Dissolved Iron

A new technique for the separation and preconcentration of dissolved Fe(III) from the ligandrich aqueous system is proposed. A solid phase extraction (SPE) system with an immobilized macrocyclic material, commonly known as molecular recognition technology (MRT) gel and available commercially, was used. Synthetic Fe(III) solution in aqueous matrices spiked with a 100-fold concentration of EDTA was used. Dissolved iron that was 'captured' by the MRT gel was eluted using hydrochloric acid and subsequently determined by graphite furnace atomic absorption spectrometry. The effect of different variables, such as pH, reagent concentration, flow rate and interfering ions, on the recovery of analyte was investigated. Quantitative maximum separation (~100%) of the dissolved Fe(III) from synthetic aqueous solutions at a natural pH range was observed at a flow rate of 0.2 mL min -1 . The extraction efficiency of the MRT gel is largely unaltered by the coexisting ions commonly found in natural water. When compared with different SPE materials, the separation performance of MRT gel is also much higher.

The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II). Here we deploy flow injection analysis to measure in-situ Fe(II) concentrations during a series of mesocosm/microcosm experiments in coastal environments in addition to the decay rate of this Fe(II) when moved into the dark. During 5 mesocosm/microcosm experiments in Svalbard and Patagonia, where dissolved (0.2 µm) Fe and Fe(II) were quantified simultaneously, Fe(II) constituted 24-65% of DFe suggesting that Fe(II) was a large fraction of the DFe pool. When this Fe(II) was allowed to decay in the dark, the vast majority of measured oxidation rate constants were retarded relative to calculated constants derived from ambient temperature, salinity, pH and dissolved O 2. The oxidation rates of Fe(II) spikes added to Atlantic seawater more closely matched calculated rate constants. The difference between observed and theoretical decay rates in Svalbard and Patagonia was most pronounced at Fe(II) concentrations <2 nM and attributed to a stabilising effect of cellular exudates upon Fe(II). This enhanced stability of Fe(II) under post-bloom conditions, and the existence of such a high fraction of DFe as Fe(II), challenges the assumption that DFe speciation is dominated by ligand bound-Fe(III) species.

The hlgh biological productivity of the Hurnboldt Current System (HCS) off Chile supports an annual fish catch of over 7 mill~on t. The area is also important biogeochemically, because the outgassing of recently upwelled water is modulated by contrasting degrees of biolopcal activity. However, very few field measurements of primary production and planktonic respiration have been undertaken within the Eastern Boundary Current (EBC) system off Chile. In this study an estimate of primary production (PP) and surface planktonic commun~ty respiration is presented from several research cruises in the HCS and adjacent oceanlc areas. The highest production levels were found near the coast correlating closely with known upwelling areas. Both gross primary production (GPP) and community respiration (CR) showed important spatial and temporal fluctuations. The highest water column integrated GPP was measured in the southern and central fishing area (19.9 g C m-2 d-l) and off the Antofagasta upwelling ecosystem (9.3 g C m-' d-'). The range of GPP agrees well with values reported for Peni (0.05 to ll.? g C m'2 d"). The Coquimbo upwehng system, despite being an area of persistent upwelling, showed lower production and community respiration values than the Antofagasta and Concepcion Shelf areas.The lowest surface PP values were measured within the oceanic region adjacent to the northern coastal upwelling zones (0.8 * 0.5 pg C 1-' h-'), though slightly enhanced biological production was found within the Nazca Ridge (1.5 * 2.1 pg C l-' h-') that separates the Chilean and Peruvian deep basins.

Iron (Fe) is an essential micronutrient for phytoplankton growth, but its scarcity in seawater limits primary productivity across much of the ocean. Most dissolved Fe (DFe) in seawater is complexed with Fe-binding organic ligands, a poorly constrained fraction of dissolved organic matter (DOM), which increase Fe residence time and impact Fe bioavailability. Here, we present the conditional concentration (LFe) and binding-strength (log K) of Fe-binding ligands in the Western Tropical South Pacific (WTSP) Ocean during the GEOTRACES TONGA cruise (GPpr14). The transect crossed the Lau basin, a region subject to shallow hydrothermal Fe inputs that fuel intense diazotrophic activity, the oligotrophic South Pacific gyre, and the Melanesian basin. Organic speciation was analyzed by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLEAdCSV)
using salicylaldoxime at 25 μM. We found a high mean LFe of 5.2 ± 1.2
nMeqFe (n = 103) across the entire transect, predominantly consisting of
intermediate strength L2 ligands (84%; mean log K of 11.6 ± 0.4),
consistent with humic-like substances. DFe correlated with the humic-like
component of the fluorescent DOM (HS-like FDOM), yet the electroactive Fe binding humic-like substances (LFeHS) accounted for only 20 ± 13% of LFe in the mixed layer and 8 ± 6% in deep waters. Ligands were in large excess compared to DFe (mean excess ligand eLFe = 4.6 ± 1.1 nMeqFe), suggesting poor stabilization of DFe inputs. High LFe (up to 9 nMeqFe) in samples close to hydrothermal sites could be due to detoxification strategies from plankton communities toward hydrothermally-fueled toxic trace metals other than Fe, with an apparent dilution of the DOM from the Lau basin into neighboring regions. We also observed a different peak potential of the Fe salicylaldoxime complex detected by CLEAdCSV between the Lau and Melanesian basins, and between surface and deep waters. To our knowledge, this change in potential has not previously been

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The oligotrophic southeastern Mediterranean Sea (SEMS) is frequently exposed to desert-dust deposition which supplies nutrients, trace metals and a wide array of viable airborne microorganisms. In this study, we experimentally examined the impact of aerosol addition, collected during an intense dust storm event in early September 2015, on the biomass and activity of pico-phytoplankton and heterotrophic bacterial populations at the sea-surface micro layer (SML) relative to the sub surface layer (SSL). Aerosol (1.5 mg L −1) was added to SML and SSL water samples in microcosms (4.5 L) and the water was frequently sampled over a period of 48 h. While the aerosol amendment triggered a moderate 1.5-2-fold increase in primary production in both the SML and the SSL, bacterial production increased by ∼3 and ∼7-folds in the SSL and SML, respectively. Concurrently, the abundance and flow-cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells showed a significant increase in the %HNA, in both SML and SSL samples following aerosol amendment. This shift in nucleic acid content took place at a much faster rate in the SML, suggesting a more active heterotrophic community. These changes were likely a result of higher rates of carbon utilizations in the SML following the dust addition, as assessed by a selected hydrocarbons and saccharides analysis. Additionally, a high absorption rate of hydrocarbons by the aerosol particles was measured following the additions, leaving less than 10% of these molecules available for potential heterotrophic microbial utilization. Our results suggest that the heterotrophic microbial community inhabiting the SML is more efficient in utilizing aerosol associated constituents than the community in the SSL.

