Iron organic speciation during the LOHAFEX experiment: Iron ligands release under biomass control by copepod grazing

The LOHAFEX iron fertilization experiment consisted in the fertilization of the closed core of a cyclonic eddy located south of the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. This eddy was characterized by high nitrate and low silicate concentrations. Despite a 2.5 fold increase of the chlorophyll-a (Chl-a) concentrations, the composition of the biological community did not change. Phytoplankton biomass was mostly formed by small autotrophic flagellates whereas zooplankton biomass was mostly comprised by the large copepod Calanus simillimus. Efficient recycling of copepod fecal pellets (the main component of the downward flux of organic matter) in the upper 100–150 m of the water column prevented any significant deep export of particulate organic carbon (POC). Before fertilization, dissolved iron (DFe) concentrations in the upper 200 m were low, but not depleted, at ~0.2 nM. High DFe concentrations appeared scattered from day 14 onwards as a result of the grazing activity. A second fertilization on day 21 had no significant effect on the DFe and Chl-a standing stocks. Work with unfiltered samples using different acidification protocols revealed that, by midway of LOHAFEX, rapid recycling of iron-replenished copepod fecal pellets explained the source of bioavailable iron that prolonged the duration of the bloom for many weeks. Here we present the evolution of the organic speciation of iron in the upper 200 m of the water column during LOHAFEX by a Competing Ligand Equilibrium method using voltammetry. During the first 12 days of the experiment, ligands of an affinity for iron similar to the ligands found before fertilization (logK′Fe′L~11.9) accumulated in fertilized waters mostly in the upper 80 m (from ~1 nM to ~2.5 nM). The restriction of ligand accumulation to the depth of Chl-a penetration points to exudation by the growing autotrophic population as the initial source of ligands. From day 5 onwards, we found in many samples a new class of ligands (L1) characterized by a significant higher conditional stability constant than the background complexation (logK′Fe′L1~12.9). During the middle section of the experiment (days 12 to 25) the accumulation of overall ligands and specifically L1, reached an upper limit in surface waters (at ~3 nM). Overall ligands and L1 accumulation was also observed below the mixed layer depth indicating that grazing was the process behind ligand release. During the last 10 days of the experiment ligands kept accumulating in deep waters but suffered a small decrease in the upper 50 m of the water column caused by the vanishing of L1. Ligand removal restricted to the euphotic layer was probably caused by photodegradation. A high correlation between [DFe] and [L1] suggested that recycled iron (released during grazing and copepod fecal pellet cycling) was in the form of FeL1 complexes. We hypothesize that the iron binding ligands released to the dissolved phase during LOHAFEX were mostly photosensitive intracellular ligands rapidly degraded in extracellular conditions (e.g.: pigments). Sloppy feeding by copepods and recycling of cells and cellular material in copepod fecal pellets caused the transfer of particulate ligands to the dissolved phase as zooplankton built up as a response to the blooming community

Exploratory evaluation of iron and its speciation in surface waters of Admiralty Bay, King George Island, Antarctica

Abstract The determination of dissolved iron concentrations and speciation was conducted for the first time in surface seawater coastline samples collected during the austral summer of 2020 in Admiralty Bay, King George Island, Antarctica. The technique of competitive ligand exchange/adsorptive cathodic stripping voltammetry with 2,3-dihydroxynaphthalene as the competing ligand was evaluated, showing a sensitivity between 14.25 and 21.05 nA nmol L-1 min-1, with an LOD of 14 pmol L-1 and a mean blank contribution of 0.248 nmol L-1. Physicochemical parameters such as pH (7.85 ± 0.2), salinity (32.7 ± 0.8) and dissolved oxygen (51.3 ± 26.6%) were compatible with those of the literature; however, the average temperature (4.2 ± 0.8 °C) was higher, possibly as a reflection of global warming. The dissolved iron mean value was 18.9 ± 6.1 nmol L-1, with a total ligand concentration of 23.6 ± 12.2 nmol L-1 and a conditional stability complex constant of 12.2 ± 0.2, indicating humic substances as possible ligands. On average, the calculated free iron concentrations were 0.7 ± 0.3 pmol L-1. Relatively high concentrations of iron indicate a possible local source of Fe, likely predominantly from upwelling sediments and secondarily from ice-melting waters, which does not limit the growth of the phytoplankton

