Southern Ocean Primary Production Research Articles

Pathways and timescales of Southern Ocean hydrothermal iron and manganese transport

Scarcity of iron and manganese limits the efficiency of the biological carbon pump over large areas of the Southern Ocean. The importance of hydrothermal vents as a source of these micronutrients to the euphotic zone of the Southern Ocean is debated. Here we present full depth profiles of dissolved and total dissolvable trace metals in the remote eastern Pacific sector of the Southern Ocean (55–60° S, 89.1° W), providing evidence of enrichment of iron and manganese at depths of 2000–4000 m. These enhanced micronutrient concentrations were co-located with 3He enrichment, an indicator of hydrothermal fluid originating from ocean ridges. Modelled water trajectories revealed the understudied South East Pacific Rise and the Pacific Antarctic Ridge as likely source regions. Additionally, the trajectories demonstrate pathways for these Southern Ocean hydrothermal ridge-derived trace metals to reach the Southern Ocean surface mixed layer within two decades, potentially supporting a regular supply of micronutrients to fuel Southern Ocean primary production.

Winter entrainment drives the mixed layer supply of manganese in the Southern Ocean

AbstractDespite the subnanomolar dissolved manganese concentrations that can co‐limit Southern Ocean primary production, their physical supply mechanisms during winter, for biological consumption in spring and summer have not yet been explored. During austral winter and spring 2019, two cruises were conducted in the Atlantic sector of the Southern Ocean, to determine the distribution and surface water supply mechanisms of dissolved manganese in the upper water column. The supply mechanisms were used to calculate the total flux of dissolved manganese to productive surface waters and were compared to biological consumption estimates. Mean dissolved manganese concentrations in the upper water column (< 500 m) during winter and spring were comparably low (≤ 0.34 nmol kg−1; p > 0.05), with seasonal mixed layer reservoir sizes averaging 65.21 ± 12.93 and 21.64 ± 19.32 μmol m−2, respectively. Winter entrainment contributed 89.33–99.99% (average 97.26% ± 5.28%) of the total dissolved manganese flux, while diapycnal diffusion contributed 0.52–10.58% (average 4.92% ± 5.14%), was identified as the dominant mechanisms for transporting dissolved manganese into the mixed layer in the subantarctic zone, polar frontal zone, and antarctic zone. Here, the winter physical supply rates were higher than the estimated consumption rates during spring, meeting phytoplankton biological demands. Whereas in the subtropical zone, the supply rates were lower than the consumption rates, indicating the presence of additional supply mechanisms such as coastal upwelling, which may help to meet the biological demands in this region.

Sea‐Ice Impacts Inter‐Annual Variability of Phytoplankton Bloom Characteristics and Carbon Export in the Weddell Sea

AbstractThe Antarctic Marginal Ice Zone (MIZ) accounts for 15% of the Southern Ocean's primary production (PP), but limited data has hindered understanding of its variability and connection to carbon export. Using a combination of gliders, biogeochemical Argo floats and satellite observations in the northeast Weddell Sea, we show that years with more sea‐ice formation over winter are followed by more intense phytoplankton blooms (∼15% greater daily PP) and export to 100 m (∼50% higher daily carbon export) the following summer. However, the carbon export beyond the deepest winter mixed layer did not vary in proportion to PP, suggesting different drivers of carbon export at depth compared to surface waters. Furthermore, across the entire MIZ, the response of blooms to sea‐ice volume was spatially variable, indicating the need to consider spatial heterogeneity in the response of the biological carbon pump to future sea‐ice changes.

