EVPP 550 Waterscape Ecology and Management

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EVPP 550Waterscape Ecology and Management

• Professor
• R. Christian Jones
• Fall 2007

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Water Chemistry CO2, alk, pH

• Global carbon cycle includes
• Photosynthesis
• Respiration
• Fossil Fuel combustion
• Ocean interactions
• Rock interactions (over long term)

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Water Chemistry CO2, alk, pH
Ice core data

• Earths atmosphere contains relatively small
  amounts of CO2 as compared to O2
• But the amount has increased greatly over the
  past several decades
• As a greenhouse gas, CO2 is a major factor in the
  warming of Earth surface temperatures
• CO2 is also intimately involved in the
  carbonate-bicarbonate buffering system that
  controls pH in most freshwaters

Direct Measurements
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Water Chemistry CO2, alk, pH

• Carbon dioxide dissolves in water to produce
  carbonic acid
• Carbonic acid dissociates to produce bicarbonate
  and hydrogen ion (1st dissociation of carbonic
  acid)
• Bicarbonate dissociates to produce carbonate and
  another hydrogen ion (2nd dissociation of
  carbonic acid)

• CO2 H20 ? H2CO3
• H2CO3 ? HCO3- H
• HCO3- ? CO3-2 H

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Water Chemistry CO2, alk, pH

• pH -log H
• pH is the negative log of the hydrogen ion
  concentration
• pH 4 means H 10-4
• pH 7 means H 10-7
• pH 10 means H 10-10

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Water Chemistry CO2, alk, pH

• The relative amounts of carbonate, bicarbonate,
  and carbon dioxide-carbonic acid change with pH
  in a predictable manner based on dissociation
  equations
• At high pH, carbonate dominates
• At intermediate pH, bicarbonate dominates
• At low pH, carbon dioxide-carbonic acid dominates

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Water Chemistry CO2, alk, pH

• Alkalinity is the ability of water to resist
  acidification
• If the carbonate-bicarbonate system is the major
  buffer, then pH change can be resisted as long as
  bicarbonate and carbonate are present since they
  can absorb hydrogen ions
• Alkalinity HCO3- 2 x CO3-2

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Water Chemistry CO2, alk, pH

• pH of rain in equilibrium with atmospheric CO2 is
  about 5.5
• Pollutants such as sulfate and NOX decrease it
  futher
• The total amount of alkalinity in a given water
  body is based, not only on the input of CO2 from
  the atmosphere, but even more so on sources of
  carbonate and bicarbonate from the watershed

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Water Chemistry CO2, alk, pH

• For some purposes we need to know the total
  amount of dissolved inorganic carbon (DIC) in a
  water body
• This determines the carbon available for
  photosynthesis and also is needed to calculate
  the photosynthetic rate using the C-14 method

• DIC H2CO3 HCO3- CO3-2
• Based on equations in handout, if we know pH,
  alkalinity, and temperature, we can derive total
  DIC and concentration of all forms of DIC

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Water Chemistry CO2, alk, pH
CO2 H20 ? H2CO3 ? HCO3- H ? CO3-2 H

• Effect of photosynthesis on pH and carbonate
  system
• Effect of respiration on pH and carbonate system

• Psyn consumes CO2, equilibrium shifts to left
  resulting in consumption of H and increase in pH
• Resp releases CO2, equilibrium shift to left
  resulting in release of H and decrease in pH

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Water Chemistry CO2, alk, pH

• Vertical profiles of pH

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Water Chemistry Dissolved Ions

• Sources
• Atmosphere
• Soil/rocks
• Dissolution
• Weathering
• Sediments

• Measurement
• Total Dissolved Solids (TDS)
• aka Filterable Residue
• Gravimetric procedure
• Filter substantial volume of water, then
  evaporate filtrate until constant weight
• Problems some residues are volatile and some
  retain water

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Water Chemistry Dissolved Ions

• Range 1 mg/L to 300,000 mg/L (saturated brine)
• Equivalent to 0.001 300 ppt
• Fresh water lt 1 ppt
• Ocean 35 ppt
• Great Salt Lake 220 ppt

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Water Chemistry Dissolved Ions

• Conductivity
• Measures the ability of water to conduct an
  electrical current
• Proportional to the number of ions in solution
• Pure water has a very low conductance (lt0.1
  umho/cm uS/cm)
• Conductance is a rough measure of TDS which can
  be calibrated more accurately for a given
  waterbody

• Conductivity
• Is a function of temperature so values need to be
  standardized to a given temperature, usually 25oC
• Conductivity increases by a factor of about 0.025
  per oC
• So to get Specific Conductance (Conductivity
  standardized to 25oC)
• Cond(25oC) Cond (T) x 1.025(25-T)

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Water Chemistry Dissolved Ions

• Anions
• CO3-2 and HCO3- (70-75 by wt)
• SO4-2 and Cl- also important
• Cations
• Ca2 (60)
• Mg2 (15-20)
• Na (15-20)
• K (5-10)

• Alkalinity
• CO3-2 HCO3-
• Acid buffering capacity
• Hardness
• Ca2 Mg2
• Reaction to soap
• More soap required in hard water because Ca and
  Mg tie some of it up

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Water Chemistry - Nitrogen

• Forms
• N2 dissolved molecular nitrogen
• NH4, NH3, NH4OH ammonia nitrogen
• NO2- nitrite ion
• NO3- nitrate ion
• Organic nitrogen includes proteins, amino acids,
  urea, etc.

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Water Chemistry - Nitrogen

• Forms
• Equilibrium between ammonia nitrogen forms is a
  function of temperature and pH

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Water Chemistry - Nitrogen

• Transformations
• Nitrogen fixation
• N2 ? reduced organic N (like amino acid)
• Three groups of organisms can do this
• Aerobic and anaerobic heterotrophic bacteria use
  organic matter as energy substrate/important in
  sediments
• Cyanobacteria use light as energy
  source/important in open water/done in
  heterocysts/may occur in large blooms in
  midsummer in enriched lakes
• Purple photosynthetic bacteria use light as
  energy source, but need anoxic conditions

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Water Chemistry - Nitrogen

• Transformations
• Nitrogen fixation
• Rate of N fixation in water column is increased
  during N limitation
• Rate of N limitation is related to light
  intensity implying that light energy is driving
  the process

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Water Chemistry - Nitrogen

• Transformations
• Assimilation of combined nitrogen
• NH4 ? reduced organic nitrogen (like amino acid)
• NO3- ? reduced organic nitrogen (like amino acid)
• NH4 is energetically more favorable as it is
  already reduced

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Water Chemistry - Nitrogen

• Transformations
• Proteolysis or ammonification
• Organic Nitrogen ? NH4
• Proteolytic bacteria use energy released from
  this transformation for metabolism
• Nitrification
• NH4 ? NO2-
• Nitrosomonas uses energy released for metabolism
• NO2- ? NO3-
• Nitrobacter uses energy released for metabolism
• Reaction occurs quickly so NO2- generally very low

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Water Chemistry - Nitrogen

• Transformations
• Denitrification
• NO3- ? N2
• Anaerobic/aerobic interface habitats such as
  mud-water interface
• Active in sediments and wetlands, may greatly
  deplete NO3 in groundwater

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Water Chemistry - Nitrogen
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Water Chemistry - Nitrogen
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Water Chemistry - Nitrogen
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Water Chemistry - Nitrogen
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Water Chemistry - Nitrogen
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Water Chemistry - Nitrogen
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Water Chemistry - Phosphorus

• Importance to organisms
• Nucleic acids
• Adenosine Triphosphate (high energy PO4 bonds)
• Bones and other solid inclusions

• Sources
• Erosion of igneous rocks
• Dissolution of phosphate-containing sedimentary
  rocks
• Guano beds, bone skeletons
• Human and animal waste, detergents

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Water Chemistry - Phosphorus

• Forms of phosphorus
• In biological systems and in water, almost all P
  is in the PO4 form
• Can be individual PO4-3 ions or PO4 group can be
  combined with organic molecules, either dissolved
  or particulate

• Analytic Forms
• Phosphate ion aka orthophosphate aka soluble
  reactive phosphorus
• Measured on filtered samples
• Total soluble phosphorus
• Measured on filtered sample after digestion
• Total phosphorus
• Measured on whole water samples after digestion

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Water Chemistry - Phosphorus

• Ortho-P
• Only directly utilizable form of inorganic P
• May be formed from organic P by enzymatic action
• Reacts with other chemicals and adsorps to
  particles and elements like Fe

• Organic P Total P Ortho P
• Often most P in lakes is tied up in organisms or
  detritus
• Can cycle between ortho-P and organic P

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Water Chemistry - Phosphorus

• P cycle in lakes

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Water Chemistry - Phosphorus

• P cycle in lakes

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Water Chemistry - Phosphorus

• P profiles in various lakes

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Water Chemistry - Iron

• Iron is a necessary requirement for all living
  organisms (enzyme systems)
• Iron has two states
• Fe3 ferric ion
• Forms insoluble compounds
• Found under oxic conditions
• Fe2 ferrous ion
• Is generally soluble
• Found under anoxic conditions

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Water Chemistry - Iron

• Even though generally insoluble in oxic
  epilimnion, Fe can be held there by chelators
  (compounds that weakly bind it to prevent
  precipitation, but may give it up to cells)

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Water Chemistry - Iron

• Generally, however, in oxic conditions Fe is
  found in a precipitated oxide form such as
  Fe(OH)3
• These iron precipitates help to bind PO4 in the
  sediments and keep it from migrating into the
  water column

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Water Chemistry - Iron

• However, when anoxic conditions set in, the
  Fe(OH)3 dissolves and PO4-3 can be rapidly
  released fueling algal growth

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Water Chemistry - Iron

• However, when anoxic conditions set in, the
  Fe(OH)3 dissolves and PO4-3 can be rapidly
  released fueling algal growth

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Water Chemistry - Iron

• However, when anoxic conditions set in, the
  Fe(OH)3 dissolves and PO4-3 can be rapidly
  released fueling algal growth

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Water Chemistry - Silicon

• Required for diatioms
• Removed from the water column during diatom
  growth and sinking
• May come to limit diatom growth during the
  growing season