have resulted in enhanced major nutrient drawdown. Our observations thus suggest that the 2010 Eyjafjallajökull eruption resulted in a significant perturbation to the biogeochemistry of the Iceland Basin.

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fig3_depo

(a) Modeled DFe enhancement (nM) as a result of ash deposition (15 April to 23 May) using midrange estimates of salt layer thickness (20 nm) of volcanic particles as obtained through leaching experiments. Contours mark 0.2nM DFe enhancement. The dashed line is the cruise track (May 2010). (b) The proportion of the Iceland Basin (assumed to be a region ~1 x 106 km2) receiving DFe inputs from ash (15 April to 23 May) using minimum (solubility 0.042%) and maximum (salt layer coating of 90nm thickness) estimates of Fe content of volcanic particles.

Ryan-Keogh T J, Macey, A.I., Lucas M., Steigenberger, S.S., Stinchcombe, M.C., Achterberg, E.P., Bibby, T.S., Moore, C.M.
Abstract

The high-latitude North Atlantic (HLNA) is characterized by a marked seasonal phytoplankton bloom, which removes the majority of surface macronutrients. However, incomplete nitrate depletion is frequently observed during summer in the region, potentially reflecting the seasonal development of an iron (Fe) limited phytoplankton community. In order to investigate the seasonal development and spatial extent of iron stress in the HLNA, nutrient addition experiments were performed during the spring (May) and late summer (July and August) of 2010. Grow-out experiments (48–120 h) confirmed the potential for iron limitation in the region. Short-term (24 h) incubations further enabled high spatial coverage and mapping of phytoplankton physiological responses to iron addition. The difference in the apparent maximal photochemical yield of photosystem II (PSII) (Fv : Fm) between nutrient (iron) amended and control treatments (Δ(Fv : Fm)) was used as a measure of the relative degree of iron stress. The combined observations indicated variability in the seasonal cycle of iron stress between different regions of the Irminger and Iceland Basins of the HLNA, related to the timing of the annual bloom cycle in contrasting biogeochemical provinces. Phytoplankton iron stress developed during the transition from the prebloom to peak bloom conditions in the HLNA and was more severe for larger cells. Subsequently, iron stress was reduced in regions where macronutrients were depleted following the bloom. Iron availability plays a significant role in the biogeochemistry of the HLNA, potentially lowering the efficiency of one of the strongest biological carbon pumps in the ocean.

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HLNA Fig 8

(a) In situ chlorophyll data (μg L-1) and relative degree of Fe stress (Δ(Fv:Fm)+2.0 Fe), (b) in situ DIN (μmol L-1) data and Δ(Fv:Fm)+2.0 Fe and in situ DIN and different in net chlorophyll growth rate following Fe addition (ΔμChl (d-1)) relative to time of peak of bloom. Superimposed on panel (c) conceptualised model of bloom dynamics, demonstrating two different post-bloom scenarios (low DIN and high DIN) associated with different degrees of Fe stress and iron limited growth rates.

Abstract

One of the important gaps in the reliable prediction of the response of the Southern Ocean carbon cycle to climate change is its sensitivity to seasonal, subseasonal forcings (in time) and mesoscales (in space). The Southern Ocean Carbon and Climate Observatory (SOCCO), a CSIR-led consortium, is planning the Southern Ocean Seasonal Cycle Experiment (SOSCEx), which will be a new type of large-scale experiment. SOSCEx reflects a shift from the historical focus on ship-based descriptive Southern Ocean oceanography and living resource conservation, to system-scale dynamics studies spanning much greater time and space scales. The experiment provides a new and unprecedented opportunity to gain a better understanding of the links between climate drivers and ecosystem productivity and climate feedbacks in the Southern Ocean. This combined high-resolution approach to both observations and modelling experiments will permit us, for the first time, to address some key questions relating to the physical nature of the Southern Ocean and its carbon cycle.

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A space–time plot showing relative scale magnitudes of a number of platforms (ships, instrumented moorings and gliders), the seasonal cycle and climate projections. This graphical representation emphasises that, even with both ships and moorings observational platforms, it is not possible to address questions on the seasonal cycle sensitivity of climate projections without using autonomous platforms. Ocean gliders are uniquely poised to bridge the spatial and temporal gap between ships and moorings – a bridge which critically covers the seasonal 'window' in the Southern Ocean Seasonal Cycle Experiment.

A space–time plot showing relative scale magnitudes of a number of platforms (ships, instrumented moorings and gliders), the seasonal cycle and climate projections. This graphical representation emphasises that, even with both ships and moorings observational platforms, it is not possible to address questions on the seasonal cycle sensitivity of climate projections without using autonomous platforms. Ocean gliders are uniquely poised to bridge the spatial and temporal gap between ships and moorings – a bridge which critically covers the seasonal ‘window’ in the Southern Ocean Seasonal Cycle Experiment.

Tagliabue, A., Mtshali T., Aumont, O., Bowie, A., Klunder, M. B. , Roychoudhury A. N., Swart S.
Abstract

Due to its importance as a limiting nutrient for phytoplankton growth in large regions of the world’s oceans, ocean water column observations of concentration of the trace-metal iron (Fe) have increased markedly over recent decades. Here we compile >13 000 global measurements of dissolved Fe (dFe) and make this available to the community. We then conduct a synthesis study focussed on the Southern Ocean, where dFe plays a fundamental role in governing the carbon cycle, using four regions, six basins and five depth intervals as a framework. Our analysis highlights depth-dependent trends in the properties of dFe between different regions and basins. In general, surface dFe is highest in the Atlantic basin and the Antarctic region. While attributing drivers to these patterns is uncertain, inter-basin patterns in surface dFe might be linked to differing degrees of dFe inputs, while variability in biological consumption between regions covaries with the associated surface dFe differences. Opposite to the surface, dFe concentrations at depth are typically higher in the Indian basin and the Subantarctic region. The inter-region trends can be reconciled with similar ligand variability (although only from one cruise), and the inter-basin difference might be explained by differences in hydrothermal inputs suggested by modelling studies (Tagliabue et al., 2010) that await observational confirmation. We find that even in regions where many dFe measurements exist, the processes governing the seasonal evolution of dFe remain enigmatic, suggesting that, aside from broad Subantarctic – Antarctic trends, biological consumption might not be the major driver of dFe variability. This highlights the apparent importance of other processes such as exogenous inputs, physical transport/mixing or dFe recycling processes. Nevertheless, missing measurements during key seasonal transitions make it difficult to better quantify and understand surface water replenishment processes and the seasonal Fe cycle. Finally, we detail the degree of seasonal coverage by region, basin and depth. By synthesising prior measurements, we suggest a role for different processes and highlight key gaps in understanding, which we hope can help structure future research efforts in the Southern Ocean.

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Distribution of iron data in the Southern Ocean with regional breakdown for different ocean regimes and basins.

Distribution of iron data in the Southern Ocean with regional breakdown for different ocean regimes and basins.

Ryan-Keogh T J, Macey, A.I., Cockshutt, A.M., Moore, C.M., Bibby, T.S.
Abstract

Iron availability limits primary production in >30% of the world’s oceans; hence phytoplankton have developed acclimation strategies. In particular, cyanobacteria express IsiA (iron-stress-induced) under iron stress, which can become the most abundant chl-binding protein in the cell. Within iron-limited oceanic regions with significant cyanobacterial biomass, IsiA may represent a significant fraction of the total chl. We spectroscopically measured the effective cross-section of the photosynthetic reaction center PSI (σPSI ) in vivo and biochemically quantified the absolute abundance of PSI, PSII, and IsiA in the model cyanobacterium Synechocystis  sp. PCC 6803. We demonstrate that accumulation of IsiA results in a 60% increase in σPSI , in agreement with the theoretical increase in cross-section based on the structure of the biochemically isolated IsiA-PSI supercomplex from cyanobacteria. Deriving a chl budget, we suggest that IsiA plays a primary role as a light-harvesting antenna for PSI. On progressive iron-stress in culture, IsiA continues to accumulate without a concomitant increase in σPSI , suggesting that there may be a secondary role for IsiA. In natural populations, the potential physiological significance of the uncoupled pool of IsiA remains to be established. However, the functional role as a PSI antenna suggests that a large fraction of IsiA-bound chl is directly involved in photosynthetic electron transport.