First report on biological iron uptake in the Antarctic sea-ice environment

Melting sea ice is a seasonal source of iron (Fe) to the Southern Ocean (SO), where Fe levels in surface waters are otherwise generally too low to support phytoplankton growth. However, the effectiveness of sea-ice Fe fertilization in stimulating SO primary production is unknown since no data exist on Fe uptake by microorganisms in the sea-ice environment. This study reports a unique dataset on Fe uptake rates, Fe-to-carbon (C) uptake ratio (Fe uptake normalized to C uptake) and Fe:C uptake rate (Fe uptake normalized to biomass) by in situ microbial communities inhabiting sea ice and the underlying seawater. Radioisotopes 55Fe and 14C were used in short-term uptake experiments during the 32-day Ice Station POLarstern (ISPOL) time series to evaluate the contributions of small (0.8–10 µm) and large (> 10 µm) microbes to Fe uptake. Overall, results show that over 90% of Fe was bound to the outside of the cells. Intracellular Fe (Feintra) uptake rates reached up to 68, 194, and 203 pmol Fe L−1d− 1 in under-ice seawater, bottom ice, and top ice, respectively. Inorganic carbon uptake ranged between 0.03 and 3.2 µmol C L−1 d−1, with the lowest rate observed in under-ice seawater. Importantly, between the start and end of ISPOL, we observed a 30-fold increase in Feintra normalized to carbon biomass in bottom sea ice. This trend was likely due to changes in the microbial community from a dominance of large diatoms at the start of the survey to small diatoms later in the season. As the Antarctic icescape and associated ecosystems are changing, this dataset will help inform the parameterisation of sea-ice biogeochemical and ecological models in ice-covered regions.

Multidecadal trend of increasing iron stress in Southern Ocean phytoplankton.

Southern Ocean primary productivity is principally controlled by adjustments in light and iron limitation, but the spatial and temporal determinants of iron availability, accessibility, and demand are poorly constrained, which hinders accurate long-term projections. We present a multidecadal record of phytoplankton photophysiology between 1996 and 2022 from historical in situ datasets collected by Biogeochemical Argo (BGC-Argo) floats and ship-based platforms. We find a significant multidecadal trend in irradiance-normalized nonphotochemical quenching due to increasing iron stress, with concomitant declines in regional net primary production. The observed trend of increasing iron stress results from changing Southern Ocean mixed-layer physics as well as complex biological and chemical feedback that is indicative of important ongoing changes to the Southern Ocean carbon cycle.

Sub‐Seasonal Forcing Drives Year‐To‐Year Variations of Southern Ocean Primary Productivity

AbstractPrimary productivity in the Southern Ocean plays a key role in global biogeochemical cycles. While much focus has been placed on phytoplankton production seasonality, non‐seasonal fluctuations exceed the amplitude of the seasonal cycle across large swaths of the Antarctic Circumpolar Current. This non‐seasonal variability comprises a broad range of timescales from sub‐seasonal (<3 months) to multi‐annual (>1 year), all of which can project onto the annual mean value. However, year‐to‐year variations of surface chlorophyll (SChl), a proxy for phytoplankton biomass, are typically attributed to ocean circulation changes associated with the Southern Annular Mode (SAM), which implicitly assumes that sub‐seasonal variability averages to near‐zero over long timescales. Here, we test this assumption by applying a timeseries decomposition method to satellite‐derived SChl in order to separate the low‐frequency and high‐frequency contributions to the non‐seasonal variability. We find that throughout most of the Southern Ocean, year‐to‐year SChl variations are dominated by the sub‐seasonal component, which is not strongly correlated with the SAM. The multi‐annual component, while correlated with the SAM, only accounts for about 10% of the total SChl variance. This suggests that changes in annual mean SChl are related to intermittent forcing at small scales, rather than low‐frequency climate variability, and thus do not remain correlated over large regions.

Iron biogeochemistry in sediments on the western continental shelf of the Antarctic Peninsula