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The distribution of dissolved iron (Fe), total organic Fe-binding ligands, and siderophores were measured between the surface and 400 m at Station ALOHA, a long term ecological study site in the North Pacific Subtropical Gyre. Dissolved Fe concentrations were low throughout the water column and strong organic Fe-binding ligands exceeded dissolved Fe at all depths; varying from 0.9 nmol L −1 in the surface to 1.6 nmol L −1 below 150 m. Although Fe does not appear to limit microbial production, we nevertheless found siderophores at nearly all depths, indicating some populations of microbes were responding to Fe stress. Ferrioxamine siderophores were most abundant in the upper water column, with concentrations between 0.1 and 2 pmol L −1 , while a suite of amphibactins were found below 200 m with concentrations between 0.8 and 11 pmol L −1. The distinct vertical distribution of ferrioxamines and amphibactins may indicate disparate strategies for acquiring Fe from dust in the upper water column and recycled organic matter in the lower water column. Amphibactins were found to have conditional stability constants (log K cond FeL 1 ,Fe ′) ranging from 12.0 to 12.5, while ferrioxamines had much stronger conditional stability constants ranging from 14.0 to 14.4, within the range of observed L 1 ligands by voltammetry. We used our data to calculate equilibrium Fe speciation at Station ALOHA to compare the relative concentration of inorganic and siderophore complexed Fe. The results indicate that the concentration of Fe bound to siderophores was up to two orders of magnitude higher than inorganic Fe, suggesting that even if less bioavailable, siderophores were nevertheless a viable pathway for Fe acquisition by microbes at our study site. Finally, we observed rapid production of ferrioxamine E by particle-associated bacteria during incubation of freshly collected sinking organic matter. Fe-limitation may therefore be a factor in regulating carbon metabolism and nutrient regeneration in the mesopelagic.

Kumar et al. (2010) 8.47 83 112.78 3.050% 3.442 Kumar et al. (2010) 4.73 83 159.00 5.220% 8.300 Kumar et al. (2010) 6.4 79.31 91.41 3.450% 3.156 Kumar et al. (2010) 65 255.08 0.030% 0.075 Bikkina et al. (2012) 22 67 152.53 0.030% 0.045 Bikkina et al. (2012) 70 296.40 0.190% 0.558 Bikkina et al. (2012) 20 72 432.04 0.070% 0.295 Bikkina et al. (2012) 15.4 73.8 371.86 0.020% 0.087 Bikkina et al. (2012) 30.65 138.68 9.72 NaN NaN Buck et al. (2006)

This work reports on the current status of the global modeling of iron (Fe) deposition fluxes and atmospheric concentrations and the analyses of the differences between models, as well as between models and observations. A total of four global 3-D chemistry transport (CTMs) and general circulation (GCMs) models participated in this intercomparison, in the framework of the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) Working Group 38, "The Atmospheric Input of Chemicals to the Ocean". The global total Fe (TFe) emission strength in the models is equal to ∼ 72 Tg Fe yr -1 (38-134 Tg Fe yr -1 ) from mineral dust sources and around 2.1 Tg Fe yr -1 (1.8-2.7 Tg Fe yr -1 ) from combustion processes (the sum of anthropogenic combustion/biomass burning and wildfires). The mean global labile Fe (LFe) source strength in the models, considering both the primary emissions and the atmospheric processing, is calculated to be 0.7 (±0.3) Tg Fe yr -1 , accounting for both mineral dust and combustion aerosols. The mean global deposition fluxes into the global ocean are estimated to be in the range of 10-30 and 0.2-0.4 Tg Fe yr -1 for TFe and LFe, respectively, which roughly corresponds to a respective 15 and 0.3 Tg Fe yr -1 for the multi-model ensemble model mean.

Melt ponds are a prominent feature of Arctic sea ice during the summer and play a role in the complex interface between the atmosphere, cryosphere and surface ocean. During melt pond formation and development, micronutrient and contaminant trace elements (TEs) from seasonally accumulated atmospheric deposition are mixed with entrained sedimentary and marine-derived material before being released to the surface ocean during sea ice melting. Here we present particulate and size-fractionated dissolved (truly soluble and colloidal) TE data from five melt ponds sampled in late summer 2015, during the US Arctic GEOTRACES (GN01) cruise. Analyses of salinity, δ 18 O, and 7 Be indicate variable contributions to the melt ponds from snowmelt, melting sea ice, and surface seawater. Our data highlight the complex TE biogeochemistry of late summer Arctic melt ponds and the variable importance of different sources for specific TEs. Dissolved TE concentrations indicate a strong influence from seawater intrusion for V, Ni, Cu, Cd, and Ba. Ultrafiltration methods reveal dissolved Fe, Zn, and Pb to be mostly colloidal (0.003-0.2 m), while Mn, Co, Ni, Cu, and Cd are dominated by a truly soluble (<0.003 m) fraction. Isotopically light dissolved Fe in some melt ponds suggests that photochemical and/or biologically driven redox cycling also takes place. Comparisons of particulate TE/Al ratios to mean crustal values indicate influences from lithogenic sources, including natural aerosols and/or sedimentary material, with significant enrichments for some elements, including Ni, Cu, Zn, Cd and Pb, that may result from anthropogenic aerosols, biogenic material, and/or in situ scavenging of dissolved TEs. Our results indicate that melt ponds represent a transitional environment in which some atmospherically-derived TEs undergo physical and/or chemical changes before their release to the surface ocean. As a result, the ongoing changes in sea ice areal extent, thickness, and melt season length are likely to influence the bioavailability of atmospheric TE input to the surface Arctic Ocean, with material released from snow and sea ice via melt ponds earlier in the summer and with more extensive direct deposition to the ocean surface.

The oligotrophic southeastern Mediterranean Sea (SEMS) is frequently exposed to desert-dust deposition which supplies nutrients, trace metals and a wide array of viable airborne microorganisms. In this study, we experimentally examined the impact of aerosol addition, collected during an intense dust storm event in early September 2015, on the biomass and activity of pico-phytoplankton and heterotrophic bacterial populations at the sea-surface micro layer (SML) relative to the sub surface layer (SSL). Aerosol (1.5 mg L −1) was added to SML and SSL water samples in microcosms (4.5 L) and the water was frequently sampled over a period of 48 h. While the aerosol amendment triggered a moderate 1.5-2-fold increase in primary production in both the SML and the SSL, bacterial production increased by ∼3 and ∼7-folds in the SSL and SML, respectively. Concurrently, the abundance and flow-cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells showed a significant increase in the %HNA, in both SML and SSL samples following aerosol amendment. This shift in nucleic acid content took place at a much faster rate in the SML, suggesting a more active heterotrophic community. These changes were likely a result of higher rates of carbon utilizations in the SML following the dust addition, as assessed by a selected hydrocarbons and saccharides analysis. Additionally, a high absorption rate of hydrocarbons by the aerosol particles was measured following the additions, leaving less than 10% of these molecules available for potential heterotrophic microbial utilization. Our results suggest that the heterotrophic microbial community inhabiting the SML is more efficient in utilizing aerosol associated constituents than the community in the SSL.

