LU6: BEHAVIOUR OF METALS IN THE NATURAL ENVIRONMENT Instructor: Farah Akmal Idrus

The importance of metals in the water system Nutrients: Macronutrients Nitrate, phosphate, silicate Micronutrients - metals Iron, manganese, cobalt, nickel, copper, zinc Metals: Essential elements Iron, manganese, cobalt, nickel, copper, zinc Potentially toxic at higher concentrations. Non-essential elements Lead, mercury, cadmium? Mostly toxic.

Extended Redfield ratio Alfred Redfield and co- workers (1963) developed a general concept of the ratio of the macronutrients composition in oceanic production: Redfield ratio C : N : P = 106 : 16 : 1 Later, researchers found that several metals are also vital, and the Redfield ratio formula was extended : Extended Redfield ratio C:N:P:S:K:Mg:Ca:Sr:Fe:Mn:Zn:Cu: Co:Cd:Mo (124:16:1:1.3:1.7:0.56:0.5)1000 5:7.5:3.0:0.8:0.38:0.19:0.21:0.03

Metal-organism interactions

Suggested Causes of HNLC: 1. Lack of micro-nutrient iron 2. Grazing suppression of blooms 3. Lack of light

Metal-phytoplankton interactions

How much metal (e.g. Mn) is needed by the phytoplankton? Only small amount of Mn is required by phytoplankton to get function for the growth. By assuming all organic C are phytoplankton, thus at ~0.6 μg Chl-a/L, 1.0 μM C is produced. Extended Redfield ratio – C:Mn is 41300:1. Tiny fraction of Mn (~25 pM) is needed.

Metals speciation Metals exist in seawater in a variety of chemical species. Strongly influences their availability to phytoplankton and their biogeochemical cycles. Free metals ion ( Fe2+/Fe3+, Mn2+, Cu2+, Zn2+ ) – RESPONSIBLE FOR BIOLOGICAL EFFECTS Complexes with organic & inorganic ligands ( OH-/CO32-, Cl-, SO42-, HS- ) – THE MAIN COMPLEXING LIGANDS BASED ON THERMODYNAMIC EQUILIBRIA Adsorbed on the particulate phases In particulate phases

The manganese speciation cycle in the marine environment MnO2(s) Mn (IV) Mn (III) Mn (II) Birnessite, Mn(III/IV) MnOOH (s) Mn(OH)n 3+ (aq) Mn(III)L (aq) Mn 2+ (aq) MnCO3 (s) Mn(II)L (aq) Hausmannite, Mn(II/III) The manganese speciation cycle in the marine environment

Concentration and distribution of metals in water system Cu in Western North Sea (Achterberg et al., 2003) Mn in tropical NE Atlantic Ocean (Idrus, 2013)

Depth profile: Classification of dissolved elements

Depth profile: Classification of dissolved elements Conservative elements: interact only weakly with the biological particle cycle & nearly all have residence times of >106 years. Recycled elements: Heavily utilised in the photic zone. Their availability can limit the primary production. Can be almost totally depleted in surface waters. Consumption & decomposition of organic matter sinking from the surface waters return the nutrients to solution.

Well-stratified surface – gravitationally stable Exercise: Why well-stratified surface waters are more likely to be rapidly depleted in nutrients than a well-mixed water column? Answer: Well-stratified surface – gravitationally stable Nutrients carried down in sinking organic debris Can’t be replaced except by slow diffusion processes. Well-mixed – Nutrients returned to deep waters due to decomposition of organic matter May be carried back up to the surface by the turbulence.

Exercise: How the general depth profile of Cd looks like?

Depth profile: Classification of dissolved elements Scavenged elements: Factors influenced the distribution: Residence time Sources : estuaries, sea-bed, aeolian dust Adsorption – desorption of metal ions onto particle surfaces (e.g. bacteria)

Sources of Scavenged elements Dissolved Al Dissolved Mn Give answer in Morpheus

Depth profile

Sergasso Sea (Shelley et al., 2012)

Over the Bouvet Triple Junction Ridge, Southern Ocean (Middag et al

Factors influencing the conc. & distribution of metals in water PBL

Recycling process of metal (e.g. Mn) in the water system The uptake of dissolved manganese is at maximum during the growth of phytoplankton, and depends on the species . growth is stopped at some stage, may be due to lack of macronutrients . dissolved manganese is adsorbed onto the outer layer of phytoplankton cells . Biogenic material is increases after the bloom event as dead phytoplankton sink down through the water column into the deep ocean, thus losing the dissolved manganese from the upper ocean. The sinking manganese is then released back into the water column by recycling processes and the vertical mixing (e.g. upwelling) brings the dissolved manganese back into the upper water column . Some fraction of the particulate manganese will be removed from the water column and deposited into the sediment.

Metals in estuary??

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