We examined iron biogeochemistry in continental margin sediments along the western side of the Antarctic Peninsula (WAP) to explore the connections between iron redox cycling and other sediment biogeochemical processes, and to specifically examine the role of sediments as a source of iron to support Southern Ocean primary productivity. Total iron (FeT) in WAP sediments ranged from ∼4 to 6 wt% Fe and showed no consistent depth variations. Total iron oxides (FeOx) represented ∼20–30% of FeT in WAP surface sediments and was comprised of two types of materials: Feox1, which is thought to contain amorphous, highly reactive, and poorly crystalline iron oxides such as ferrihydrite and lepidocrocite, and Feox2, which is thought to contain magnetite and less reactive, more crystalline iron oxides such as goethite and hematite. Absolute and relative concentrations of FeOx declined with sediment depth, presumably due to reductive dissolution, and there was also a major depth change in the speciation of FeOx. Near the sediment surface ∼30% of FeOx was Feox1, while below ∼30 cm Feox1 was almost completely consumed and therefore here virtually all FeOx was Feox2. Consistent with these observations, a simple reaction-transport model applied to the depth distributions of Feox1 and Feox2 iron in WAP shelf sediments suggested a ∼20-fold difference in the relative reactivity of these different oxides towards reductive dissolution (Feox1 > Feox2). At several stations profiles of pore water dissolved iron and solid phase Feox1 can be explained by steady state iron redox cycling. In contrast, profiles at other stations showed evidence of non steady-state iron redox cycling. Seasonal changes in the sediment organic carbon rain rate at these sites may explain this latter behavior. Differences in the behavior of iron at different sites may be related to the interplay between changes in sediment redox conditions and their impact on bioturbation and iron redox cycling. Sediment iron sources to the water column include dissolved iron (∼Fe2+) benthic fluxes as well as sediment resuspension processes that mix iron oxides (i.e., Feox1) found in surface sediments into the bottom waters. The impact these benthic iron sources will have on surface water iron concentrations (and eventually primary production) will depend on transport processes that move this iron from deeper waters to surface waters. They will also depend on chemical transformations in the water column that affect iron bioavailability. While sediment processes (in general) appear to be an important source of iron to Southern Ocean surface waters, there is large uncertainty in their magnitude. However, the results presented here demonstrate that the suspended sediment flux of amorphous iron oxides needs to be considered when discussing sediment iron sources to Southern Ocean surface waters.

Springtime phytoplankton responses to light and iron availability along the western Antarctic Peninsula

AbstractLight and iron availability are intertwined in controlling Southern Ocean primary production because several photosynthetic proteins require iron. Changes in light and iron availability can also affect phytoplankton species composition, which impacts nutrient cycling, carbon drawdown, and food web structure. To investigate the interactive effects of light and iron on phytoplankton growth, photosynthesis, photoacclimation strategy, micronutrient stress‐induced protein expression, and species composition, we conducted five bioassay experiments during spring in waters along the western Antarctic Peninsula with four treatments: low light (LL) or high light (HL) combined with or without iron addition. This region has rarely been studied in spring. We found that light limits growth while iron does not, despite overall low iron concentrations. Our results demonstrate that phytoplankton were LL acclimated in situ but photosynthetically optimized for higher light than they were experiencing, likely due to a highly dynamic light regime. Expression patterns of micronutrient stress‐induced proteins were consistent with iron stress in off‐shelf regions, but remarkably this iron stress did not result in lower carbon fixation and growth rates. Notably, manganese drawdown was highest under elevated light, suggesting a possible role in managing HL, although high flavodoxin expression indicated that Phaeocystis antarctica may not have been manganese‐limited. Although light and iron treatments did not impact species composition, high methionine synthase indicated that diatoms could have experienced stress induced by low vitamin B12, potentially contributing to P. antarctica's general dominance throughout the experiments. Our results indicate that P. antarctica may be better adapted to spring conditions than diatoms.