The high N 2 fixation rate observed in the Lau Basin of the western tropical South Pacific Ocean (WTSP) is fueled by iron (Fe) released from shallow hydrothermal systems. Understanding Fe bioavailability is crucial but the controls on the stability and bioavailability of hydrothermal Fe inputs are still poorly understood. Here, we provide new data on the spatial and vertical distribution of the soluble ubiquitous humic-like ligands (L FeHS) and their associated dissolved Fe (DFe) in the WTSP, including in samples near hydrothermal vents. Our data show that L FeHS are heterogenous ligands with binding sites of both strong and intermediate strengths. These ligands are primarily produced in surface waters and partially mineralized in mesopelagic waters. A substantial fraction of DFe was complexed by L FeHS (mean~30%). The DFe complexed by L FeHS is likely bioavailable to phytoplankton and L FeHS stabilized Fe released by the mineralization of sinking biomass. However, unsaturation of L FeHS by Fe suggest that part of DFe is not available for complexation with L FeHS. Possible reasons are competition between DFe and other metals, such as dissolved copper, or the inability of L FeHS to access colloidal DFe. The study of two volcanic sites indicates that L FeHS were not produced in these hydrothermal systems. At the active site (DFe~50 nmol L-1), L FeHS can only partially solubilize the hydrothermal DFe released in this area (1~5.5% of the total DFe). We performed controlled laboratory experiments which show that the observed low solubilization yield result from the inability of L FeHS to solubilize aged Fe oxyhydroxides (FeOx-a kinetically mediated process) and to form stable complexes with Fe(II) species. Our study provides new understanding of the role of L FeHS on the bioavailability and stabilization of hydrothermal DFe.

This work reports on the current status of the global modeling of iron (Fe) deposition fluxes and atmospheric concentrations and the analyses of the differences between models, as well as between models and observations. A total of four global 3-D chemistry transport (CTMs) and general circulation (GCMs) models participated in this intercomparison, in the framework of the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) Working Group 38, "The Atmospheric Input of Chemicals to the Ocean". The global total Fe (TFe) emission strength in the models is equal to ∼ 72 Tg Fe yr −1 (38-134 Tg Fe yr −1) from mineral dust sources and around 2.1 Tg Fe yr −1 (1.8-2.7 Tg Fe yr −1) from combustion processes (the sum of anthropogenic combustion/biomass burning and wildfires). The mean global labile Fe (LFe) source strength in the models, considering both the primary emissions and the atmospheric processing, is calculated to be 0.7 (±0.3) Tg Fe yr −1 , accounting for both mineral dust and combustion aerosols. The mean global deposition fluxes into the global ocean are estimated to be in the range of 10-30 and 0.2-0.4 Tg Fe yr −1 for TFe and LFe, respectively, which roughly corresponds to a respective 15 and 0.3 Tg Fe yr −1 for the multi-model ensemble model mean.

The atmospheric cycle of phosphorus (P) is here parameterized in a global 3-D chemistry-transport model, taking into account primary emissions of total P (TP) and dissolved P (DP) associated with mineral dust, combustion particles of natural and anthropogenic sources, bioaerosols, sea-spray and volcanic aerosols. Global TP emissions are calculated to amount roughly 1.33 Tg-P yr-1 with mineral sources (about 1.10 Tg-P yr-1) contributing more than 80% to these emissions. Additionally, under acidic atmospheric conditions, for the present study we take into account the P mobilization from 20 mineral dust, that is calculated to contribute about one third (0.14 Tg-P yr-1) to the global DP atmospheric source. The calculated global annual DP deposition flux equals to 0.43 Tg-P yr-1 (about 40% enters the ocean), and shows a strong spatial and temporal variability. Considering that all bioaerosol P is bioavailable (BP) and accounting for all other sources of DP, a flux of 0.16 Tg-P yr-1 BP to the ocean is derived. Present day simulations of atmospheric P aerosol concentrations and deposition fluxes are satisfactory compared with available observations, indicating however a 50% uncertainty of current 25 knowledge on primary and secondary sources of P that drive its atmospheric cycle. Sensitivity simulations using preindustrial (year 1850) and future (2100) anthropogenic and biomass burning emission scenarios, showed a present-day increase of 75% in the dissolution flux of P present in dust aerosol compared to the 1850 dissolution flux due to increasing atmospheric acidity over the last 150 years. Future reductions in air pollutants, due to the implementation of air-quality regulations, are expected to decrease P mobilization flux by about 30% for the year 2100 compared to the present-day. A 30 striking result is that more than 50% of the BP deposition flux to the ocean originates from biological particle and this contribution is found to maximize in summer when atmospheric deposition impact on the marine ecosystem is the highest due to ocean stratification. These findings reveal the largely unknown but important role of terrestrial bioaerosols as suppliers of bioavailable P to the oceanwith very important implications for past and future responses of ecosystems to global change. Therefore, our study provides new insights to the atmospheric P cycle by demonstrating that bioaerosols are 35 as important carriers of bioavailable P as dust aerosol, that was up to now considered as the only large source of DP external to the open ocean.

The global tropospheric budget of gaseous and particulate non-methane organic matter (OM) is reexamined to provide a holistic view of the role that OM plays in transporting the essential nutrients nitrogen and phosphorus to the ocean. A global 3-dimensional chemistry-transport model was used to construct the first global picture of atmospheric transport and deposition of the organic nitrogen (ON) and organic phosphorus (OP) that are associated with OM, focusing on the soluble fractions of these nutrients. Model simulations agree with observations within an order of magnitude. Depending on location, the observed water soluble ON fraction ranges from $3% to 90% (median of $35%) of total soluble N in rainwater; soluble OP ranges from $20-83% (median of $35%) of total soluble phosphorus. The simulations suggest that the global ON cycle has a strong anthropogenic component with $45% of the overall atmospheric source (primary and secondary) associated with anthropogenic activities. In contrast, only 10% of atmospheric OP is emitted from human activities. The model-derived present-day soluble ON and OP deposition to the global ocean is estimated to be $16 Tg-N/yr and $0.35 Tg-P/yr respectively with an order of magnitude uncertainty. Of these amounts $40% and $6%, respectively, are associated with anthropogenic activities, and 33% and 90% are recycled oceanic materials. Therefore, anthropogenic emissions are having a greater impact on the ON cycle than the OP cycle; consequently increasing emissions may increase P-limitation in the oligotrophic regions of the world's ocean that rely on atmospheric deposition as an important nutrient source.

