The fraction of atmospherically deposited iron which dissolves in seawater, or becomes available to phytoplankton for growth, is a key determinant of primary productivity in many open ocean regions. ...As such this parameter plays an important part in the global oceanic cycles of iron and carbon, and yet the factors that control iron dissolution from aerosol are very poorly understood. In this manuscript we seek to synthesise the available knowledge of these factors, which operate in the atmosphere and in seawater. A conceptual model of the overall aerosol iron solubility is presented, in which we liken the various controls on iron solubility to sets of parallel electrical resistors. We also discuss experimental methods for the determination of iron solubility and make recommendations for future studies in this area.
The current socio-economic, environmental and public health challenges that countries are facing clearly need common-defined strategies to inform and support our transition to a sustainable economy. ...Here, the technology-critical elements (which includes Ga, Ge, In, Te, Nb, Ta, Tl, the Platinum Group Elements and most of the rare-earth elements) are of great relevance in the development of emerging key technologies—including renewable energy, energy efficiency, electronics or the aerospace industry. In this context, the increasing use of technology-critical elements (TCEs) and associated environmental impacts (from mining to end-of-life waste products) is not restricted to a national level but covers most likely a global scale. Accordingly, the European COST Action TD1407: Network on Technology-Critical Elements (NOTICE)—from environmental processes to human health threats, has an overall objective for creating a network of scientists and practitioners interested in TCEs, from the evaluation of their environmental processes to understanding potential human health threats, with the aim of defining the current state of knowledge and gaps, proposing priority research lines/activities and acting as a platform for new collaborations and joint research projects. The Action is focused on three major scientific areas: (i) analytical chemistry, (ii) environmental biogeochemistry and (iii) human exposure and (eco)-toxicology.
Iron is a key micronutrient for phytoplankton growth in the surface ocean. Yet the significance of volcanism for the marine biogeochemical iron-cycle is poorly constrained. Recent studies, however, ...suggest that offshore deposition of airborne ash from volcanic eruptions is a way to inject significant amounts of bio-available iron into the surface ocean. Volcanic ash may be transported up to several tens of kilometers high into the atmosphere during large-scale eruptions and fine ash may stay aloft for days to weeks, thereby reaching even the remotest and most iron-starved oceanic regions. Scientific ocean drilling demonstrates that volcanic ash layers and dispersed ash particles are frequently found in marine sediments and that therefore volcanic ash deposition and iron-injection into the oceans took place throughout much of the Earth's history. Natural evidence and the data now available from geochemical and biological experiments and satellite techniques suggest that volcanic ash is a so far underestimated source for iron in the surface ocean, possibly of similar importance as aeolian dust. Here we summarise the development of and the knowledge in this fairly young research field. The paper covers a wide range of chemical and biological issues and we make recommendations for future directions in these areas. The review paper may thus be helpful to improve our understanding of the role of volcanic ash for the marine biogeochemical iron-cycle, marine primary productivity and the ocean-atmosphere exchange of CO2 and other gases relevant for climate in the Earth's history.
Iron (Fe) is a limiting nutrient for phytoplankton productivity in many different oceanic regions. A critical aspect underlying iron limitation is its low solubility in seawater as this controls the ...distribution and transport of iron through the ocean. Processes which enhance the solubility of iron in seawater, either through redox reactions or organic complexation, are central to understanding the biogeochemical cycling of iron. In this work we combined iron solubility measurements with parallel factor (PARAFAC) data analysis of Coloured Dissolved Organic Matter (CDOM) fluorescence along a meridional transect through the Atlantic (PS ANT XXVI‐4) to examine the hypothesis that marine humic fluorescence is a potential proxy for iron solubility in the surface ocean. PARAFAC analysis revealed 4 components (C1‐4), two humic like substances (C2&4) and two protein‐like (C1&3). Overall none of the 4 components were significantly correlated with iron solubility, though humic‐like components were weakly correlated with iron solubility in iron replete waters. Our analysis suggests that the ligands responsible for maintaining iron in solution in the euphotic zone are sourced from both remineralisation processes and specific ligands produced in response to iron stress and are not easily related to bulk CDOM properties. The humic fluorescence signal was sharply attenuated in surface waters presumably most likely due to photo bleaching, though there was only a weak correlation with the transient photo product H2O2, suggesting longer lifetimes in the photic zone for the fluorescent components identified here.
Key Pointshumic-like components correlated with Fe solubility in iron repleted waterligands are sourced from remineralisation processes produced to Fe stresshumic flu sharply attenuated in surface waters, but only weak corr. with H2O2
The paper presents the current status of the Maritime Aerosol Network (MAN), which has been developed as a component of the Aerosol Robotic Network (AERONET). MAN deploys Microtops handheld Sun ...photometers and utilizes the calibration procedure and data processing (Version 2) traceable to AERONET. A web site dedicated to the MAN activity is described. A brief historical perspective is given to aerosol optical depth (AOD) measurements over the oceans. A short summary of the existing data, collected on board ships of opportunity during the NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project is presented. Globally averaged oceanic aerosol optical depth (derived from island‐based AERONET measurements) at 500 nm is ∼0.11 and Angstrom parameter (computed within spectral range 440–870 nm) is calculated to be ∼0.6. First results from the cruises contributing to the Maritime Aerosol Network are shown. MAN ship‐based aerosol optical depth compares well to simultaneous island and near‐coastal AERONET site AOD.
