quick review of the redox hierarchy

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Iron Manganese

• quick review of the redox hierarchy
• iron
• manganese

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Redox Hierarchy in Lakes
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Iron in Lakes

• Fe3 e- ? Fe2 E7 0.30 to 0.20 v
• Fe3 ferric iron or iron (III), oxidized state
• Fe3 3OH- ? Fe(OH)3
• Fe3 PO4-3 ? FePO4
• Fe2 ferrous iron or iron (II), reduced state
• Fe2 2HCO3- ? Fe(HCO3-) or Fe(HCO3)2
• Fe2 S-2 ? FeS

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SeasonalIron Cycle
Figure 10-7 in Horne and Goldman
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Manganese in Lakes

• very similar to iron but
• reduces more easily
• more difficult to oxidize
• Thus, Mn2, as Mn(HCO3)2 can remain in solution
  with 1 mg/L of DO at pH7

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Distribution of Fe and Mn
From Wetzel, 1983
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Lake Management

• Iron has been to used in attempts to control
  eutrophication because it will bind with
  phosphate forming insoluble Fe(III)PO4
• Some have suggested manganese can also be used
  for eutrophication management

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Major Ions of Inland Waters

• salinity
• hard and soft waters
• specific ions Ca, S, Si, and minor metallic
  elements

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Salinity

• Salinity is the sum of anions and cations
• cations anions
• Ca2 CO3-
• Mg2 SO4-2
• Na Cl-
• K HCO3-
• Conductivity/specific conductance the measure of
  electrical flow through water
• - high salinity high conductivity

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Sources of Salinity

• weathering of rock or soil leaching
• chemical reactions
• redox
• acid-base reactions
• formation of complexes
• atmospheric precipitation and fallout
• evaporation

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Hard and Soft Water

• Hardness reflects the concentration of
  multivalent ions (i.e. usually Ca2 and Mg2),
  thus
• hard water has high salinity
• soft water has low salinity

BUT high salinity waters are not necessarily hard!
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Specific Ions - Calcium

• metabolism
• skeletal-strengthening material, cell walls of
  some alage
• HCO3-CO3 equilibrium
• main buffering system
• marl formation

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Specific Ions - Sulfer

• abundance in natural waters
• importance
• sources
• rain water
• drainage basin
• sulfur bacteria

Sulfur cycle is driven primarily by
microorganisms!
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Sulfur Cycle

• Active uptake of SO4-2 by photoplankton and
  planktonic bacteria

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Sulfur Cycle

• Return by zooplankton and fish excretion

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Sulfur Cycle

• Bacterial decomposition

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Sulfur Cycle

• Chemical and/or biological oxidation

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Sulfur Cycle

• Anaerobic reduction of SO4-2 by bacteria

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Sulfur Cycle

• 6 7. Precipitation of S-compounds

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Sulfur Cycle

• Inflows and outflows of SO4-2

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Sulfur Cycle

• Inflows, outflows, and loss to sediments of
  organic S compounds

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Four Groups of Sulfur Bacteria

• organic S decomposers (line 3)
• sulfate (SO4-2) reducers (line 5)
• sulfur oxidizers
• photosynthetic sulfur bacteria

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Specific Ions - Silicon

• sources
• inflows from weathered rock
• from below the photic zone
• insignificant amounts animal recycling
• ecological significance
• diatom frustules up to half the cells dry
  weight!

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Diatom Frustules in Lake Tahoe
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SEM Pictures of Diatom Frustules
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Seasonal Fluctuations of Silica
Figure 12-6 from Horne and Goldman
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Minor Metallic ElementsCu, Zn, Mo, and Co

• utilization by phytoplankton
• Fe gt Zn gt Cu gt Co gt Mn gt Mo
• rate of mineralization
• Fe gt Mn gt Co gt Zn gt Cu gt Mo