Importance of deep mixing and silicic acid in regulating phytoplankton biomass and community in the iron-limited Antarctic Polar Front region in summer

En prens

Controls of primary production in two phytoplankton blooms in the Antarctic Circumpolar Current

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Comparison and combined use of linear and non-linear fitting for the estimation of complexing parameters from metal titrations of estuarine samples by CLE/AdCSV

11 páginas, 4 tablas, 3 figurasDespite the need to determine the concentration and conditional stability constants (K′) of natural ligands, we are far from achieving a consensus about the mathematical procedure to use with metal titrations due to the complexity of the samples and the wide range of fitting procedures and problems associated with the selection of the sensitivity (S) of the method. Here, we used Competitive Ligand Exchange/Adsorptive Cathodic Stripping Voltammetry (CLE/AdCSV) empiric data from estuarine waters and computer generated titration sets to compare linear methods with iterative correction of S with non-linear fitting adding S as a parameter. We demonstrate for the first time that, independent of the fitting procedure, S cannot be retrieved if all the ligands present in the sample are not included in the speciation model. We also investigated the variables, apart from analytical noise, that can cause flawed non-linear fittings of titration data. Computer generated data under multiple combinations of analytical conditions showed that a long extension of the titration (at least twice the total ligand concentration for estuarine conditions) and an analytical window (as the side coefficient ∝′) centered below the complexing strength of the natural ligands are essential to produce reliable complexing parameters. We verified, using for the first time a combination of experimental and computer generated data, that faulty estimations of S and K′ obtained in empiric titrations of estuarine samples were artifacts of non-linear fitting. Non-linear fitting flaws were caused by a combined effect of the analytical error, the analytical window and the ratio in between the copper concentration and the concentration of the strongest ligands. Here, we recommend for the study of estuarine waters to complement non-linear fitting with iterative linear fitting in order to avoid severe overestimations of S and the conditional stability constant of strong metal ligands.This work was funded by the MINECO of Spain (CGL2010-11846-E) and the Government of the Balearic Islands (project AAEE083/09). LML was supported by a Ramon y Cajal (MINECO) fellowship. J. Santos-Echeandía was funded with a JAE(CSIC) grantPeer reviewe

Quantification of iron in seawater at the low picomolar range based on optimization of bromate/ammonia/dihydroxynaphtalene system by catalytic adsorptive cathodic stripping voltammetry

7 páginas, 4 figuras, 2 tablasA new analytical protocol for the challenging analysis of total dissolved iron at the low picomolar level in oceanic waters suitable for onboard analysis is presented. The method is based on the revision of the adsorptive properties of the iron/2,3-dihydroxynaphthalene (Fe/DHN) complexes on the hanging mercury drop electrode with catalytic enhancement by bromate ions. Although it was based on a previously proposed reagent combination, we show here that the addition of an acidification/alkalinization step is essential in order to cancel any organic complexation, and that an extra increment of the pH to 8.6–8.8 leads to the definition of a preconcentration-free procedure with the lowest detection limit described up to now. For total dissolved iron analysis, samples were acidified to pH 2.0 in the presence of 30 μM DHN and left to equilibrate overnight. A 10 mL sample was subsequently buffered to a pH of 8.7 in the presence of 20 mM bromate: a 60 s deposition at 0 V led to a sensitivity of 34 nA nM–1 min–1, a 4-fold improvement over previous methods, that translated in a limit of detection of 5 pM (2–20 fold improvement). Several tests proved that a nonreversible reaction in the time scale of the analysis, triggered by the acidification/alkalinization step, was behind the signal magnification. The new method was validated onboard via the analysis of reference material and via intercalibration against flow injection analysis-chemiluminescence on Southern Ocean surface samples.This work was funded by the MINECO of Spain (Grant CGL2010-11846-E) and the Government of the Balearic Islands (Grant AAEE083/09). L.M.L. was supported by a Ramon y Cajal (MINECO) fellowship. J.S.E. was supported by the JAEDoc program of the CSICPeer reviewe