The decay kinetics of superoxide (O2−) reacting with organic matter was examined in oligotrophic waters at, and nearby, the TENATSO ocean observatory adjacent to the Cape Verde archipelago. ...Superoxide is the short‐lived primary photochemical product of colored dissolved organic matter (CDOM) photolysis and also reacts with CDOM or trace metals (Cu, Fe) to form H2O2. In the present work we focused our investigations on reactions between CDOM and superoxide. O2− decay kinetics experiments were performed by adding KO2 to diethylenetriaminepentaacetic acid (DTPA) amended seawater and utilizing an established chemiluminescence technique for the detection of O2− at nM levels. In Cape Verdean waters we found a significant reactivity of superoxide with CDOM with maximal rates adjacent to the chlorophyll maximum, presumably from production of new CDOM from bacteria/phytoplankton. This work highlights a poorly understood process which impacts on the biogeochemical cycling of CDOM and trace metals in the open ocean.
► The suitability of ozone for water quality improvement was evaluated by investigating the ozone-based removal of nitrite, ammonia, yellow substances and total bacterial biomass. ► Ozone can be ...efficiently utilized to simultaneously remove nitrite and yellow substances without risking the formation of toxic oxidant concentrations. ► An effective reduction of bacterial biomass was achieved by short-term ozonation in combination with foam fractionation. ► Ammonia oxidation in seawater by ozonation is independent from pH. ► Nitrogen gas is the primary end product during the ozone-based ammonia removal in seawater. ► Ammonia oxidation by ozonation in seawater presupposes an initial accumulation of ozone-produced oxidants to highly toxic amounts, restricting a safe application in aquaculture.
The high levels of water-reuse in intensive recirculating aquaculture systems (RAS) require an effective water treatment in order to maintain good water quality. In order to reveal the potential and limitations of ozonation for water quality improvement in marine RAS, we tested ozone's ability to remove nitrite, ammonia, yellow substances and total bacterial biomass in seawater, considering aspects such as efficiency, pH-dependency as well as the formation of toxic ozone-produced oxidants (OPO). Our results demonstrate that ozone can be efficiently utilized to simultaneously remove nitrite and yellow substances from process water in RAS without risking the formation of toxic OPO concentrations. Contemporaneously, an effective reduction of bacterial biomass was achieved by ozonation in combination with foam fractionation. In contrast, ammonia is not oxidized by ozone so long as nitrite and yellow substances are present in the water, as the dominant reaction of the ozone-based ammonia-oxidation in seawater requires the previous formation of OPO as intermediates. The oxidation of ammonia in seawater by ozone is basically a bromide-catalyzed reaction with nitrogen gas as end product, enabling an almost complete removal of ammonia-nitrogen from the aquaculture system. Results further show that pH has no effect on the ozone-based ammonia oxidation in seawater. Unlike in freshwater, an effective removal of ammonia even at pH-values as low as 6.5 has been shown to be feasible in seawater. However, as the predominant reaction pathway involves an initial accumulation of OPO to toxic amounts, we consider the ozone-based removal of ammonia in marine RAS as risky for animal health and economically unviable.
With the common goal of more accurately and consistently quantifying ambient concentrations of free metal ions and natural organic ligands in aquatic ecosystems, researchers from 15 laboratories that ...routinely analyze trace metal speciation participated in an intercomparison of statistical methods used to model their most common type of experimental dataset, the complexometric titration. All were asked to apply statistical techniques that they were familiar with to model synthetic titration data that are typical of those obtained by applying state-of-the-art electrochemical methods – anodic stripping voltammetry (ASV) and competitive ligand equilibration-adsorptive cathodic stripping voltammetry (CLE-ACSV) – to the analysis of natural waters. Herein, we compare their estimates for parameters describing the natural ligands, examine the accuracy of inferred ambient free metal ion concentrations (Mf), and evaluate the influence of the various methods and assumptions used on these results.
The ASV-type titrations were designed to test each participant's ability to correctly describe the natural ligands present in a sample when provided with data free of measurement error, i.e., random noise. For the three virtual samples containing just one natural ligand, all participants were able to correctly identify the number of ligand classes present and accurately estimate their parameters. For the four samples containing two or three ligand classes, a few participants detected too few or too many classes and consequently reported inaccurate ‘measurements’ of ambient Mf. Since the problematic results arose from human error rather than any specific method of analyzing the data, we recommend that analysts should make a practice of using one's parameter estimates to generate simulated (back-calculated) titration curves for comparison to the original data. The root–mean–squared relative error between the fitted observations and the simulated curves should be comparable to the expected precision of the analytical method and upon visual inspection the distribution of residuals should not be skewed.