Atmospheric deposition of labile iron (Fe) to the ocean has been suggested to modulate primary ocean productivity and thus indirectly affect the climate. A key process contributing to atmospheric sources of labile Fe is associated with atmospheric acidity, which leads to Fe transformation from insoluble to soluble forms. Significant progress has been made in our understanding of atmospheric inputs of labile Fe from natural and anthropogenic sources to the oceans. However, there are still large uncertainties regarding the relative importance of different sources of Fe and effects of atmospheric processing on the bioavailability of the delivered Fe. Here, we investigate the effects of atmospheric processing on Fe solubility and contribution of different sources of Fe to labile Fe in the atmosphere. We compiled Fe loading and solubility in aerosols from four atmospheric chemistry transport models and a number of field measurements. Fe-containing aerosols from combustion sources are cha...

This manuscript describes the development of a detailed multiphase chemistry scheme within EC-Earth with the worthy aim to realistically represent the atmospheric iron cycle. This can improve understanding of marine biogeochemistry perturbations, and thus carbon and nitrogen cycles, under past and future climate scenarios. The model contains iron from both natural and anthropogenic sources and contains schemes for the dissolution of insoluble iron to a soluble form which account kinetically for the solution's acidity, oxalic acid, and irradiation. The chemistry required is discussed in great detail and results from two sets of simulation are compared to observations of oxalate, sulphate, and iron aerosol. The analysis is very good, and results are interesting given that representing the full atmospheric aerosol iron cycle within a global Earth System Model is still a new development. In my opinion this manuscript is very well written and in particular the description of the chemistry was very well made. I feel this manuscript is thus suitable for publication in Geoscientific Model Development after addressing a series of, mainly minor, comments.

Understanding how multiphase processes affect the iron-containing aerosol cycle is key to predicting ocean biogeochemistry changes and hence the feedback effects on climate. For this work, the EC-Earth Earth system model in its climate-chemistry configuration is used to simulate the global atmospheric oxalate (OXL), sulfate (SO 2− 4), and iron (Fe) cycles after incorporating a comprehensive representation of the multiphase chemistry in cloud droplets and aerosol water. The model considers a detailed gasphase chemistry scheme, all major aerosol components, and the partitioning of gases in aerosol and atmospheric water phases. The dissolution of Fe-containing aerosols accounts kinetically for the solution's acidity, oxalic acid, and irradiation. Aerosol acidity is explicitly calculated in the model, both for accumulation and coarse modes, accounting for thermodynamic processes involving inorganic and crustal species from sea salt and dust. Simulations for present-day conditions (2000-2014) have been carried out with both EC-Earth and the atmospheric composition component of the model in standalone mode driven by meteorological fields from ECMWF's ERA-Interim reanalysis. The calculated global budgets are presented and the links between the (1) aqueous-phase processes, (2) aerosol dissolution, and (3) atmospheric composition are demonstrated and quantified. The model results are supported by comparison to available observations. We obtain an average global OXL net chemical production of 12.615 ± 0.064 Tg yr −1 in EC-Earth, with glyoxal being by far the most important precursor of oxalic acid. In comparison to the ERA-Interim simulation, differences in atmospheric dynamics and the simulated weaker oxidizing capacity in EC-Earth overall result in a ∼ 30 % lower OXL source. On the other hand, the more explicit representation of the aqueous-phase chemistry in EC-Earth compared to the previous versions of the model leads to an overall ∼ 20 % higher Published by Copernicus Publications on behalf of the European Geosciences Union. 3080 S. Myriokefalitakis et al.: Multiphase processes in the EC-Earth model sulfate production, but this is still well correlated with atmospheric observations. The total Fe dissolution rate in EC-Earth is calculated at 0.806 ± 0.014 Tg yr −1 and is added to the primary dissolved Fe (DFe) sources from dust and combustion aerosols in the model (0.072 ± 0.001 Tg yr −1). The simulated DFe concentrations show a satisfactory comparison with available observations, indicating an atmospheric burden of ∼0.007 Tg, resulting in an overall atmospheric deposition flux into the global ocean of 0.376 ± 0.005 Tg yr −1 , which is well within the range reported in the literature. All in all, this work is a first step towards the development of EC-Earth into an Earth system model with fully interactive bioavailable atmospheric Fe inputs to the marine biogeochemistry component of the model.

Subantarctic Southern Ocean surface waters in the austral summer and autumn are characterized by high concentrations of nitrate and phosphate but low concentrations of dissolved iron (Fe,-0.05 nM) and silicic acid (Si, <1 gM). During the Subantarctic Zone AU9706 cruise in March 1998 we investigated the relative importance of Fe and Si in controlling phytoplankton growth and species composition at a station within the subantarctic water mass (46.8øS, 142øE) using shipboard bottle incubation experiments. Treatments included unamended controls; 1.9 nM added iron (+Fe); 9 [tM added silicic acid (+Si); and 1.9 nM added iron plus 9 [tM added silicic acid (+Fe+Si). We followed a detailed set of biological and biogeochemical parameters over 8 days. Fe added alone clearly increased community growth rates and nitrate drawdown and altered algal community composition relative to control treatments. Surprisingly, small, lightly silicified pennate diatoms grew when Fe was added either with or without Si, despite the extremely low ambient silicic acid concentrations. Pigment analyses suggest that lightly silicified chrysophytes (type 4 haptophytes) may have preferentially responded to Si added either with or without Fe. However, for many of the parameters measured the +Fe+Si treatments showed large increases relative to both the +Fe and +Si treatments. Our results suggest that iron is the proximate limiting nutrient for chlorophyll production, photosynthetic efficiency, nitrate drawdown, and diatom growth, but that Si also exerts considerable control over algal growth and species composition. Both nutrients together are needed to elicit a maximum growth response, suggesting that both Fe and Si play important roles in structuring the subantarctic phytoplankton community.

