Chemical Processes II

1
Chemical Processes II
Lesson 4
2
Summary

• Chemical Processes II
• inorganic carbon
• nitrogen
• phosphorus
• sulfur
• silica
• iron

3
Inorganic Carbon

• CO2-diffusion across air/water interface (Henrys
  law) dissolution and stepwise dissociation
• speciation is pH-dependent usually HCO3-
  -dominated
• dissolution of calcium carbonate (limestone) by
  carbon dioxide-containing water
• CaCO3CO2H2O ? Ca(HCO3)2
• bicarbonate only, if dissolved carbon dioxide
  present (equilibrium carbon dioxide), otherwise
  re-precipitation of calcium carbonate

4
Inorganic Carbon
Fraction of total C species
pH
C-species change with pH
5
Inorganic Carbon
Reaction rates and amounts of CO2 in a typical
lake at pH 7 and 15C the slowest rate,
hydration and dehydration of CO2, will
potentially limit photosynthesis. ! Note the
small amount of dissolved CO2 available for
photosynthesis relative to the large HCO3- and
atmospheric CO2 pools.
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Inorganic Carbon
Depth-related changes in a typical eutrophic lake
in mid summer (left). !Disequilibrium of
atmosphere and water process speeds
Anoxic sediments
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Disturbances to Equilibrium

• Removal of CO2 CaCO3 precipitation, pH rise
• pressure decrease (e.g. travertine), temperature
  increase
• photosynthesis (marl lakes, lake whitening, e.g.
  5-15 g m-3 after primary production maximum
  phosphate co-precipitation possible)
• Addition of CO2 CaCO3 dissolution, pH decrease
• respiration (hypolimnion, soil)
• CO2 increase in atmosphere
• Carbonate acts as important buffer!

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Nitrogen

• nitrate input from inflow, terrestrial runoff and
  precipitation (previously underestimated!)
• in situ nitrogen fixation by blue-green algae,
  bacteria (Azotobacter, Chlostridium) or Alnus
• nitrate predominant in the epilimnion, can be
  used up during blooms ammonia predominant in the
  eutrophic hypolimnion and reducing sediments
• nitrite only at low concentrations unstable

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Nitrogen
Microbial N-cycle in a lake (after Kusnezow in
Bringmann 1970)
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Nitrogen
Oligotrophic
Mesotrophic
Generalized vertical distribution of NH3 and
NO3-nitrogen in stratified lakes of low and high
productivity
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Phosphorus

• rare and limiting nutrient (in water only few
  µg/l bioavailable, needed for ATP)

• naturally from minerals, e.g. apatite
  precipitation, atmospheric particles not from
  natural soils!

• anthropogenic from agriculture and detergents

• total phosphate (TP)
• dissolved inorganic phosphate (DIP,
  orthophosphate)
• dissolved organic phosphate (DOP)
• total particulate phosphate (TPP, not
  biologically available).

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Phosphorus cycles

• small P-cycle, ca. 80 in lake
• uptake of dissolved inorganic phosphate by
  organisms
• redissolution within water column
• large P-cycle, ca. 20 in lake
• dead organic matter sediments on lake bottom
• adsorption to particles, not bioavailable
• redissolution in reducing (ltlt0,5 mg/l O2)
  conditions
• uptake of dissolved inorganic phosphate by
  organisms

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Phosphorus cycle in lakes
Thick lines external loading, dashed lines
internal loading other lines internal
recycling. Most P is organic (living and dead
biomass) BOP biologically available P PP
particulate P
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Sulphur

• important nutrient, e.g., proteins
• mineralisation mainly by microbial action
• aerobic to sulfate SO42-
• anaerobic to hydrogen sulfide H2S other sulfides
• pathway depends on stratification/redox potential
• sedimentation
• sulfide /pyrite oxidation acidification of lakes

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Microbial Sulphur-cycle in a lake
After Kusnezow in Bringmann 1970
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Silica

• silicate (SiO2) in diatom frustule
• bioavailable as reactive silica H2SiO4
• no H2SiO4 during diatom blooms spring/autumn
• remineralisation in winter (and summer)
• sedimentation of frustules
• redissolution from sediments also bio-mediated

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Silica-cycling
The spring bloom of the holoplanktonic diatom
Asterionella (in lake Windermere). Note the
maximum bloom population (500 to 2000 times the
winter population typical for holoplankton with
high growth rates (redrawn from Lund 1964 and
Heron 1961)
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Iron

• important e.g. for haemoglobine oxygen transport
• dissolved iron Fe2 only at
• low oxygen (hypolimnion, groundwater)
• pH lt 7,5
• complexation by humic/fulvic acids
• oxidised species Fe3 insoluble (oxy/hydroxides)
• dissolution/reduction in hypolimnion, sediments,
  photoreduction
• precipitation in springs, wells, hypolimnion
  overturn oxidising bacteria

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The Iron, S and P interaction
in a eutrophic lake
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Questions

• The carbonate system and its equilibrium
  conditions play a major role in aquatic systems.
  a) Describe the equilibria and b) explain its
  relevance
• a) What are the most important nutrients in
  aquatic systems? List them and b) talk about
  sources, concentrations, and relevance
• Phosphorus is often referred to as a limiting
  nutrient in aquatic systems. a) Explain this
  idea, and b) argue for other elements that might
  be limiting nutrients under certain conditions
• Which role do N-species have in aquatic
  environments? B) List as many species as you can
  and describe their role. C) Give tolerable
  concentration (ranges) of these species
• How do you assess the chemical quality of an
  aquatic system? B) how do you reconstruct the
  chemical characteristics of an aquatic system? C)
  how do you sample water for chemical analysis in
  a lake?
• What are typical concentrations of S, P, and
  N-species in water? B) what Fe-concentrations do
  you expect?

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Table of Contents
Lecture
Contents, Syllabus, Scope, Field training,
References
Lesson 1
Introduction
Lesson 2
Physical processes I
Lesson 3
Physical processes II and chemical processes I
Lesson 4
Chemical processes II
Lesson 5
Major groups of organisms
Lesson 6
Habitats and communities
Lesson 7
Energy fluxes
Lesson 8
Reservoirs
Lesson 9
Selected limnological methods
Lesson 10
Extraction and mining lakes
Lesson 11
Degradation and rehabilitation of streams and
rivers