Iron Incorporation From Seawater Into Antarctic Sea Ice: A Model Study

Sea ice acts as an iron (Fe) reservoir in the Southern Ocean (SO) where primary productivity is largely Fe limited. The mechanisms leading to Fe enrichment in sea ice result from the combination of poorly understood and largely unexplored physical and biological processes. We analyze the biogeochemical impacts of three plausible idealized formulations of dissolved Fe (DFe) incorporation into sea ice corresponding to (i) constant Fe concentration in sea ice, (ii) constant ocean‐ice Fe flux, and (iii) ocean‐ice Fe flux linearly varying with seawater Fe concentration in a global ocean‐sea‐ice‐biogeochemical model, focusing on the SO. The three formulations simulate different geographical distributions of DFe concentrations in sea ice. Iron in sea ice remains largely uncertain due to the limited number of spatial and seasonal observations, poorly constrained Fe sources and sinks, and significant uncertainties in simulated sea ice and hydrography. Despite these differences, the fertilization effect by sea ice on phytoplankton photosynthesis is qualitatively similar regardless of the formulation considered. Iron incorporation during sea‐ice formation, transport, and melt release, common to all formulations, dominates over differences in sea‐ice Fe concentrations. Formulating the Fe incorporation rate as proportional to seawater Fe concentrations gives the closest agreement to field observations. With this formulation, sediments work in synergy with Fe transport to fertilize the waters north of the continental shelf. Southern Ocean primary production and export production increase by 5–10% and 9–19%, respectively, when Fe incorporation into sea ice is considered, suggesting a moderate effect of Fe‐bearing sea ice on marine productivity.

Sensitivity of ocean biogeochemistry to the iron supply from the Antarctic Ice Sheet explored with a biogeochemical model

Abstract. Iron (Fe) delivery by the Antarctic Ice Sheet (AIS) through ice shelf and iceberg melting enhances primary productivity in the largely iron-limited Southern Ocean (SO). To explore this fertilization capacity, we implement a simple representation of the AIS iron source in the global ocean biogeochemical model NEMO-PISCES. We evaluate the response of Fe, surface chlorophyll, primary production, and carbon (C) export to the magnitude and hypothesized vertical distributions of the AIS Fe fluxes. Surface Fe and chlorophyll concentrations are increased up to 24 % and 12 %, respectively, over the whole SO. The AIS Fe delivery is found to have a relatively modest impact on SO primary production and C export, which are increased by 0.063±0.036 PgC yr−1 and 0.028±0.016, respectively. However, in highly fertilized areas, primary production and C export can be increased by up to 30 % and 42 %, respectively. Icebergs are predicted to have a much larger impact on Fe, surface chlorophyll, and primary productivity than ice shelves in the SO. The response of surface Fe and chlorophyll is maximum in the Atlantic sector, northeast of the tip of the Antarctic Peninsula, and along the East Antarctic coast. The iceberg Fe delivery below the mixed layer may, depending on its assumed vertical distribution, fuel a non-negligible subsurface reservoir of Fe. The AIS Fe supply is effective all year round. The seasonal variations of the iceberg Fe fluxes have regional impacts that are small for annual mean primary productivity and C export at the scale of the SO.

Physiology and iron modulate diverse responses of diatoms to a warming Southern Ocean

Climate change-mediated alteration of Southern Ocean primary productivity is projected to have biogeochemical ramifications regionally, and globally due to altered northward nutrient supply1,2. Laboratory manipulation studies that investigated the influence of the main drivers (CO2, light, nutrients, temperature and iron) on Southern Ocean diatoms revealed that temperature and iron exert major controls on growth under year 2100 conditions3,4. However, detailed physiological studies, targeting temperature and iron, are required to improve our mechanistic understanding of future diatom responses. Here, I show that thermal performance curves of bloom-forming polar species are more diverse than previously shown5, with the optimum temperature for growth (Topt) ranging from 5–16 °C (the annual temperature range is −1–8 °C). Furthermore, iron deficiency probably decreases polar diatom Topt and Tmax (the upper bound for growth), as recently revealed for macronutrients and temperate phytoplankton6. Together, this diversity of thermal performance curves and the physiological interplay between iron and temperature may alter the diatom community composition. Topt will be exceeded during 2100 summer low iron/warmer conditions, tipping some species close or beyond Tmax, but giving others a distinct physiological advantage. Future polar conditions will enhance primary productivity2–4, but will also probably cause floristic shifts, such that the biogeochemical roles and elemental stoichiometry of dominant diatom species will alter the polar biogeochemistry and northwards nutrient supply. Climate change will alter primary productivity in the Southern Ocean, and warming and iron limitation will influence the composition of diatoms in the region. Optimum growth temperatures are wider than expected, but limited iron will affect which species flourish.