Trace metals (TMs) are important to the functioning of marine ecosystems, with a range of TMs required as micronutrients by phytoplankton and serving as essential cofactors in metalloenzymes (Sunda, 1989, 2012). Iron (Fe), cobalt (Co), and manganese (Mn) are potentially (co-)limiting oceanic primary production (Browning

Air-water CO 2 fluxes were up-scaled to take into account the latitudinal and ecosystem diversity of the coastal ocean, based on an exhaustive literature survey. Marginal seas at high and temperate latitudes act as sinks of CO 2 from the atmosphere, in contrast to subtropical and tropical marginal seas that act as sources of CO 2 to the atmosphere. Overall, marginal seas act as a strong sink of CO 2 of about À0.45 Pg C yr À1. This sink could be almost fully compensated by the emission of CO 2 from the ensemble of near-shore coastal ecosystems of about 0.40 Pg C yr À1. Although this value is subject to large uncertainty, it stresses the importance of the diversity of ecosystems, in particular near-shore systems, when integrating CO 2 fluxes at global scale in the coastal ocean.

Metal partitioning between the dissolved and particulate phases is still poorly constrained within the early mixing of hydrothermal fluids and deep seawater. In this study, in situ filtration has been used to collect early buoyant plume fluids. This has provided the unique opportunity to reassess precisely metal partitioning along the mixing gradient by limiting chemical exchange processes between the dissolved (<0.45 μm) and particulate (>0.45 μm) phases during sampling. We report on the partitioning of three major metals (Fe, Cu, Zn) in the early buoyant plume of six black and clear smokers from the Lucky Strike hydrothermal field (37°N, MAR; EMSO-Azores deep sea observatory). We show that chemical changes are limited in the warmest part of the plume [50-150°C, dMn > 40 μM; dilution factor (DF) of ~1-10 by NADW] as metal partitioning displays a chemical signature similar to the end-member one. However, as the dilution ratio between the hydrothermal fluid and North-Atlantic Deep-water (NADW) increases (4-50°C, dMn < 40 μM; DF of 10-100 by NADW), metal partitioning is affected by different precipitation and oxidation processes. Molar ratios normalized to Fe in the particles highlight the onset of Fe oxides formation, the precipitation of barite and the decreasing contribution of sulfide minerals (mainly Cu-Fe sulfides and sphalerite/wurtzite) along with fluid dilution. Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Here we report on the seasonal productivity cycle at a fixed station in the Puyuhuapi Channel (44ºS, 73ºW), Chilean Patagonia. The analysis of in situ water column data and longer-term records of satellitederived surface ocean color (Chl-a) highlighted two contrasting seasons. A more productive period occurred between August and April, where depth-integrated gross primary production (GPP) estimates ranged from 0.1 to 2.9 g C m-2 d-1 , and a shorter, less-productive season lasted from May to July with GPP ranging from 0.03 to 0.3 g C m-2 d-1. Diatoms of the genera Pseudo-nitzschia, Skeletonema and Chaetoceros dominated the phytoplankton, and showed a pronounced seasonality greatly influenced by prevailing environmental conditions. Warmer waters were associated with high concentrations of Pseudo-nitzschia spp., while high abundances of Skeletonema spp. and Chaetoceros spp. were associated with fresher silicate-rich waters. A marked Skeletonema spp. bloom characterized the onset of the productive season in August 2008, and with some exceptions, the highest levels of GPP (1.5 to 2.9 g C m-2 d-1) were measured when Skeletonema dominated the phytoplankton community. Reduced production (low GPP) was observed during periods of increased discharge from the Cisnes River, whereas the "spring" diatom bloom (or late winter bloom in Puyuhuapi Channel) observed during August 2008 coincided with a ~50% drop in the freshwater discharge. We therefore suggest that periods of intensive freshwater Input, that increase the silicic acid concentrations in the upper layers, provide ideal growing conditions for diatoms as the freshwater flow subsequently recedes. Principal component analysis (PCA) suggests that a decrease in salinity, increase in silicic acid concentration, and growth of Skeletonema spp. and Chaetoceros spp., are probably key factors in the annual cycle of GPP in Puyuhuapi Channel.

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Metal partitioning between the dissolved and particulate phases is still poorly constrained within the early mixing of hydrothermal fluids and deep seawater. In this study, in situ filtration has been used to collect early buoyant plume fluids. This has provided the unique opportunity to reassess precisely metal partitioning along the mixing gradient by limiting chemical exchange processes between the dissolved (<0.45 μm) and particulate (>0.45 μm) phases during sampling. We report on the partitioning of three major metals (Fe, Cu, Zn) in the early buoyant plume of six black and clear smokers from the Lucky Strike hydrothermal field (37°N, MAR; EMSO-Azores deep sea observatory). We show that chemical changes are limited in the warmest part of the plume [50-150°C, dMn > 40 μM; dilution factor (DF) of ~1-10 by NADW] as metal partitioning displays a chemical signature similar to the end-member one. However, as the dilution ratio between the hydrothermal fluid and North-Atlantic Deep-water (NADW) increases (4-50°C, dMn < 40 μM; DF of 10-100 by NADW), metal partitioning is affected by different precipitation and oxidation processes. Molar ratios normalized to Fe in the particles highlight the onset of Fe oxides formation, the precipitation of barite and the decreasing contribution of sulfide minerals (mainly Cu-Fe sulfides and sphalerite/wurtzite) along with fluid dilution. Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Air-water CO 2 fluxes were up-scaled to take into account the latitudinal and ecosystem diversity of the coastal ocean, based on an exhaustive literature survey. Marginal seas at high and temperate latitudes act as sinks of CO 2 from the atmosphere, in contrast to subtropical and tropical marginal seas that act as sources of CO 2 to the atmosphere. Overall, marginal seas act as a strong sink of CO 2 of about À0.45 Pg C yr À1. This sink could be almost fully compensated by the emission of CO 2 from the ensemble of near-shore coastal ecosystems of about 0.40 Pg C yr À1. Although this value is subject to large uncertainty, it stresses the importance of the diversity of ecosystems, in particular near-shore systems, when integrating CO 2 fluxes at global scale in the coastal ocean.

Carbon system parameters measured during several expeditions along the coast of Chile (23°S-56°S) have been used to show the main spatial and temporal trends of air-sea CO 2 fluxes in the coastal waters of the eastern South Pacific. Chilean coastal waters are characterized by strong pCO 2 gradients between the atmosphere and the surface water, with high spatial and temporal variability. On average, the direction of the carbon flux changes from CO 2 outgassing at the coastal upwelling region to CO 2 sequestering at the nonupwelling fjord region in Chilean Patagonia. Estimations of surface water pCO 2 along the Patagonian fjord region showed that, while minimum pCO 2 levels (strong CO 2 undersaturation) occurs during the spring and summer period, maximum levels (including CO 2 supersaturation) occur during the austral winter. CO 2 uptake in the Patagonia fjord region during spring-summer is within the order of −5 mol C m −2 yr −1 , indicating a significant regional sink of atmospheric CO 2 during that season. We suggest that the CO 2 sink at Patagonia most probably exceeds the CO 2 source exerted by the coastal upwelling system off central northern Chile.

A dense winter bloom of the dinoflagellate Heterocapsa triquetra was observed at a fixed station (44°35.3'S; 72°43.6'W) in the Puyuhuapi Fjord in Chilean Patagonia during July 2015. H. triquetra dominated the phytoplankton community in the surface waters between 2 and 15 m (13-58 × 10 9 cell m-2), with abundances some 3 to 15 times higher than the total abundance of the diatom assemblage, which was dominated by Skeletonema spp. The high abundance of dinoflagellates was reflected in high rates of gross primary production (GPP; 0.6-1.6 g C m-2 d-1) and chlorophyll-a concentration (Chl-a; 70-199.2 mg m-2) that are comparable to levels reported in spring diatom blooms in similar Patagonian fjords. We identify the main forcing factors behind a pulse of organic matter production during the non-productive winter season, and test the hypothesis that low irradiance levels are a key factor limiting phytoplankton blooms and subsequent productivity during winter. Principal Component Analysis (PCA) indicated that GPP rates were significantly correlated (r =-0.8, p< 0.05) with a decrease in salinity/temperature and the presence of the Heterocapsa bloom. The bloom occurred under low surface irradiance levels characteristic of austral winter and was accompanied by strong northern winds, associated with the passage of a low-pressure system, and a water column dominated by double diffusive layering. To our knowledge, this is the first report of a dense dinoflagellate bloom during deep austral winter in a Patagonian fjord, and our data challenge the paradigm of light limitation as a factor controlling phytoplankton blooms in this region in winter.

The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II). Here we deploy flow injection analysis to measure in-situ Fe(II) concentrations during a series of mesocosm/microcosm experiments in coastal environments in addition to the decay rate of this Fe(II) when moved into the dark. During 5 mesocosm/microcosm experiments in Svalbard and Patagonia, where dissolved (0.2 µm) Fe and Fe(II) were quantified simultaneously, Fe(II) constituted 24-65% of DFe suggesting that Fe(II) was a large fraction of the DFe pool. When this Fe(II) was allowed to decay in the dark, the vast majority of measured oxidation rate constants were retarded relative to calculated constants derived from ambient temperature, salinity, pH and dissolved O 2. The oxidation rates of Fe(II) spikes added to Atlantic seawater more closely matched calculated rate constants. The difference between observed and theoretical decay rates in Svalbard and Patagonia was most pronounced at Fe(II) concentrations <2 nM and attributed to a stabilising effect of cellular exudates upon Fe(II). This enhanced stability of Fe(II) under post-bloom conditions, and the existence of such a high fraction of DFe as Fe(II), challenges the assumption that DFe speciation is dominated by ligand bound-Fe(III) species.

In order to improve the area and time coverage of seawater pCO2 (pCO2 sw) observations along the eastern Pacific coast, MBARI’s and LDEO’s pCO2 databases were merged and averaged by cruise and 5-degree latitudinal bins. After extracting the first 100 km from coast, a final pCO2 database close to one million underway observations was obtained, with data from 418 cruises distributed along the Pacific coast of the Americas from Alaska to Chile. Higher pCO2sw values were found in the tropics (~500 ppm) compared to pCO2sw at high latitudes (~250-300 ppm). On the other hand, wind speeds were found to be lower near the tropical zone (~3 m/s) compared to high latitudes (&gt; 6m/s). High CO2 fluxes into the ocean were found at high latitudes (reaching -10 mol m y near the tip of Chile) and lower CO2 fluxes to the atmosphere were found in the Peruvian and Chilean upwelling systems (+4 mol m y). In order to explain the pattern of pCO2sw found for the western coast of the Americas, we correlate...

Patagonia, due to its geographic position and the dominance of westerly winds, is a key area that contributes to the supply of nutrients to the Southern Ocean, both through mineral dust and through the periodic deposits of volcanic ash. Here we evaluate the characteristics of Fe dissolved (into soluble and colloidal species) from volcanic ash for three recent southern Andes volcanic eruptions having contrasting features and chemical compositions. Contact between cloud waters (wet deposition) and end-members of andesitic (Hudson volcano) and rhyolitic (Chaitén volcano) materials was simulated. Results indicate higher Fe release and faster liberation rates in the andesitic material. Fe release during particle-seawater interaction (dry deposition) has higher rates in rhyolitic-type ashes. Rhyolitic ashes under acidic conditions release Fe in higher amounts and at a slower rate, while in those samples containing mostly glass shards, Fe release was lower and faster. The 2011 Puyehue eruption was observed by a dust monitoring station. Puyehue-type eruptions can contribute soluble Fe to the ocean via dry or wet deposition, nearly reaching the limit required for phytoplankton growth. In contrast, the input of Fe after processing by an acidic eruption plume could raise the amount of dissolved Fe in surface ocean waters several times, above the threshold required to initiate phytoplankton blooms. A single eruption like the Puyehue one represents more than half of the yearly Fe flux contributed by dust.

name was later changed to Scientific Committee on Oceanic Research. DEDICATION. This article is dedicated to the late Keith Hunter, who passed away on October 24, 2018. Keith was a complex and wonderful person, a full-blooded researcher and brilliant scientist, and also the most down-to-earth person-digging up some potatoes in his garden or sitting down at home with a glass of good whiskey looking over the sea. Keith was one of the first chemical oceanographers to embrace John Martin's hypothesis that iron is a limiting nutrient in many parts of the global surface ocean. In 1996, Keith and David Turner initiated and then co-chaired the very first of SCOR's iron-related working groups, WG 109 on the Biogeochemistry of Iron in Seawater. Keith was an inspiring supervisor and mentor, a staunch supporter of great research, and an outstanding scientist, as reflected in his more than 150 publications, software innovations, and receipt of numerous science awards and prizes. He will live on in our memories, and the ideas he inspired in many of us will be his legacy.

A dense winter bloom of the dinoflagellate Heterocapsa triquetra was observed at a fixed station (44°35.3'S; 72°43.6'W) in the Puyuhuapi Fjord in Chilean Patagonia during July 2015. H. triquetra dominated the phytoplankton community in the surface waters between 2 and 15 m (13-58 × 10 9 cell m -2 ), with abundances some 3 to 15 times higher than the total abundance of the diatom assemblage, which was dominated by Skeletonema spp. The high abundance of dinoflagellates was reflected in high rates of gross primary production (GPP; 0.6-1.6 g C m -2 d -1 ) and chlorophyll-a concentration (Chl-a; 70-199.2 mg m -2 ) that are comparable to levels reported in spring diatom blooms in similar Patagonian fjords. We identify the main forcing factors behind a pulse of organic matter production during the non-productive winter season, and test the hypothesis that low irradiance levels are a key factor limiting phytoplankton blooms and subsequent productivity during winter. Principal Component Analysis (PCA) indicated that GPP rates were significantly correlated (r = -0.8, p< 0.05) with a decrease in salinity/temperature and the presence of the Heterocapsa bloom. The bloom occurred under low surface irradiance levels characteristic of austral winter and was accompanied by strong northern winds, associated with the passage of a low-pressure system, and a water column dominated by double diffusive layering. To our knowledge, this is the first report of a dense dinoflagellate bloom during deep austral winter in a Patagonian fjord, and our data challenge the paradigm of light limitation as a factor controlling phytoplankton blooms in this region in winter.