Modeling the synthetic, CLE-ACSV-type titration dataset, which comprises 5 titration curves generated at different analytical windows or levels of competing ligand added to the virtual sample, proved to be more challenging due to the random measurement error that was incorporated. Comparison of the submitted results was complicated by the participants' differing interpretations of their task. Most adopted the provided ‘true’ instrumental sensitivity in modeling the CLE-ACSV curves, but several estimated sensitivities using internal calibration, exactly as is required for actual samples. Since most fitted sensitivities were biased low, systematic error in inferred ambient Mf and in estimated weak ligand (L2) concentrations resulted.
The main distinction between the mathematical approaches taken by participants lies in the functional form of the speciation model equations, with their implicit definition of independent and dependent or manipulated variables. In ‘direct modeling’, the dependent variable is the measured Mf (or Ip) and the total metal concentration (MT) is considered independent. In other, much more widely used methods of analyzing titration data – classical linearization, best known as van den Berg/Ružić, and isotherm fitting by nonlinear regression, best known as the Langmuir or Gerringa methods – Mf is defined as independent and the dependent variable calculated from both MT and Mf. Close inspection of the biases and variability in the estimates of ligand parameters and in predictions of ambient Mf revealed that the best results were obtained by the direct approach. Linear regression of transformed data yielded the largest bias and greatest variability, while non-linear isotherm fitting generated results with mean bias comparable to direct modeling, but also with greater variability.
Participants that performed a unified analysis of ACSV titration curves at multiple detection windows for a sample improved their results regardless of the basic mathematical approach taken. Overall, the three most accurate sets of results were obtained using direct modeling of the unified multiwindow dataset, while the single most accurate set of results also included simultaneous calibration. We therefore recommend that where sample volume and time permit, titration experiments for all natural water samples be designed to include two or more detection windows, especially for coastal and estuarine waters. It is vital that more practical experimental designs for multi-window titrations be developed.
Finally, while all mathematical approaches proved to be adequate for some datasets, matrix-based equilibrium models proved to be most naturally suited for the most challenging cases encountered in this work, i.e., experiments where the added ligand in ACSV became titrated. The ProMCC program (Omanović et al., this issue) as well as the Excel Add-in based KINETEQL Multiwindow Solver spreadsheet (Hudson, 2014) have this capability and have been made available for public use as a result of this intercomparison exercise.
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•An inter-comparison of approaches to analyzing complexometric titration data is reported.•Error of fit and parameter estimates depended on data transformation, with untransformed<non-linear isotherm form<linearized.•Simple internal calibration resulted in low-biased sensitivities and L2 parameters.•Simultaneous analysis of multiple analytical windows increased accuracy of both calibration and ligand parameter estimation.•Titration of added ligands must be accounted for in modeling CLE-ACSV titration data.
It is well demonstrated that the major source of iron (Fe) to the Tropical Atlantic is through aeolian deposition of Saharan dust. However presently we know very little about the dissolution ...processes of these aerosols in the sunlit productive surface waters of this region. Candidate processes identified as being potentially critical to dust dissolution include thermal dissolution, direct photochemical reduction, ligand induced dissolution and reductive dissolution by superoxide (O
2
−), formed predominantly by photochemical reactions with dissolved organic matter in seawater. O
2
− is short lived (half life: 1–100
s) in seawater as it reacts rapidly with both the iron (Fe(II)/Fe(III)) and copper (Cu(I)/Cu(II)) redox couples and to a less extent with dissolved organic matter. In the euphotic zone where high fluxes of sunlight and potentially O
2
− exist there exists the possibility of a kinetic controlled reduction of Fe in colloids and particles to the more soluble and bioavailable Fe(II). However, presently no information is available from the open ocean on the rates that this process may be occurring. Here we present experimental data using a new methodological approach to assess trace metal speciation which involves the measurement of O
2
− decay kinetics. Our method allows an assessment of changes in both the organic reactivity with O
2
− simultaneously with apparent changes in the speciation of iron and/or copper. Our approach allows evaluation of the importance of each of the different reaction pathways for O
2
− and thus the factors which control metal redox speciation in seawater. In the present work we applied this technique using seawater collected in the vicinity of the island of Sao Vicente, Cape Verde, to dust incubation experiments to ascertain rapidly if dust dissolution had altered the
in situ iron or copper speciation in seawater. Our results show the importance of Cu and CDOM to O
2
− reactivity in the Tropical Atlantic and highlight the natural variability in dust dissolution processes in the ocean that is still poorly understood. Our data also indicate that reactions with O
2
− in the surface Tropical Ocean are not a significant source of soluble iron.
►Superoxide reactivity as a probe for metal speciation in seawater. ►Processes influencing dust dissolution in the surface ocean. ►Superoxide is not a significant reductant for aerosol iron in the open ocean. ►Copper is the dominant sink for superoxide at TENATSO.
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