The influence of Fe on the distribution and kinetic speciation of Zn, Cd, Co and Ni in the Southern Ocean

The role of Fe as the key limiting nutrient for the growth of phytoplankton in the high-nitrate-low-chlorophyll (HNLC) waters of the Southern Ocean is now without question after a series of mesoscale Fe enrichment experiments conducted there over the last few years. Through its influence on phytoplankton physiology changes in Fe have an effect on the biological demand and the distribution of other nutrients. Some of the other key bio-elements have the potential to co-limit the growth of plankton species and in turn influence the phytoplankton community structure and the drawdown of macronutrients. In this context the trace metals Zn, Cd, Co and Ni are needed for the uptake and metabolisation of the macronutrients N, Si, C and P (Figure 1) and have been identified as prime candidates for further studies. While Zn, Co and Ni are important cofactors in various enzymes the role of Cd is ambivalent as it is toxic in relatively low concentrations. However recently it could be shown that a variety of marine diatoms have the ability to use Cd as a substitute for Zn in isoforms of the Carbonic Anhydrase. Furthermore a deepened knowledge regarding the biogeochemistry of Zn and Cd is fundamental to establish their use as paleo-tracers for carbon-export (Zn) and primary productivity (Cd).


Figure 1: Schematic diagram showing the influence of Fe availability and other trace metals on the uptake of macronutrients by diatoms in the Southern Ocean. The width of the arrows indicate the size of the uptake flux and dashed lines denote processes that are known to occur in culture studies but the evidence is limited from the field.

The chemical form of these trace metals in the marine environment determines their interactions with the environment and controls important parameters, e.g. solubility, particle reactivity and bio-availability. Thus it is important to analytically assess not only their distribution but also the speciation in order shed light on their biogeochemical cycle and their effect on the biological environment (Figure 2).


Figure 2: Schematic diagram of effect of Fe on speciation of other key bio-elements: The width of the arrow is proportional to the flux and the size of the symbols to the pool sizes of the inorganic (e.g. Zn) or organic pools (e.g. ZnL).

Central to this work will be investigations into the distribution and speciation of Zn, Cd, Co and Ni in the context of Fe availability in the Southern Ocean. In the proposed work there are 3 main objectives:

1. Analysis and interpretation of samples collected during the ZERO and DRAKE research cruise onboard the Polarstern (ANTXXIV-3: Cape Town to Punta Arenas Feb 6 – Apr 17, 2008).

2. To examine the effect of Fe limitation on the chemical speciation of Zn, Cd, Co and Ni.

3. To develop a budgetary scheme for Zn, Cd, Co and Ni in the Southern Ocean, including both concentrations of various inorganic and organic pools, size ranges and the fluxes between them.

The overall aim of this work is to combine the results of the objectives listed above into a comprehensive model of the key processes affecting the biogeochemistry of the 4 selected metals in the Southern Ocean.

The principal field work was carried out during Polarstern ANTXXIV-3 ZERO and DRAKE as part of the IPY project GEOTRACES. Presently the speciation and distribution of Zn, Cd, Co and Ni is examined across the gradients of iron concentration and bioavailabilty in seawater; found in the open ocean region along the prime meridian, the Weddell Sea and Drake Passage and near to Antarctic islands. Further laboratory work will focus on kinetic aspects of the speciation of these elements at the ambient temperatures found in Southern Ocean waters.

The proposed work is performed using state of the art techniques: (1) Competitive ligand exchange with detection by cathodic stripping voltammetry for the measurement of Co2+, Ni2+ kinetics and speciation. (2) Anodic stripping voltammetry for the measurement of Cd2+ and Zn2+ kinetics and speciation. (3) Standard solvent extraction methods with graphite furnace atomisation for the measurement of Zn, Cd, Co and Ni in seawater. The accumulated data will be incorporated into a comprehensive model for trace metal biogeochemistry in the Southern Ocean.

Scientists

Peter L. Croot
Chemical oceanography
IFM-GEOMAR, Kiel

Oliver Baars
Chemical oceanography
IFM-GEOMAR, Kiel

Peter Streu
Chemical oceanography
IFM-GEOMAR, Kiel

Research areas

Greenwich Meridian, Weddell Sea and Drake Passage

Publications

Baars O, Croot PL, 2011. The speciation of dissolved zinc in the Atlantic sector of the Southern Ocean. Deep Sea Research Part II: Topical Studies in Oceanography. in press

Baars O, Croot PL, 2011. Comparison of Alternate Reactants for pM Level Cobalt Analysis in Seawater by the Use of Catalytic Voltammetry. Electroanalysis. in press

Croot PL, Baars O, Streu P, 2011. The distribution of dissolved Zinc in the Atlantic sector of the Southern Ocean. Deep Sea Research II. in press

Croot PL, Bluhm K, Schlosser C Streu P, Breitbarth E, Frew R, Ardelan MV, 2008. Regeneration of Fe(II) during EIFeX and SOFeX. Geophysical Research Letters 35, L19606, doi:10.1029/2008GL035063.

Croot PL, Frew RD, Hunter KA, Sander S, Ellwood MJ, Abraham ER, Law CS, Smith MJ, Boyd PW, 2007. The effects of physical forcing on iron chemistry and speciation during the FeCycle experiment in the South West Pacific. Journal of Geophysical Research - Oceans 112, C06015, doi:10.1029/2006JC003748.

Croot PL, Passow U, Assmy P, Jansen S, Strass VH, 2007. Surface-active substances in the upper water column during a Southern Ocean Iron Fertilization Experiment (EIFEX). Geophysical Research Letters 34, L03612, doi:10.1029/2006GL028080.

Croot PL, Laan P, Nishioka J, Strass V, Cisewski B, Boye M, Timmermans K, Bellerby R, Goldson L de Baar HJW 2005. Spatial and Temporal distribution of Fe(II) and H2O2 during EISENEX, an open ocean mesoscale iron enrichment. Marine Chemistry 95, 65-88.

Croot PL, Andersson K, Ötztürk M, Turner D, 2004. The Distribution and Speciation of Iron along 6° E, in the Southern Ocean. Deep-Sea Research II 51, 2857-2879.

Croot PL, Laan P, 2002. Continuous shipboard determination of Fe(II) in Polar waters using flow injection analysis with chemiluminescence detection. Analytica Chimica Acta 466, 261-273.

Croot PL, Laan P, 2001. Ferrous Wheels in the Ocean: The Southern Ocean Fairground. Berichte zur Polarforschung, Alfred-Wegener-Institut für Polar- und Meeresforschung 400: 149-158.

Croot PL, Bowie AR, Frew RD, Maldonado M, Hall JA, Safi KA, la Roche J, Boyd PW, Law CS, 2001. Retention of dissolved iron and FeII in an iron induced Southern Ocean phytoplankton bloom. Geophysical Research Letters 28, 3425-3428.

Croot PL, de Jong JTM, 2000. Field distribution of iron in a section of the Antarctic Polar Frontal Zone. Berichte zur Polarforschung, Alfred-Wegener-Institut für Polar- und Meeresforschung 364, 44-50.

Croot PL, Johansson M, 2000. Determination of iron speciation by cathodic stripping voltammetry in seawater using the competing ligand 2-(2-Thiazolylazo)-p-cresol (TAC). Electroanalysis 12, 565-576.

Homepage

Homepage IFM-GEOMAR

Research funding organisation

German Research Foundation

Project numbers: CR 10-1
Funding period: July 2007 - June 2009