The hlgh biological productivity of the Hurnboldt Current System (HCS) off Chile supports an annual fish catch of over 7 mill~on t. The area is also important biogeochemically, because the outgassing of recently upwelled water is modulated by contrasting degrees of biolopcal activity. However, very few field measurements of primary production and planktonic respiration have been undertaken within the Eastern Boundary Current (EBC) system off Chile. In this study an estimate of primary production (PP) and surface planktonic commun~ty respiration is presented from several research cruises in the HCS and adjacent oceanlc areas. The highest production levels were found near the coast correlating closely with known upwelling areas. Both gross primary production (GPP) and community respiration (CR) showed important spatial and temporal fluctuations. The highest water column integrated GPP was measured in the southern and central fishing area (19.9 g C m-2 d-l) and off the Antofagasta upwelling ecosystem (9.3 g C m-' d-'). The range of GPP agrees well with values reported for Peni (0.05 to ll.? g C m'2 d"). The Coquimbo upwehng system, despite being an area of persistent upwelling, showed lower production and community respiration values than the Antofagasta and Concepcion Shelf areas.The lowest surface PP values were measured within the oceanic region adjacent to the northern coastal upwelling zones (0.8 * 0.5 pg C 1-' h-'), though slightly enhanced biological production was found within the Nazca Ridge (1.5 * 2.1 pg C l-' h-') that separates the Chilean and Peruvian deep basins.

It is well recognized that the atmospheric deposition of iron (Fe) affects ocean productivity, atmospheric CO 2 uptake, ecosystem diversity, and overall climate. Despite significant advances in measurement techniques and modeling efforts, discrepancies persist between observations and models that hinder accurate predictions of processes and their global effects. Here, we provide an assessment report on where the current state of knowledge is and where future research emphasis would have the highest impact in furthering the field of Fe atmosphere-ocean biogeochemical cycle. These results were determined through consensus reached by diverse researchers from the oceanographic and atmospheric science communities with backgrounds in laboratory and in situ measurements, modeling, and remote sensing. We discuss i) novel measurement methodologies and instrumentation that allow detection and speciation of different forms and oxidation states of Fe in deliquesced mineral aerosol, cloud/rainwater, and seawater; ii) oceanic models that treat Fe cycling with several external sources and sinks, dissolved, colloidal, particulate, inorganic, and organic ligand-complexed forms of Fe, as well as Fe in detritus and phytoplankton; and iii) atmospheric models that consider natural and anthropogenic sources of Fe, mobilization of Fe in mineral aerosols due to the dissolution of Fe-oxides and Fe-substituted aluminosilicates through proton-promoted, organic ligand-promoted, and photo-reductive mechanisms. In addition, the study identifies existing challenges and disconnects (both fundamental and methodological) such as i) inconsistencies in Fe nomenclature and the definition of bioavailable Fe between oceanic and atmospheric disciplines, and ii) the lack of characterization of the processes controlling Fe speciation and residence time at the atmosphere-ocean interface. Such challenges are undoubtedly caused by extremely low concentrations, short lifetime, and the myriad of physical, (photo)chemical, and biological processes affecting global biogeochemical cycling of Fe. However, we also argue that the historical division (separate treatment of Fe biogeochemistry in oceanic and atmospheric disciplines) and the classical funding structures (that often create obstacles for transdisciplinary collaboration) are also hampering the advancement of knowledge in the field. Finally, the study

The distribution of dissolved iron (Fe), total organic Fe-binding ligands, and siderophores were measured between the surface and 400 m at Station ALOHA, a long term ecological study site in the North Pacific Subtropical Gyre. Dissolved Fe concentrations were low throughout the water column and strong organic Fe-binding ligands exceeded dissolved Fe at all depths; varying from 0.9 nmol L -1 in the surface to 1.6 nmol L -1 below 150 m. Although Fe does not appear to limit microbial production, we nevertheless found siderophores at nearly all depths, indicating some populations of microbes were responding to Fe stress. Ferrioxamine siderophores were most abundant in the upper water column, with concentrations between 0.1 and 2 pmol L -1 , while a suite of amphibactins were found below 200 m with concentrations between 0.8 and 11 pmol L -1 . The distinct vertical distribution of ferrioxamines and amphibactins may indicate disparate strategies for acquiring Fe from dust in the upper water column and recycled organic matter in the lower water column. Amphibactins were found to have conditional stability constants (log K cond FeL 1 ,Fe ′ ) ranging from 12.0 to 12.5, while ferrioxamines had much stronger conditional stability constants ranging from 14.0 to 14.4, within the range of observed L 1 ligands by voltammetry. We used our data to calculate equilibrium Fe speciation at Station ALOHA to compare the relative concentration of inorganic and siderophore complexed Fe. The results indicate that the concentration of Fe bound to siderophores was up to two orders of magnitude higher than inorganic Fe, suggesting that even if less bioavailable, siderophores were nevertheless a viable pathway for Fe acquisition by microbes at our study site. Finally, we observed rapid production of ferrioxamine E by particle-associated bacteria during incubation of freshly collected sinking organic matter. Fe-limitation may therefore be a factor in regulating carbon metabolism and nutrient regeneration in the mesopelagic.

The northern Red Sea (NRS) is a low-nutrient, low-chlorophyll (LNLC) ecosystem with high rates of atmospheric deposition due to its proximity to arid regions. Impacts of atmospheric deposition on LNLC ecosystems have been attributed to the chemical constituents of dust, while overlooking bioaerosols. Understanding how these vast areas of the ocean will respond to future climate and anthropogenic change hinges on the response of microbial communities to these changes. We tested the impacts of bioaerosols on the surface water microbial diversity and the primary and bacterial production rates in the NRS, a system representative of other LNLC oceanic regions, using a mesocosm bioassay experiment. By treating NRS surface seawater with dust, which contained nutrients, metals, and viable organisms, and “UV-treated dust” (which contained only nutrients and metals), we were able to assess the impacts of bioaerosols on local natural microbial populations. Following amendments (20 and 44 h) th...