The Biological Pump and Seasonal Variability of pCO2 in the Southern Ocean: Exploring the Role of Diatom Adaptation to Low Iron

AbstractIron is known to limit primary production in the Southern Ocean (SO). To cope with the lack of this micronutrient, diatoms, a dominant phytoplankton group in this oceanic region, have been shown in cultures to have developed an original adaptation strategy to maintain efficient growth rates despite very low cellular iron quotas, even in low light conditions. Using a global ocean biogeochemical model, we explored the consequences of this physiological adaptation for the biological pump and the seasonal variability of both surface chlorophyll concentrations and surface partial pressure of carbon dioxide (pCO2) in this key region for global climate. In the model, we implemented a low intracellular Fe:C requirement in the SO for diatoms uniquely. This results in an increase of 10% in the relative contribution of diatoms to total SO primary production. The biological pump is also strengthened, which increases the biological contribution to the seasonal evolution of pCO2 relative to the thermodynamic component. Therefore, the seasonal evolution of both surface chlorophyll and surface pCO2 is significantly impacted, with a marked improvement, in our model, in the SO polar zone compared to the observations. Our model study underscores the potentially important consequences that this adaptive physiological behavior of diatoms could have on marine biogeochemistry in the SO. It is thus critical to improve our understanding of the physiology of this key phytoplankton group, in particular in the SO.

Antarctic pack ice algal distribution: Floe‐scale spatial variability and predictability from physical parameters

AbstractAntarctic pack ice serves as habitat for microalgae which contribute to Southern Ocean primary production and serve as important food source for pelagic herbivores. Ice algal biomass is highly patchy and remains severely undersampled by classical methods such as spatially restricted ice coring surveys. Here we provide an unprecedented view of ice algal biomass distribution, mapped (as chlorophyll a) in a 100 m by 100 m area of a Weddell Sea pack ice floe, using under‐ice irradiance measurements taken with an instrumented remotely operated vehicle. We identified significant correlations (p < 0.001) between algal biomass and concomitant in situ surface measurements of snow depth, ice thickness, and estimated sea ice freeboard levels using a statistical model. The model's explanatory power (r2 = 0.30) indicates that these parameters alone may provide a first basis for spatial prediction of ice algal biomass, but parameterization of additional determinants is needed to inform more robust upscaling efforts.

A preliminary model of iron fertilisation by baleen whales and Antarctic krill in the Southern Ocean: Sensitivity of primary productivity estimates to parameter uncertainty

Large marine animals may play a crucial role in storing and recycling bioavailable iron in surface waters by consuming iron-rich prey and subsequent defecation of iron that is excess to their requirements. This biological recycling of iron could enhance primary productivity in iron-limited waters. However, quantifying the effects of marine animals on ocean primary productivity remains challenging because of a limited understanding of the key biogeochemical processes involved. In this paper, we develop a preliminary model that explores these uncertainties and examines the potential effects of historical populations of blue, fin and humpback whales, and the biomass of Antarctic krill required to support the whale populations, on primary productivity in the Southern Ocean.Our results suggest that, despite conservative estimates for key processes in our model, pre-exploitation populations of blue whales and, to a lesser extent fin and humpback whales, could have contributed to iron recycling, resulting in enhanced phytoplankton production in iron-limited Southern Ocean waters. Iron-rich defecation by un-exploited whale populations in the Southern Ocean, and the biomass Antarctic krill required to support them, could have resulted in a contribution to primary productivity of between 1.5×10−4 to 23.4gCm−2yr−1 (blue whales), 1.4×10−4 to 13.9gCm−2yr−1 (fin whales), and 2.4×10−5 to 1.7gCm−2yr−1 (humpback whales). However, only when all parameter estimates are at their upper limits does there appear to be this significant role for whales in enhancing primary productivity, and thus we need to assess the likelihood of these values arising.The high degree of uncertainty around the magnitude of these increases in primary productivity is mainly due to our limited quantitative understanding of key biogeochemical processes. To reduce uncertainty regarding the effect of whales on Southern Ocean primary productivity, future research will need to refine our understanding of five influential model parameters: iron content in krill; krill consumption rates by whales; persistence of whale faecal iron in the photic zone; bioavailability of this retained iron; and the carbon-to-iron ratio of phytoplankton.