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Isia_figure3

The in vivo effective absorption cross-section of PSI (σPSI) measured on Synechocystis PCC 6803 under iron-replete (+Fe) and iron-deplete (-Fe) conditions. Displayed are results averaged from triplicates from three independent experiments with ±standard errors.

Abstract

In the Ocean, the seasonal cycle is the mode that couples climate forcing to ecosystem response in production, diversity and carbon export. A better characterisation of the ecosystem’s seasonal cycle therefore addresses an important gap in our ability to estimate the sensitivity of the biological pump to climate change. In this study, the regional characteristics of the seasonal cycle of phytoplankton biomass in the Southern Ocean are examined in terms of the timing of the bloom initiation, its amplitude, regional scale variability and the importance of the climatological seasonal cycle in explaining the overall variance. The seasonal cycle was consequently defined into four broad zonal regions; the subtropical zone (STZ), the transition zone (TZ), the Antarctic circumpolar zone (ACZ) and the marginal ice zone (MIZ). Defining the Southern Ocean according to the characteristics of its seasonal cycle provides a more dynamic understanding of ocean productivity based on underlying physical drivers rather than climatological biomass. The response of the biology to the underlying physics of the different seasonal zones resulted in an additional classification of four regions based on the extent of inter-annual seasonal phase locking and the magnitude of the integrated seasonal biomass. This regionalisation contributes towards an improved understanding of the regional differences in the sensitivity of the Southern Oceans ecosystem to climate forcing, potentially allowing more robust predictions of the effects of long term climate trends.

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A schematic summarising the response of phytoplankton biomass to the underlying physics of the different seasonal regimes. Regions in blue represent regions of low ( 0.4) (Region A, light blue) or low seasonal cycle reproducibility (R2  0.25 mgm−3) with either high seasonal cycle reproducibility (Region C, dark green) or low seasonal cycle reproducibility (Region D, light green). Mean (1998–2007) frontal positions are shown for the STF (red), the SAF (black), the PF (orange) and the SACCF (blue).

A schematic summarising the response of phytoplankton biomass to the underlying physics of the different seasonal regimes. Regions in blue represent regions of low (< 0.25 mgm−3) chlorophyll concentration with either high seasonal cycle reproducibility (R2 > 0.4) (Region A, light blue) or low seasonal cycle reproducibility (R2 < 0.4) (Region B, dark blue). Regions in green represent regions of high chlorophyll concentration ( > 0.25 mgm−3) with either high seasonal cycle reproducibility (Region C, dark green) or low seasonal cycle reproducibility (Region D, light green). Mean (1998–2007) frontal positions are shown for the STF (red), the SAF (black), the PF (orange) and the SACCF (blue).

Pollard, R.T., Salter, I.R.J., Lucas M., Moore, C.M., Mills, R.A., Statham, P.J., Allen, J.T., Bakker, D.C.E., Charette, M.A., Fielding, S., Thomalla S.J., Fones, G.R. et al.
Abstract

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon1, 2, 3. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified3. Here we report data from the CROZEX experiment4 in the Southern Ocean, which was conducted to test the hypothesis that the observed north–south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean5. The CROZEX sequestration efficiency6 (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600molmol-1 was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron7, but 77 times smaller than that of another bloom8 initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom6. The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

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Chlorophyll a images of Crozet region. a, Chlorophyll a in October for the whole of the Southern Ocean, showing location of Crozet. Colour indicates concentration as shown in b. b, Merged SeaWiFS/MODIS chlorophyll a image for the eight-day peak bloom period 23–30 October 2004. Solid and dashed lines show mean and eddy circulations, respectively13, with the sub-Antarctic Front (SAF, the northern boundary of the Antarctic Circumpolar Current) and the Agulhas Return Current (ARC) shown bold. Main sampling (1) and coring (N) sites are labelled. Thin white lines are the 2,000-m depth contour, with the main Crozet Islands (Iˆle de la Possession, I ˆ le de l’Est) seen at 46.5u S, 52u E.

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