The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist almost exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II), the speciation of which is uncertain. Here we deploy flow injection analysis to measure in situ Fe(II) concentrations during a series of mesocosm/microcosm/multistressor experiments in coastal environments in addition to the decay rate of this Fe(II) when moved into the dark. During five mesocosm/microcosm/multistressor experiments in Svalbard and Patagonia, where dissolved (0.2 µm) Fe and Fe(II) were quantified simultaneously, Fe(II) constituted 24 %-65 % of DFe, suggesting that Fe(II) was a large fraction of the DFe pool. When this Fe(II) was allowed to decay in the dark, the vast majority of measured oxidation rate constants were less than calculated constants derived from ambient temperature, salinity, pH, and dissolved O 2. The oxidation rates of Fe(II) spikes added to Atlantic seawater more closely matched calculated rate constants. The difference between observed and theoretical decay rates in Svalbard and Patagonia was most pronounced at Fe(II) concentrations < 2 nM, suggesting that the effect may have arisen from organic Fe(II) ligands. This apparent enhancement of Fe(II) stability under post-bloom conditions and the existence of such a high fraction of DFe as Fe(II) challenge the assumption that DFe speciation in coastal seawater is dominated by ligand bound-Fe(III) species.

The speciation of dissolved iron (DFe) in the ocean is widely assumed to consist exclusively of Fe(III)-ligand complexes. Yet in most aqueous environments a poorly defined fraction of DFe also exists as Fe(II). Here we deploy flow injection analysis to measure in-situ Fe(II) concentrations during a series of mesocosm/microcosm experiments in coastal environments in addition to the decay rate of this Fe(II) when moved into the dark. During 5 mesocosm/microcosm experiments in Svalbard and Patagonia, where dissolved (0.2 µm) Fe and Fe(II) were quantified simultaneously, Fe(II) constituted 24-65% of DFe suggesting that Fe(II) was a large fraction of the DFe pool. When this Fe(II) was allowed to decay in the dark, the vast majority of measured oxidation rate constants were retarded relative to calculated constants derived from ambient temperature, salinity, pH and dissolved O 2. The oxidation rates of Fe(II) spikes added to Atlantic seawater more closely matched calculated rate constants. The difference between observed and theoretical decay rates in Svalbard and Patagonia was most pronounced at Fe(II) concentrations <2 nM and attributed to a stabilising effect of cellular exudates upon Fe(II). This enhanced stability of Fe(II) under post-bloom conditions, and the existence of such a high fraction of DFe as Fe(II), challenges the assumption that DFe speciation is dominated by ligand bound-Fe(III) species.

Understanding carbon fluxes in shelf systems is an important aspect of quantifying global carbon budgets. Three years of pCO 2 observations are analysed from spring to autumn during 2005, 2007 and 2008 at L4, a seasonally stratified station in the Western English Channel. A general trend from low to high seawater pCO 2 during each year was observed, punctuated by episodic low seawater pCO 2 events. Air-sea CO 2 flux dynamics derived from seawater pCO 2 showed spring and summer to be times of atmospheric CO 2 drawdown during stratified water conditions while autumn saw the breakdown of stratification and CO 2 outgassing. The largest CO 2 instantaneous drawdown was observed during high wind events. Seawater pCO 2 at L4 is controlled by metabolic processes, solubility and advection processes, although to a varying extent between years. While tidal influence, movement of water masses and rapid phytoplankton blooms contribute to large pCO 2 fluctuations between adjacent samples, distinct quasi-seasonal phases are observed due to the natural physical and biological cyclic controls on seawater pCO 2 .

Understanding carbon fluxes in shelf systems is an important aspect of quantifying global carbon budgets. Three years of pCO 2 observations are analysed from spring to autumn during 2005, 2007 and 2008 at L4, a seasonally stratified station in the Western English Channel. A general trend from low to high seawater pCO 2 during each year was observed, punctuated by episodic low seawater pCO 2 events. Air-sea CO 2 flux dynamics derived from seawater pCO 2 showed spring and summer to be times of atmospheric CO 2 drawdown during stratified water conditions while autumn saw the breakdown of stratification and CO 2 outgassing. The largest CO 2 instantaneous drawdown was observed during high wind events. Seawater pCO 2 at L4 is controlled by metabolic processes, solubility and advection processes, although to a varying extent between years. While tidal influence, movement of water masses and rapid phytoplankton blooms contribute to large pCO 2 fluctuations between adjacent samples, distinct quasi-seasonal phases are observed due to the natural physical and biological cyclic controls on seawater pCO 2 .

Precipitation is an important component of the interaction between Earth's atmosphere and oceans, modifying air-sea fluxes of momentum, heat, and gas. It has been hypothesized that rain's suppression of ocean surface gravity waves and centimeter-scale wave enhancement should alter the nature of air-sea momentum flux, resulting in increased near-surface current. Here, we use field observations to describe this impact and measure the very near-surface current response to rainfall. During heavy rain, surface-roughening ring waves were generated and longer gravity waves were suppressed; immediately following, the magnitude of the near-surface current increased in response to wind forcing but died as the rain subsided and long waves recovered. These first-of-their-kind field observations indicate that rain reduces ocean wave form drag in favor of tangential stress, resulting in the acceleration of current near the sea surface. Plain Language Summary Rain is known to be an important component of atmosphere-ocean interactions. However, the specific response of short-scale waves and very near-surface current to precipitation has not yet been described in the Earth's ocean. We observe that rainfall suppresses long waves and grows centimeter-scale ring waves. This alteration facilitates the acceleration of current, which ceases in favor of longer wave growth as the rain subsides. This is the first observation of this phenomenon in the Earth's real ocean.

Sea surface fugacity of carbon dioxide (fCO2ssw) was measured across the Weddell gyre and the eastern sector in the Atlantic Southern Ocean in autumn. During the occupation between February and April 2019, the region of the study transect was a potential ocean CO2 sink. A net CO2 flux (FCO2) of −6.2 (± 8; sink) mmol m–2 d–1 was estimated for the entire study region, with the largest average CO2 sink of −10.0 (± 8) mmol m–2 d–1 in the partly ice-covered Astrid Ridge (AR) region near the coast at 68°S and −6.1 (± 8) mmol m–2d–1 was observed in the Maud Rise (MR) region. A CO2 sink was also observed south of 66°S in the Weddell Sea (WS). To assess the main drivers describing the variability of fCO2ssw, a correlation model using fCO2 and oxygen saturation was considered. Spatial distributions of the fCO2 saturation/O2 saturation correlations, described relative to the surface water properties of the controlling variables (chlorophyll a, apparent oxygen utilization (AOU), sea surface tem...