Phytoplankton–bacterial interactions mediate micronutrient colimitation at the coastal Antarctic sea ice edge

Southern Ocean primary productivity plays a key role in global ocean biogeochemistry and climate. At the Southern Ocean sea ice edge in coastal McMurdo Sound, we observed simultaneous cobalamin and iron limitation of surface water phytoplankton communities in late Austral summer. Cobalamin is produced only by bacteria and archaea, suggesting phytoplankton-bacterial interactions must play a role in this limitation. To characterize these interactions and investigate the molecular basis of multiple nutrient limitation, we examined transitions in global gene expression over short time scales, induced by shifts in micronutrient availability. Diatoms, the dominant primary producers, exhibited transcriptional patterns indicative of co-occurring iron and cobalamin deprivation. The major contributor to cobalamin biosynthesis gene expression was a gammaproteobacterial population, Oceanospirillaceae ASP10-02a. This group also contributed significantly to metagenomic cobalamin biosynthesis gene abundance throughout Southern Ocean surface waters. Oceanospirillaceae ASP10-02a displayed elevated expression of organic matter acquisition and cell surface attachment-related genes, consistent with a mutualistic relationship in which they are dependent on phytoplankton growth to fuel cobalamin production. Separate bacterial groups, including Methylophaga, appeared to rely on phytoplankton for carbon and energy sources, but displayed gene expression patterns consistent with iron and cobalamin deprivation. This suggests they also compete with phytoplankton and are important cobalamin consumers. Expression patterns of siderophore- related genes offer evidence for bacterial influences on iron availability as well. The nature and degree of this episodic colimitation appear to be mediated by a series of phytoplankton-bacterial interactions in both positive and negative feedback loops.

Effects of whaling on the structure of the Southern Ocean food web: insights on the "krill surplus" from ecosystem modelling.

The aim of this study was to examine the ecological plausibility of the “krill surplus” hypothesis and the effects of whaling on the Southern Ocean food web using mass-balance ecosystem modelling. The depletion trajectory and unexploited biomass of each rorqual population in the Antarctic was reconstructed using yearly catch records and a set of species-specific surplus production models. The resulting estimates of the unexploited biomass of Antarctic rorquals were used to construct an Ecopath model of the Southern Ocean food web existing in 1900. The rorqual depletion trajectory was then used in an Ecosim scenario to drive rorqual biomasses and examine the “krill surplus” phenomenon and whaling effects on the food web in the years 1900–2008. An additional suite of Ecosim scenarios reflecting several hypothetical trends in Southern Ocean primary productivity were employed to examine the effect of bottom-up forcing on the documented krill biomass trend. The output of the Ecosim scenarios indicated that while the “krill surplus” hypothesis is a plausible explanation of the biomass trends observed in some penguin and pinniped species in the mid-20th century, the excess krill biomass was most likely eliminated by a rapid decline in primary productivity in the years 1975–1995. Our findings suggest that changes in physical conditions in the Southern Ocean during this time period could have eliminated the ecological effects of rorqual depletion, although the mechanism responsible is currently unknown. Furthermore, a decline in iron bioavailability due to rorqual depletion may have contributed to the rapid decline in overall Southern Ocean productivity during the last quarter of the 20th century. The results of this study underscore the need for further research on historical changes in the roles of top-down and bottom-up forcing in structuring the Southern Ocean food web.

Annual primary production in Antarctic sea ice during 2005–2006 from a sea ice state estimate

AbstractUsing the data‐bounded Sea Ice Ecosystem State (SIESTA) model, we estimate total Antarctic sea ice algal primary production to be 23.7 Tg C a−1 for the period July 2005–June 2006, of which 80% occurred in the bottom 0.2 m of ice. Simulated sea ice primary production would constitute 12% of total annual primary production in the Antarctic sea ice zone, and ∼1% of annual Southern Ocean primary production. Model sea ice algal growth was net nutrient limited, rather than light limited, for the vast majority of the sunlit season. The seasonal distribution of integrated ice algal biomass matches available observations. The vertical algal distribution was weighted toward the ice bottom compared to observations, indicating that interior ice algal communities may be under‐predicted in the model, and that nutrient delivery via gravity‐induced convection is not sufficient to sustain summertime algal biomass. Bottom ice algae were most productive in ice of 0.36 m thickness, whereas interior algal communities were most productive in ice of 1.10 m thickness. Sensitivity analyses that tested different atmospheric forcing inputs, sea ice parameterizations, and nutrient availability caused mean and regional shifts in sea ice state and ice algal production even when sea extent and motion was specified. The spatial heterogeneity of both ice state and algal production highlight the sensitivity of the sea ice ecosystem to physical perturbation, and demonstrate the importance of quality input data and appropriate parameterizations to models of sea ice and associated biology.

Antarctic ice sheet fertilises the Southern Ocean

Abstract. Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability of iron in surface waters, which is thought to exert a significant control upon atmospheric CO2 concentrations on glacial/interglacial timescales. The zone bordering the Antarctic Ice Sheet exhibits high PP and seasonal plankton blooms in response to light and variations in iron availability. The sources of iron stimulating elevated SO PP are in debate. Established contributors include dust, coastal sediments/upwelling, icebergs and sea ice. Subglacial meltwater exported at the ice margin is a more recent suggestion, arising from intense iron cycling beneath the ice sheet. Icebergs and subglacial meltwater may supply a large amount of bioavailable iron to the SO, estimated in this study at 0.07–0.2 Tg yr−1. Here we apply the MIT global ocean model (Follows et al., 2007) to determine the potential impact of this level of iron export from the ice sheet upon SO PP. The export of iron from the ice sheet raises modelled SO PP by up to 40%, and provides one plausible explanation for seasonally very high in situ measurements of PP in the near-coastal zone. The impact on SO PP is greatest in coastal regions, which are also areas of high measured marine PP. These results suggest that the export of Antarctic runoff and icebergs may have an important impact on SO PP and should be included in future biogeochemical modelling.

Surface-water iron supplies in the Southern Ocean sustained by deep winter mixing

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. Low levels of iron limit primary productivity across much of the Southern Ocean. Measurements of dissolved iron levels combined with hydrographic data suggest that much of the iron in the surface waters of the Southern Ocean is supplied by deep mixing during winter.

Photoprotection in Southern Ocean phytoplankton: missing key to low primary productivity

AbstractThe Southern Ocean is considered a zone of high nutrients and low chlorophyll (HNLC). Despite the year round availability of nutrients, phytoplankton biomass remains low. Key modes of control are now thought to be light, grazing, iron and supply of silicic acid^1^. Physiological photoprotective mechanisms used by phytoplankton to regulate photosynthesis versus rapid light fluctuations have not been considered important. In diatoms and haptophytes, which are the major component of phytoplankton biodiversity in the Southern Ocean, the one step de-epoxidation of diadinoxanthin (Dd) into diatoxanthin (Dt) of the Dd-cycle constitutes the main photoprotective mechanism^2,3^. Here we show that Southern Ocean diatoms and haptophytes have higher concentrations of cellular Dd-cycle pigments than other oceanic regions. The Dd-cycle is activated under low irradiance conditions and the pool of photoprotective pigments increases under intermittent light conditions. The high cellular concentration of Dd-cycle pigments and our observations of an increase of the pool size of Dd-cycle pigments towards the surface implies that the Dd-cycle pigments bind to light harvesting proteins other than with fucoxanthin and chlorophyll c. Within photosystem II, enrichment of Dd and the physiological acclimation to light changes constrains photosynthetic activity, efficiency and potentially growth rates^3-7^. These results raise important questions about the role of photoprotective mechanisms in limiting Southern Ocean primary productivity and how these mechanisms will respond to predicted climate change impacts^8^.