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

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

The Antarctic Circumpolar Current has a high potential for primary production and carbon sequestration through the biological pump. In the current study, two large-scale blooms observed in 2012 during a cruise with R.V. Polarstern were investigated with respect to phytoplankton standing stocks, primary productivity and nutrient budgets. While net primary productivity was similar in both blooms, chlorophyll a –specific photosynthesis was more efficient in the bloom closer to the island of South Georgia (39 °W, 50 °S) compared to the open ocean bloom further east (12 °W, 51 °S). We did not find evidence for light being the driver of bloom dynamics as chlorophyll standing stocks up to 165 mg m-2 developed despite mixed layers as deep as 90 m. Since the two bloom regions differ in their distance to shelf areas, potential sources of iron vary. Nutrient (nitrate, phosphate, silicate) deficits were similar in both areas despite different bloom ages, but their ratios indicated more pronounced iron limitation at 12 °W compared to 39 °W. While primarily the supply of iron and not the availability of light seemed to control onset and duration of the blooms, higher grazing pressure could have exerted a stronger control toward the declining phase of the blooms

Capillary filling with pseudo-potential binary Lattice-Boltzmann model

We present a systematic study of capillary filling for a binary fluid by using a mesoscopic lattice Boltzmann model for immiscible fluids describing a diffusive interface moving at a given contact angle with respect to the walls. The phenomenological way to impose a given contact angle is analysed. Particular attention is given to the case of complete wetting, that is contact angle equal to zero. Numerical results yield quantitative agreement with the theoretical Washburn law, provided that the correct ratio of the dynamic viscosities between the two fluids is used. Finally, the presence of precursor films is experienced and it is shown that these films advance in time with a square-root law but with a different prefactor with respect to the bulk interface.Comment: 13 pages, 8 figures, accepted for publication on The European journal of physics

Changes in meltwater chemistry over a 20-year period following a thermal regime switch from polythermal to cold-based glaciation at Austre Broggerbreen, Svalbard

Our long-term study gives a rare insight into meltwater hydrochemistry following the transition of Austre Brøggerbreen from polythermal to cold-based glaciation and its continued retreat. We find that the processes responsible for ion acquisition did not change throughout the period of records but became more productive. Two regimes before and after July/August 2000 were identified from changes in solute concentrations and pH. They resulted from increased chemical weathering occurring in ice-marginal and proglacial environments that have become progressively exposed by glacier retreat. Carbonate carbonation nearly doubled between 2000 and 2010, whilst increases in the weathering of silicate minerals were also marked. In addition, the end of ablation season chemistry was characterized by reactions in long residence time flow paths like those in subglacial environments, in spite of their absence in the watershed. Furthermore, the retreat of the glacier caused the sudden re-routing of meltwaters through its immediate forefield during 2009, which more than doubled crustal ion yields in this particular year and influenced chemical weathering in 2010 regardless of a low water flux. Such a “flush” of crustally derived ions can be meaningful for downstream terrestrial and marine ecosystems. We therefore find that, during glacier retreat, the recently exposed forefield is the most chemically active part of the watershed, making high rates of weathering possible, even when ice losses have caused a switch to cold-based conditions with no delayed subglacial drainage flowpaths. In addition, the drainage system reorganization events result in significant pCO2 depletion in an otherwise high pCO2 system

Lattice Gas Automata: Drying Simulation in Heterogeneous Models

Moisture flow in porous media is the driving force behind early age drying shrinkage. Cracking in the ITZ, between cement paste and aggregate-inclusion, is related to restraint caused by, among others, aggregates that obstruct free deformation of the paste. ESEM test results are used as a base for the developed method for measuring shrinkage deformations during drying. Modeling of moisture flow in the heterogeneous samples is numerically performed with Lattice Gas Automaton (LGA). Fracture simulation with the LGA, requests coupling with ESEM tests regarding shrinkage coefficient.Structural EngineeringCivil Engineering and Geoscience

Buffering, stoichiometry, and the sensitivity of pH to biogeochemical and physical processes: a proton-based model perspective (AGGREGATION CH 5)

Mechanistic understanding of factors governing pH is essential, given growing concern about ocean acidification. The classical approach to carbonate chemistry and pH calculations is centred around the alkalinity concept. In this publication, the classical approach is taken one step further: the in uences of biogeochemical and physical processes on the pH are calculated directly, without the detour of alkalinity. The in uence of a given process on pH is expressed by its process rate, modulated by a sensitivity factor. Here, we provide the necessary tools and procedures to calculate this sensitivity factor analytically for an arbitry process. Moreover, we show that the sensitivity can be decomposed as the ratio of a particular stoichiometric coefficient of protons over a buffer factor. The stoichiometric coefficient can be derived by describing the process using a fractional reaction equation at ambient pH and it represents the protons that are released by the process without accounting for re-equilibration. The presented approach thus provides a chemical interpretation of the mechanisms underlying pH changes in aquatic systems. Applying the concept of buffer capacity and pH sensitivities to an averaged global ocean shows that towards the end of the century the ocean will be around four times less buffered than it is today. This demonstrates how the concepts and tools presented here can make a complementary contribution to the existing modelling approaches of ocean acidification

Buffering, stoichiometry, and the sensitivity of pH to biogeochemical and physical processes: a proton-based model perspective (THESIS VERSION)

Mechanistic understanding of factors governing pH is essential, given growing concern about ocean acidi cation. The classical approach to carbonate chemistry and pH calculations is centred around the alkalinity concept. In this publication, the classical approach is taken one step further: the in uences of biogeochemical and physical processes on the pH are calculated directly, without the detour of alkalinity. The in uence of a given process on pH is expressed by its process rate, modulated by a sensitivity factor. Here, we provide the necessary tools and procedures to calculate this sensitivity factor analytically for an arbitry process. Moreover, we show that the sensitivity can be decomposed as the ratio of a particular stoichiometric coe cient of protons over a bu er factor. The stoichiometric coe cient can be derived by describing the process using a fractional reaction equation at ambient pH and it represents the protons that are released by the process without accounting for re-equilibration. The presented approach thus provides a chemical interpretation of the mechanisms underlying pH changes in aquatic systems. Applying the concept of bu er capacity and pH sensitivities to an averaged global ocean shows that towards the end of the century the ocean will be around four times less bu ered than it is today. This demonstrates how the concepts and tools presented here can make a complementary contribution to the existing modelling approaches of ocean acidification

An introduction to poloidal and toroidal fields

SIGLECopy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman